Weather Break -- Tehuantepecers, Part 2

The following is a transcript of the Weather Break radio show for Thursday, January 31, 2008. This episode was written by Dr. Jon Schrage.

On yesterday’s episode of Weather Break, we talked about a weather phenomenon called “tehuantepecers”. Tehuantepecers are extreme wind events in Central America, when cold air masses from the arctic slide south all the way across the United States into the Gulf of Mexico. The cold, dense air is trapped on the Atlantic side of Central America by the fairly high, continuous mountain ranges found there. These mountain ranges make a very effective wall that keeps this air from crossing Central America and reaching the Pacific side, except in the area of Chivela Pass, which is a narrow gap in the mountains in Mexico. The Chivela Pass acts like a nozzle, squirting the cold, dense air from the Atlantic side to the Pacific side. The air accelerates through the Pass, sometimes reaching speeds of 100 miles per hours as it crosses the Pacific coast of Mexico and heads out to sea. Tehuantepecers are a particularly spectacular example of what meteorologists call “gap flow”, where air masses accelerate as they are squeezed through narrow gaps in mountain ranges.

Tehuantepecers get their name from the fact that the part of the coast of Mexico where they occur is called the Gulf of Tehuantepec. The Gulf of Tehuantepec is a particularly important fishing area; it’s where much of the world’s tuna catch comes from, for example. The Gulf of Tehuantepec is such a productive part of the ocean because of the upwelling of cold, nutrient-rich water right along the coast of Central America. These nutrients support various forms of plankton in the Gulf of Tehuantepec, which are the food source for small fish. The small fish are eaten by bigger fish like tuna, which, in turn, we eat.

During a tehuantepecer, strong surface winds of as much as 100 miles per hour race out over the Gulf of Tehuantepec. These strong winds generate huge waves that cause strong mixing in the ocean. The mixing of surface water with deep, cold, nutrient-rich waters is one of the hallmarks of a tehuantepecer. Sea surface temperatures in the Gulf of Tehuantepec can fall by as much as 15 degrees Fahrenheit in a matter of hours. Satellites in orbit can actually detect sea surface temperatures from space, and monitoring and forecasting the dramatic changes in water temperature is the work of synoptic oceanographers.

When the nutrient-rich water gets to the surface of the ocean, plankton in the ocean take advantage of the food and the sunlight, and they reproduce wildly. A so-called “plankton bloom” sets up for a matter of days or weeks, when the surface waters of the Gulf of Tehuantepec suddenly teem with many times more plankton than usual. The increased numbers of plankton form the base of the food chain in the region, so more plankton means more small fish, and maybe eventually more tuna.

Because the population of plankton is so important to life in the ocean, scientists have developed ways to monitor plankton worldwide. Plankton change the color of the ocean slightly, so special satellites have been developed that can detect subtle changes in the color of the ocean and, from that information, determine how large the plankton population of a region is. One such satellite is called Sea-WIFS. Scientists can actually watch the growth and development of plankton blooms after tehuantepecers using Sea-WIFS imagery; we’ll have examples of what can be seen using Sea-WIFS on our website.

Tehuantepecers don’t happen every time a cold air mass slips down from the arctic, but they do typically happen a few times each year. One particularly famous example of a tehuantepecer happened in March of 1993 in association with the so-called Storm of the Century, which, in addition to causing an impressive tehuantepecer and plankton bloom, also brought as much as 4 feet of snow in the eastern US.

Weather Break -- Tehuantepecers, Part 1

The following is a transcript of the Weather Break radio show for Wednesday, January 30, 2008. The program was written by Dr. Jon Schrage.

In mid January, much of the US was suffering under the influence of a large, arctic air mass that brought unseasonably cold temperatures and low wind chills. In Nebraska and South Dakota, nighttime low temperatures were well below zero several days in a row. Now, during this time, I was actually down in New Orleans attending the Annual Meeting of the American Meteorological Society. All of the local people that I met there were complaining about the cold weather there, too. Now, Louisiana was also under the same cold air mass that the upper-midwest was, but by the time the air mass had reached New Orleans, the air mass had “modified”—nighttime low temperature in New Orleans were running right around 32 degrees during that cold air outbreak. To folks in Nebraska and South Dakota, that might not seem so cold, but that is far below normal in southern Louisiana. In TX and LA, these cold air outbreaks are called “northers”.

It’s often the case that a cold air mass slipping south across North America will stall right along the Gulf Coast. Most of the time, meteorological conditions prevent a cold air mass from continuing south across the Gulf of Mexico—that’s part of the reason why the islands of the Gulf and the Caribbean are nice places to take vacations this time of year. However, sometimes the processes that keep cold air masses from sliding out in the Gulf of Mexico aren’t in place, and the atmosphere is set up for an interesting weather phenomenon called a tehuantepecer.

Cold air masses are relatively dense. This dense, heavy air is hard to lift, so it tends to stay trapped right near the surface. It resists being lifted up to go over a mountain range, for example. Now, when a cold air mass moves out over the warm waters of the Gulf of Mexico, the process of modification continues and even accelerates; the cold air is touching the warm surface waters, so heat from the gulf is transferred up into the cold air mass, increasing its temperature. A cold air mass over the Gulf of Mexico is certainly far warmer than it was when it crossed NE and SD, but it is still unseasonably cold for these tropical and subtropical locations.

As the cold air mass presses farther and farther to the south, it is hemmed in on the west by the mountain ranges of Central America. These countries are very mountainous, and the mountain ranges act like a wall, keeping the cold air mass in the Gulf of Mexico and the Caribbean from crossing Central America and pushing westward into the Pacific Ocean. The cold, dense, arctic air mass piles up against this wall of mountains all along the east coast of Central America.

These mountain ranges form a very effective barrier between surface weather in on the Atlantic side of Central America and the Pacific side, except in one location: Chivela Pass in Mexico. This is a narrow pass through the mountain range. It’s only about 25 miles wide, but it’s a little gap in the mountain ranges of Central America—in fact, it’s one of the ONLY such gaps. Cold, dense, arctic air masses that have been piled up against the wall of mountain ranges in Mexico find this narrow gap, and the air begins to race through Chivela Pass, much like air races out of a hole in a tire. Winds accelerate through the Pass and reach speeds of up to 100 miles per hour as they exit the Pass on the Pacific side. The body of water on the Pacific coast of Chivela Pass is called the Gulf of Tehuantepec, so these extreme wind events are known as “tehuantepecers”.

During a tehuantepecer, the Chivela Pass is like a nozzle squirting relatively dense, cold air of arctic origin from the Atlantic side of Central America to the Pacific side. These winds don’t just stop at the coast; rather, they race out to side with surprising speed, where they can be a serious hazard to unsuspecting ships. There are many reports of ships being sandblasted by these winds and the dust and sand that they picked up on the Pacific coast of Mexico.

Tehuantepers typically happen a few times each year, and they have important effects on the Pacific ocean and the environment. On tomorrow’s episode of Weather Break, we’ll talk more about tehuantepers and what they mean for fishing and sea life in the Gulf of Tehuantepec.

Weather Break -- Snowmaking

This is a transcript of the Weather Break radio show for Tuesday, January 29, 2008. This episode was written by Dr. Jon Schrage.

On Monday's episode of Weather Break, we talked about mankind's ongoing desire to be able to actually control the weather and the climate of his environment. Meteorologists know that it isn't really possible to have some kind of "weather control satellite" or anything like that--the atmosphere is just too big and too complex to be able to regulate like that for human purposes.

However, there are some ways in which scientists and engineers CAN control -- or at least INFLUENCE -- the weather of a region, especially if the region is pretty small. One example of this kind of work is snowmaking.

In the winter, ski resorts really only make money when there is enough snow on the ground for people to be able to ski. However, there really doesn't need to be snow all over the mountain; really, just on the ski trails and ski runs themselves is enough, especially very early and very late in the ski season. Since the total amount of land that they are trying to cover with snow is pretty small, ski resorts will typically practice a certain amount of snowmaking to either extend the skiing season or to help cover up bare or icy patches that may form on the runs.

Strictly speaking, snowmaking isn't exactly a form of weather modification, since the snow that is produced isn't falling from the clouds. In fact, snowmaking works just as well if the skies are clear as if they are cloudy! When a ski resort practices snowmaking, they pump water up the mountain through pipes. The water is combined with cooled and compressed air and then shot through a nozzle called a gun into cold, mountain air. By adjusting the mixture of air and water, the operator is able to produce tiny droplets of liquid water that are small enough to freeze into little ice crystals before they hit the ground. Sometimes, chemicals are added to the water, too, that facilitate the freezing.

If snowmaking sounds like they are pretty much just spraying water into the air and hoping that it lands in the form of ice, you're pretty much right. The process gets a little bit more scientific than that as newer snowmaking equipment can make use of weather information like temperatures and humidities to adjust the mixture of air and water correctly to optimize the efficiency of the process. Still, even under the best of conditions, snowmaking is an inefficient and expensive process. In the Western states in particular, simply securing the rights to the large amounts of water needed can be difficult or even impossible. That's why, even though snowmaking is technologically possible, it still only really makes sense to operate ski resorts in locations where the vast majority of the snowfall will be natural.

We have lots more information about snowmaking and weather at ski resorts on our website. As always, you can find us online at weatherbreak.creighton.edu. If you have questions or comments about anything that you have heard on Weather Break -- or anything to do with the weather, for that matter -- you can always call and leave us a message at 302-258-1617. We'd love to hear from you.

By the way, today is a very special day for us here at Weather Break. One year ago today, we aired our very first episode of this show! Today's is the 242nd episode, and we hope that we will be able to continue producing this program and sharing our enthusiasm for the atmosphere for a long time to come. I want to take this opportunity to thank all of the people that make this program possible, both at Creighton and at the stations that so kindly allow us to have a few minutes of their air time each day: KINI in St. Francis, SD and KBRB in Ainsworth, NE. On behalf of everyone here at the Creighton University Department of Atmospheric Sciences, thanks for tuning in, and we hope that you tune in again tomorrow.

Weather Break -- Can't We Do Something About All This Cold Weather?

The following is a transcript of the Weather Break radio show for Monday, January 28, 2008. The program was written by Dr. Jon Schrage.

All last week, thousands of meteorologists from around the world gathered in New Orleans, Louisiana for the Annual Meetings of the American Meteorological Society. Of course, this also coincided with the outbreak of extremely cold weather across most of the United States. You know, Mark Twain wrote that, "Everyone talks about the weather, but no does anything about it." Well, obviously, there was a LOT of "talking about the weather" last week, and you can probably imagine that lots of people quipped that we should "do something about" the subzero temperatures. So, why can't scientists control the weather?

Well, firstly, in some small ways, we CAN at least INFLUENCE the weather and climate of a region. Changes in the use of the land by people produce changes in the climate and weather patterns that a region experiences. For example, building cities like Phoenix and Las Vegas out in the desert has produced a moistening of the climate of those regions. While those particular cities do a reasonably good job of managing water and encouraging conservation, there IS more evaporation from those cities than there is in the surrounding countryside, and this increased humidity increases annual rain rates and cloud cover. Similarly, breaking the sod to raise cash crops in the Midwest is believed to have produced some climate change in the 19th century-- in fact, the pioneers were counting on it! Most of the pioneers believed that the lands of the central United States were too dry to raise crops, but conventional wisdom at that time was that "the rain follows the plow"; in other words, they believed that the land would become more moist once the sod was broken. We'll have more information about the idea that the rain follows of the plow on Friday's episode of Weather Break.

That being said, I think that we're all in agreement that this isn't what people mean when they jokingly ask the meteorologists if they could do something about the weather. Rather, they mean that they are wondering why it isn't possible to just flip a switch and make the weather do something that we want it to do, like increase the temperature at the surface on a cold day.

Well, the primary reason why we can't control the weather is that the atmosphere is really big. Air may seem light, but there's a lot it in the atmosphere, and it would take a lot of energy to heat the air up. For example, let's suppose that you wanted to make a system that somehow used a satellite in space to heat up the air and make cold winter days a little warmer. For example, maybe there are lasers on the satellite or something like that. Okay, let's say that you wanted to heat up all of the air over the United States by 10°F, and just for the sake of argument let's say that you wanted to heat the air from the surface up to a depth of just 10 m. That would mean that you would need to heat up a volume of air the size of the United States and as deep as a four story building. The amount of heat needed to increase the temperature by just 10°F would be 5 times 10 to the 17th Joules--that's the equivalent of 144 megatons of TNT, or about the same about of energy as would be released by detonating about 430 ballistic missiles! I don't know about you, but I'm GLAD that we don't have satellites in orbit that can deliver that kind of energy!

That kind of "brute force" weather modification strategy really can only work on the very smallest of areas. For example, farmers in the South can protect their orchards from frost by setting up so-called "orchard heaters", which help keep the temperature from falling too far below freezing on a chilly night. It works, but it's expensive--farmers in the Midwest couldn't afford to use a strategy like that to protect corn and soybeans, for example, because an acre of soybeans doesn't produce nearly as much money as an acre of oranges.

Even if there was a way in which somehow meteorologists could control the weather, it wouldn't be a very good idea, simply because it would be impossible to predict all of the unintended consequences of weather modification. Besides, just because a particular weather pattern is unpleasant or uncomfortable for people, that doesn't mean that it isn't important for some other aspect of the environment. Winter storms may cause travel headaches for people trapped at airports, but the moisture from these storms can be very effective at soaking down into the soil, providing moisture to crops in the summer. Subzero temperatures are unpleasant, but some plants have seeds that need cold temperatures in the winter in order to germinate in the spring. Meteorologists and atmospheric scientists tend to be pretty smart, but there's no way they could possibly anticipate all of the possible things that would go wrong if they tried to control the weather on any large scale.

Weather Break -- Are Cold Fronts and Warm Fronts the Same Thing?

The follow is a transcript of the Weather Break radio show for Friday, January 25, 2008. The episode was written by Dr. Jon Schrage.

The recent cold temperatures across much of the country happened as an extremely cold, Arctic airmass slipped south across North America behind a strong cold front. Anyone who spent much time outside recently probably found himself wishing that a warm front would pass through, bringing spring weather. You might be puzzled to know that students of meteorology often find themselves wondering if cold fronts and warm fronts are actually the same thing.

It’s not as silly of a question as it sounds. Cold fronts and warm fronts certainly do have a lot of features in common. Both cold fronts and warm fronts separate air masses with different temperatures, for example. In that sense, both cold fronts and warm fronts are examples of what meteorologists call polar fronts; polar fronts are the boundary between cold air coming from the north and warm air coming from the south.

Another way in which cold fronts and warm fronts are alike has to do with their shape. Both cold fronts and warm fronts are three-dimensional. Think of fronts as like a wall in the atmosphere that separates two air masses with different temperatures. In general, this wall or boundary is not straight up and down, however; rather, it leans to the north. In other words, cold fronts and warm fronts are both three-dimensional boundaries that start at the surface of the earth and slope to the north with respect to height. In fact, they slope a LOT. A typical frontal surface rises only 1 km for every 50-200 kilometers of distance to the north! That’s a really shallow slope!

Another way in which cold fronts and warm fronts are alike is their relationship to the jet stream. One of the fundamental rules of meteorology is called the Thermal Wind Relationship, and one of the consequences of the Thermal Wind Relationship is that jet streams are found aloft directly above locations where temperature is changing rapidly at the surface. Since both cold fronts and warm fronts are places where temperature is changing rapidly, both types of fronts have jet streams directly above them.

In fact, the differences between cold fronts and warm fronts are really in the details. The main way in which they are different has to do with the direction in which the frontal boundary is moving. In a cold front, the colder air mass is advancing and pushing the warmer air out the way. In a warm front, however, the warmer air mass is pushing the colder air out of the way. Think about a “front” in the military sense of the word, where the “front” is the boundary between two competing armies. In many ways, the military front is same regardless of which army is advancing. However, on the ground, it makes a BIG difference which army is advancing, right? In the same way, the main difference between cold fronts and warm fronts is just which air mass is pushing which out of the way. In that sense, you can think of a warm front as just a cold front that is “backing up”.

The cold fronts and warm fronts are different in a number of smaller ways, too. Cold fronts move much faster than warm fronts do, and they are somewhat more steeply sloped. Cold fronts tend to bring more showery weather, whereas warm fronts tend to bring more gentle, continuous rains. There are different varieties of clouds that tend to be seen along each type of front too.

So the next time that you are out in the driveway, shoveling snow and cursing the cold front that just passed through, now you know what to wish for—just hope that the atmosphere finds a way to make that cold front “back up”, and you’ll have a nice warm front on your hands!

Weather Break -- Development of the Wind Chill Temperature

This is a transcript of the Weather Break radio show for Wednesday, January 23, 2008. The episode was written by Dr. Jon Schrage.

On yesterday's episode of Weather Break, we talked about the meaning of the wind chill temperature. I think pretty much everyone knows that the air FEELS colder when the wind is blowing than when it is calm outside. As we discussed yesterday, this is because our sense of "how cold it is" is really a measure of how fast your body is losing heat. When the wind is blowing, your body loses heat more rapidly that it does under calm conditions, and so you feel colder. Meteorologists have found complex mathematical formulas to determine these rates of heat loss, which they summarize as the wind chill temperature.

The modern way in which wind chill temperatures are computed involves a complex understanding of the thermodynamics and the fluid mechanics involved, but the original formulation was purely empirical, meaning that it was based on experimentation and observation. The original inventors of the wind chill factor were Paul Siple and Charles Passell, who were American research scientists working in near the South Pole as part of the third Antarctic Expedition from 1939 to 1940. Siple and Passell came up with a clever way to simulate the way a body loses heat when exposed to very cold temperatures and strong winds. They filled containers made of pyrene, which is a type of plastic, with water and placed them outside in the brutal Antarctic conditions. To Siple and Passell, these plastic, water-filled containers were a good approximation to the human body, which is almost made mostly of water held together by a flexible container. Using a system of sensors, Siple and Passell were able to observe how look it took before the container of water froze. Not surprisingly, when the temperatures were colder, the water frozen more quickly, but the plastic container of water also froze more quickly the stronger the wind was. In one case, the entire container froze in about 35 seconds, whereas under milder conditions the water sometimes took more than an hour to freeze. Using a large number of these observations, Siple and Passell were able to use complex mathematics and statistics to come up with a system of equations that could estimate how long it would take for the containers of water to freeze under any combination of temperature and wind conditions. This eventually became a way to calculate a "wind chill temperature", which is a type of "apparent temperature" and has become familiar to most people.

If it seems to you that the wind chill temperatures aren’t as low as they used to be in years passed, you’re right. Since the development of the original wind chill temperature scale, there has been rumblings in the meteorological community that wind chill temperatures seemed to exaggerate the apparent coldness of the air. An official weather report, for example, might say that the wind chill temperature is thirty degrees below zero, and while there is not doubt that it FEELS very cold out there, does it really FEEL like it is thirty below? Probably not. In the 1990s, more rigorous calculations of the rate of heat loss were made, and it was found that Siple and Passell’s original method of computing wind chill temperature did have a significant cold bias. Starting in the year 2001, the National Weather Service adopted a new way of computing the wind chill temperature, and the result is that the wind chill is never as low as it used to be on the old scale. The Weather Service is more confident now that these wind chill temperatures are a meaningful way of expressing the rate at which bodies lose heat when exposed to extreme temperature conditions. This information is used in part to help the Weather Service issue various winter weather advisories that help the public make good decisions about travel and school closures.

Weather Break -- The Wind Chill Temperature

This is a transcript of the Weather Break radio show for Tuesday, January 22, 2008. The episode was written by Dr. Jon Schrage.

As a kid growing up on a farm in rural Nebraska, I remember standing out at the end of the lane in the winter, waiting for the schoolbus. As a kid, I'm sure that I didn't know anything really about "wind chill temperatures", but I'm sure that I understood that it seemed a lot colder outside on days when the wind was blowing. Today, the wind chill temperature is one of several different so-called "apparent temperatures" that meteorologists can use to describe the weather conditions. Another example of an apparent temperature that you might have heard of is the heat index.

You'll often hear TV meteorologists say that the wind chill temperature is the "feels like" temperature, and that's not a bad description of the number. For example, suppose that the temperature outside right now is 30°. If there is no wind at all, the wind chill temperature will be the same as the air temperature; it FEELS like it is 30° outside because it IS 30° outside. However, if it were windy out, when you go outside it feels much colder than that--let's say 10°. In that case, the meteorologist would say that the air temperature is 30° but the wind chill temperature is 10°--it feels like it is 10° outside.

You might think that it would difficult to very accurately describe how cold it "feels like" outside, but actually that's a pretty well-understood problem. How cold it feels has to do with how quickly your body is losing heat. In the winter, your body is always warmer than the outside air, so heat is always moving from your body to the air around your body; effectively, your warm body is trying to heat up the air around you. There's a technical term for this process; it's called the "sensible heat flux", meaning that there is a flux or transfer of heat from your body to the air around you. That rate of heat flux depends on two things: how great the difference in between is between your body and the air around you, and how fast the wind is at the surface. These two factors come together in terms of the wind chill temperature. Thinking back on our example, when the air temperature is 30° and the wind chill temperature is 10°, what the meteorologist is saying is that your body is losing heat at the same as it would if the air temperature were 10°, rather than the real temperature of 30°. Your body is losing heat faster, of course, because the wind is blowing, just like blowing on a hot spoonful of soup helps cool the soup down more quickly than if you just sip the soup right out of the bowl.

Notice that this means that the wind chill temperature cannot be greater than the air temperature. If there is no wind at all, the wind chill temperature is equal to the air temperature, and as the wind increases the difference between the air temperature and the wind chill temperature increases, too.

The physics and the thermodynamics of how fast heat is transferred between two objects of different temperature under different wind conditions are pretty complicated, but it's a well-known and well-studied problem in the atmospheric sciences. Based on this information, organizations like that National Weather Service have published tables that show the wind chill temperature for various combinations of air temperature and wind speed; we have links to these tables on our website, which is weatherbreak.creighton.edu. These are the numbers that you see on the news or hear about on the radio.

When talking about the wind chill temperature, however, it's important to understand that this number doesn't describe the actually the real temperature of any object. For example, suppose that the air temperature is 35° and the wind chill temperature is 30°--in other words, the air temperature is ABOVE freezing but the wind chill temperature is BELOW freezing. Now, suppose that you set a glass of water outside under these conditions; would the water freeze? The answer is no. The water would cool to the AIR temperature, not the wind chill, so the water wouldn't get any colder than the air temperature, which we said was 35°. Where the wind chill DOES come into play, however, is in HOW FAST the water got to that temperature. When the wind chill is low, the objects are losing heat fast due to the higher wind speeds. So even though the water will only cool to a temperature of 35°, it will cool at a rate AS IF it were cooling all the way down to the wind chill temperature of 30°.

On tomorrow's episode of Weather Break, we'll take a look back at how the wind chill temperature was initially developed on an expedition to Antarctica.

Weather Break -- The Siberian Express

This is a transcript of the Weather Break radio show for Monday, January 21, 2008. The episode was written by Dr. Jon Schrage.

Late last week, very cold air began to penetrate in the continental United States from the north. TV meteorologists were calling this weather pattern “The Siberian Express”. This term isn’t particularly scientific or formal; rather, it’s just an expression for an outbreak of extremely cold air over North America. This cold air mass didn’t necessarily come from Siberia, of course. Siberia, which is in extreme northeastern Russia, certainly is a very good source region for extremely cold air masses, but it is fairly uncommon for these airmasses to travel all the way from Russia to the United States without significantly modifying. “Modification” is the technical term meteorologists use to describe the processes that adjust the temperature and moisture in an air mass once it has left its original source region. In the case of a Siberian air mass, there are lots of ways in which that extremely cold, extremely dry air would modify between Russia and the United States. As the air mass passes over bodies of open water, evaporation and the conduction of heat would change the moisture and the temperature properties of the air, making it less dry and less cold, for example.

On the other hand, the Siberian Express weather pattern can be the result of very cold air masses pushing their way south from northern Canada or even the Arctic. There is relatively little opportunity for the air mass to modify between northern Canada and the United States, since the surface over which it will be passing is probably very cold and covered with snow at this time of year.

The Siberian Express is an example of a weather pattern in which air masses are in motion. Air masses don’t just move themselves; they are moved by the weather patterns around them. Meteorologists talk about air masses as being moved by the jet streams. The jet streams are like rivers in the atmosphere. They are fast-moving channels of wind found about 5-6 miles above the surface of the earth. All of the jet streams that are important to weather in the United States are generally from the west, although they typically meander north and south quite a bit. Picture the way a river meanders from side to side on its way to the sea. In the same way, jet streams meander north and south as they head off to the east. Meteorologists call these meanders troughs and ridges, and the position and movement of troughs and ridges in the jet stream cause air masses to move.

The weather pattern known as the Siberian Express is often set up by a particular pattern in the jet stream known as PNA. PNA, or the Pacific-North America pattern is actually one of the most common patterns in the atmosphere. In the PNA pattern, the jet stream swings far to the south of Alaska before turning sharply to the north. The jet reaches almost all of the way to the North Pole over western Canada before making a sharp right turn and heading south across the central United States. At the coast of the Gulf of Mexico, the jet stream makes a left turn and heads east across Mississippi, Alabama and Georgia before turning even farther to the left, going northeast up along the east coast of the US. A pattern like this in the jet stream creates conditions at the surface that are favorable for the coldest air up in northwestern Canada to slide down across the plains of Canada and the central US, producing extremely low temperatures across much of the country.The Siberian Express and the PNA pattern illustrate one of the most important principles in meteorology—namely, that the weather and climate at one location are strongly influenced by weather patterns at locations that might be thousands of miles away. In the case of the Siberian Express, the conditions that favor this cold weather pattern are set up by the PNA pattern in the jet stream, which itself is set up by patterns of thunderstorm activity in parts of the tropical Pacific ocean. The best forecast models incorporate global weather features into their depiction of the atmospheric state so that they can produce the most accurate representation of how the atmosphere will evolve over the next few days.

Weather Break -- The Highest Clouds in the Atmosphere

This is a transcript of the episode of the Weather Break radio show for Friday, January 18, 2008. The episode was written by Dr. Jon Schrage.

Long-time listeners to Weather Break know that I’m a huge fan of clouds. When you tell someone that you’re a meteorologist, you get two questions right off the bat: what’s the weather going to be like tomorrow, and what’s you’re favorite cloud? The forecast for tomorrow’s weather may or may not be difficult, but it’s always a tough call to pick my favorite cloud, because so there are so many difficult types of clouds and they all have different characteristics that make them fascinating to atmospheric scientists.

When you think back over all the different kinds of clouds that you have seen in your lifetime, all of these clouds had one thing in common that you probably didn’t realize at the time—they were all located in the layer of the atmosphere called the troposphere. The troposphere is the lowest layer of the atmosphere, typically occupying the lowest 6 miles or so. Pretty much everything that you or I would call “weather” happens in the troposphere, including almost all clouds. Even the tops of the very tallest thunderstorms only barely reach out of the top of the troposphere and into the next layer of the atmosphere, the stratosphere.

That means that almost all clouds in the atmosphere occur at heights less than about 6 or 7 miles above the earth surface, with most of the clouds occurring at far lower heights than that. The puffy, cumulus clouds that you would see drifting by on a warm summer day? They might be no more than a few hundred to a few thousand FEET above the ground.

All of these clouds are practically on the ground compared to the very highest clouds in the atmosphere—the noctilucent clouds. Noctilucent clouds occur in a layer of the atmosphere knows as the mesosphere, meaning that these clouds are typically about 30 to 60 miles above the earth’s surface. But these clouds aren’t much like the regular clouds that you see closer to the surface of the earth, and they are pretty rare. At the extreme heights where noctilucent clouds occur, the air is very thin—at most, just a few percent as dense as it is near the surface. At these very low pressures, air can’t hold very much water vapor, so you might think that the clouds would be common; after all, down in the troposphere, clouds normally form when air can no longer hold as much water vapor as the air currently is holding. But actually the noctilucent clouds are very rare, mainly because it’s extremely difficult for water vapor to GET to the mesosphere. The source of water vapor for the atmosphere is evaporation off of bodies of water at the earth’s surface. In order to get into the mesosphere, that water vapor would somehow have to rise 30-60 miles without condensing out and becoming liquid water drops or ice crystals that would fall back to the surface as precipitation.

The name “noctilucent” tells us something important about these clouds; they glow at night, or at least in the evening. Noctilucent clouds are very thin; in the harsh light of day, they are too thin to be seen by the human eye. (The same is often true of cirrostratus clouds.) But shortly after sunset, the rays of light from the sun can still be hitting these extremely high clouds, even though the sun appears to be below the horizon to an observer at the surface. At these times, noctilucent clouds glow brightly for a period of minutes before they, too, pass into darkness.

The exact cause of noctilucent clouds is not known. There are several reasons to believe that they may be caused by human activity, even though, of course, they occur at altitudes where humans don’t spend much time. It’s possible that these clouds are somehow seeded by the exhaust from rockets or the space shuttle as they pass through the mesosphere on their way to orbit. Regardless of how they form, they are unquestionably the highest clouds known to form in the atmosphere.

Weather Break -- Winter in Siberia

This is a transcript of today's episode of the Weather Break radio show. The episode was written by Dr. Jon Schrage.

Meteorologists define “winter” as the three month period from December through February. So far, the winter of 2007-2008 has been quite warm across most of the continental United States. Sure, there have been some bitterly cold days in Nebraska and South Dakota (especially right around New Years), but the warm days have more than made up for it. Right now, most of Nebraska is running about 2°F above normal for the winter, with South Dakota being more like three to three and a half degrees above normal. Nationwide, only the state of Nevada is running below normal, with parts of Montana and the Ohio Valley averaging about 5°F above normal so far this winter.

Actually, temperature have been running a few degrees above normal in Siberia, too, although that’s probably not particularly comforting to a place where the average air temperature in the winter is about forty degrees below zero. Siberia, of course, is the vast, largely unpopulated region that makes up about three quarters of the area of Russia. It’s larger than the continental United States, and it makes up most of northeastern Asia.

I looked up some recent weather observations from stations in Siberia. On the morning of January 10, 2008, the weather station at Jakutsk in central Siberia reported a morning low temperature of a whopping -53°F! Twelve hours later, the HIGH temperature for the day was -45°F!

There are certain codes that weather observers use to report aspects of the current weather conditions aside from the temperature, pressure, humidity and winds. For example, there are codes that can be sent out that report blowing snow, or patchy fog, or dusty conditions. There are special codes to report thunder and lightning in the area, hail, freezing rain, or whatever whether is presently occurring. One code that meteorologists don’t get to use very often represents “visibility obstructed by smoke”. Typically in the United States, weather observers use this code when there are forest fires in the area; basically, they are just informing the other meteorologists that the observations of visibility in the area might be faulty due to problems with smoke in the air. However, “visibility obstructed by smoke” is actually a pretty common weather observation in Siberia; just today I looked at the charts, and that was the observation from a number of stations in eastern Russia and Mongolia. I certainly hope that there are no forest fires right now in Siberia—I can’t even imagine fighting a forest fire when the air temperature is more than forty degrees below zero!

However, the smoke that is obstructing visibility in Siberia probably isn’t due to forest fires. It’s probably coming from chimneys and smokestacks in eastern Russia. Now, of course, there aren’t a LOT of people and industries out there in Siberia, but there are SOME. When the air is as cold as it is in Siberia in the winter, the air is extremely dense and heavy compared to the air aloft. That means that the air is hard to lift up from the surface and mix with the warmer, lighter air aloft. Meteorologists describe a situation like this as being “stable”. Stable conditions happen in the US, too—it’s part of the reason why thunderstorms don’t generally happen in the winter, for example, since stable conditions mean that the dense, heavy air at the surface is hard to lift into the updraft of thunderstorms. However, the air in Siberia is EXTREMELY stable and dense, meaning that there is no way for the air near the surface to get away from the surface and mix with clean air aloft. Any pollutants that get added to the air over Siberia are stuck near the surface for days or even weeks before diluting into the atmosphere aloft. Environmental engineers sometimes say that the solution to air pollution is dilution, meaning that no one really notices air pollution problems when air at the surface can easily dilute or mix with cleaner air aloft. In winter in Siberia, the residents don’t just have to battle some of the most brutal temperature conditions on earth, but they are also stuck with terrible air quality for months at a time.

Weather Break: Tornadoes in January

This is a transcript of the Weather Break radio show for Wednesday, January 16, 2008. This episode was written by Dr. Jon Schrage.

With all the coverage of politics and elections in the news last week, you could be forgiven if you missed hearing about the amazing series of storms that moved across the United States. A series of areas of low pressure--known as cyclones--moved ashore on the West Coast, bringing heavy rains at low altitudes and up to 11 feet of snow in the mountains. Out ahead of these storms, the counterclockwise rotation of the winds around the cyclones pulled very warm air from the Gulf of Mexico northward all along the Eastern Seaboard. On just one day last week, over 80 cities in the eastern US set record high temperatures for the day, with people showing up for the New Hampshire primaries in short-sleeved shirts.

Of course, whenever you have two very different air masses meeting like they did last week in the central United States, a meteorologists starts thinking about severe weather. The boundary between two air masses is known as a front, and the front separating cold, Canadian air in the west from tropical air in the east was unusually strong for the month of January. A strong jet stream aloft above the front provided the necessary wind shear to produce tornadic thunderstorms in five states--Oklahoma, Arkansas, Missouri, Illinois, and, of all places, Wisconsin. The Storms Prediction Center in Oklahoma is saying that there reports from 42 tornadoes on Monday, January 7.

Now, January isn't exactly the peak of tornado season. Tornadoes certainly can happen in January, especially during years of El Nino conditions in the Pacific. However, these tornadoes generally occur about as far south as they can in the US, in the southern states of Mississippi, Alabama, Georgia, for example. Farther north, tornadoes are much more rare in the winter. The news media were reporting that northern Illinois hadn't had a confirmed tornado almost sixty years. The Milwaukee Journal Sentinel reported that there hadn't been a January tornado in Wisconsin since 1844.

Why are tornadoes so rare in northern states in the winter? There are a couple of factors, actually.

Firstly, a strong thunderstorm generally requires warm, moist air at the surface. Air like that is relatively easy to lift into the updraft of a supercell thunderstorm or a squall line. In the winter, we all know that warm, moist air just isn't that common in states like Wisconsin and Illinois, due to the likely snowcover on the ground, the low sun angles during the day, and the short length of day.

Secondly, as mentioned earlier, most tornadic thunderstorms happen along a frontal boundary that separates different air masses. In the spring and summer, tornadoes are particularly common along a type of front called a dryline, which separates hot, dry air coming from places like TX, New Mexico, and northern Mexico from moist air coming from the Gulf of Mexico. Drylines are not likely to form in winter, however, because the usual source regions for hot, dry air are not particulary warm. Without strong drylines to foster the growth of tornadic thunderstorms, only the less-favorable cold fronts and warm fronts are available for tornadogenesis, and these fronts mainly occur farther south during the winter--not surprisingly, in the southern states of Mississippi, Alabama and Georgia.

Last week, the unusually strong cyclones coming ashore along the west coast set up conditions across the central US that somewhat recreated the pattern we would see in the spring, with warm, moist air surging northward and colliding with cooler air along a strong frontal boundary. All the pieces were in place for an extremely rare, January outbreak of tornadoes.

Weather Break: Snow Rollers

The following is a transcript of an episode of the Weather Break radio show, written by Dr. Jon Schrage.

Recent heavy snowfalls in the Rocky Mountains have caused a number of recent avalanches, which have been in the news quite a bit lately. Avalanches can happen under a number of conditions, but they generally happen when a heavy, sticky layer of snow falls on a weaker, older, crusty layer. If the snow is heavy enough and the slope is steep enough, the older layer of the snow cannot bear the weight and the top layer of snow begins to slide down the hill. Other avalanches happen when an overhanging ledge of snow, known as a cornice, breaks free and tumble down a mountainside.

All types of avalanches, of course, involve steep terrain—the snow won’t move if the land is flat, right? Or will it? While avalanches certainly are potentially destructive and dangerous, under the right weather conditions snow WILL move across flat surfaces in the form of what meteorologists call “snow rollers”.

Snow rollers are like snowballs, but they are barrel-shaped. They are generally quite rare, but they can happen whenever the snow is light but sticky and the winds are quite strong. When the wind starts rolling a small quantity of the snow across the surface, the tiny snowball begins to pick up more snow. Picture the way most people make a snowman; they start with a fairly small snowball and roll it across the yard. The snowball grows and grows until it forms the base of the snowman. Now, when you are making a snowman, you want the big snowball to be ROUND, so if the snowball becomes lop-sided or barrel-shaped or whatever, you push the snowball a different direction for a while. In the case of snow rollers, however, the wind just keeps pushing them in the same direction, so instead of forming round snowballs the snow roller become shaped like a barrel or a log.

Snow rollers have been reported that were as much as 3 feet in diameter, although most are much smaller than that. Generally, they occur in groups; you don’t usually see just ONE snow roller, but rather hundreds or even thousands of them may form over an open, snow-covered surface like a field or a frozen lake.

Snow rollers are fascinating, but they aren’t dangerous or severe. Here at Weather Break, we weren’t able to find any documentation about how fast these snow rollers form and tumble across the surface, but it certainly isn’t fast enough to cause damage or injury to anyone. Maybe we’ll eventually find some video online of snow rollers forming.

One of the biggest snow roller events in recent years happened on February 11 and 12, 2003, when gusty winds produced large, impressive snow rollers over much of Illinois. We have links to lots of pictures of snow rollers from that event and other similar storms on our website. You find that and much more at weatherbreak.creighton.edu.

Weather Break: Snow Where You'd Least Expect It

The following is a transcript of the Weather Break radio show for Monday, January 14, 2008. The episode was written by Dr. Jon Schrage.

Last week, a series of low pressure systems called “cyclones” dumped snow across much of the western United States. In some cases, it was a LOT of snow, with Kirkwood, CA reporting over 11 FEET of snow during these storms. Now, when most people think about CA, they are picturing warm beaches and deserts, but Kirkwood is northeast of San Francisco, just south of Lake Tahoe. It’s mountainous there, and as the moist air from the Pacific flows over the lee side of the mountains, the air is forced to rise, producing condensation that falls to the ground as snow at these high altitudes. In fact, the snowiest city in the US is Truckee, CA, which averages over 203 inches of snowfall a year—that’s almost 17 FEET of snow! Some of the ski resorts in CA actually have trouble staying open in particularly wet years because the snow gets so deep that chairs on the ski lift have to be dug out of the snowdrifts!

Of course, snowfall generally does become less common the farther south you go, simply because the average temperatures are increasing. However, snow can fall at high altitudes in the tropics, and along the equator itself there are even some examples of glaciers, where the snowpack doesn’t even melt in the summer!

In the tropics, just like other parts of the world, one of the major factors that control the climate is the altitude of a location. In general, the higher a location is, the colder its climate tends to be. The reason why this happens has to due with the fact that the heat and light from the sun tend to pass THROUGH the atmosphere without heating it. The light and heat from the sun reach the ground, heating the ground. In meteorology courses, we professors always need to remind our students that the sun heats the ground, not the atmosphere. The air is then heated by the ground, either by conduction or by radiation. On a mountain top, the sun can and does heat the ground itself, but the mountaintop is surrounded by air that may be thousands of heat above the ground in the valleys. Therefore, the mountain tops remain cold. In contrast, the valleys and the lower elevations have warm ground around them on all sides, heating the surrounding air. The results are warmer temperatures at the lower elevations.

A few tropical mountains receive considerable snowfall—sometimes enough snowfall that it doesn’t melt in the cool summer season, creating glaciers. Fans of Hemmingway will immediately think of his short story The Snows of Kilimanjaro. Mt. Kilimanjaro is a volcano and it’s the highest point in Africa. At over 19,000 feet, it’s one of the highest mountains in the world. It’s also just a few degrees south of the equator in the east African nation of Tanzania. Since that’s obviously a very tropical location, you might be surprised to realize that there are glaciers on Mt. Kilimanjaro. These glaciers, however, are rapidly receding, and they will probably be completely gone in the next few decades, according to researchers. The reasons for the retreating glaciers on Mt. Kilimanjaro are not completely clear, but it could be due to either warming temperatures or decreasing precipitation in the region.

Mt. Chimborazo is the highest point in the South American nation of Ecuador, rising to over 20,500 feet. At just more than 1° south of the equator, Mt. Chimborazo is covered with glaciers, whose annual meltwater is an important source of fresh drinking water in the relatively dry provinces of Ecuador. Just like in Africa, the glaciers of Mt. Chimborazo are retreating, although the reasons why are complex and include Global Warming, El Nino, and ashfall from nearby volcanoes.

Indonesia is the nation made up of over 17,000 islands north of Australia. While most of these islands are very small, the island of New Guinea is the world’s second largest island, with about twice the area of the state of California. New Guinea is extremely tropical, with lush rain forests at low elevations. However, some of the mountains on New Guinea are more than 16,000 feet high, and they receive enough snow to form permanent glaciers as well.

Whether tropical or from a higher latitude, almost all glaciers on earth are currently retreating. It’s a matter of some debate in scientific communities about whether or not this is attributable to Global Warming. A delicate balance of precipitation and melting is necessary to maintain a glacier, and studying these factors and their role in climate is the work of a branch of earth science known as glaciology.

Getting Ready for ATS 113!

Hello, ATS 113 students!

As you may know, the ATS 113 course uses a lot of electronic resources, like websites and course CD. Rather than spend a bunch of time on the first day of class trying to make sure that everyone understands what's going on, it seems like a better idea to do this using the course blog this year.

Course CD. I'm currently putting the final touches on the course CD, so it will definitely be ready by the first day of class. Every student is required to have a copy of the course CD. However, you are under no obligation to BUY the CD; you can borrow a friend's copy, or you can check the CD out from the library. If you choose to buy the CD, I'll bring a bunch of copies to class on the first day and they are $5 each. (The proceeds from the sale of the CD go to Weather Break, Creighton's daily radio program about weather, climate and the environment.) You can run the CD directly, or there are installation instructions with the CD that will help you copy the disk to your hard drive, which will result in much smoother performance.

What kind of stuff is on the course CD?

• Outlines of every lecture. (These are not NOTES; they are outlines.)
• Many of the figures and handouts for each lecture.
• A daily video called the "Reading Coach". On each Reading Coach video, I walk you through that day's assigned readings, pointing out the most important (and least important) parts of the reading.
• Several dozen short videos produced here at Creighton for use in ATS 113.
• Many PowerPoint files and related materials.
  
• And much more!
  

Course Website. This course has a website that is maintained in two convenient locations. You can either access the course at ats113.com, or you can use Blueline. The same content is available on both sites, although grades are only available on Blueline this year.

What kind of stuff is available on the course websites?

• All of the daily outlines and the Reading Coach videos.
• Most of the PowerPoint files and related materials.
• All "news" and "announcements".

In other words, the course websites have largely the same content as the course CD. However, because of possible copyright conflicts, many of the images on the website have been disabled; those resources are only available on the course CD.

Course Wiki. Electronic participation is a small but important part of your total grade in ATS 113. One of the ways in which many students choose to participate electronically in the course is to add to the course wiki. I'm guessing that most of you know what a wiki is; wikis are made of web pages that users can modify. We maintain two kinds of course wikis in this class:

• "Group Lecture Notes Wikis" are written by students based on their notes from the lecture.
• "Group Reading Notes Wikis" are similar, but they are based on the readings from each day.

Now, maybe YOU will NEVER contribute to the Wikis, and that's okay (maybe you'll get your participation points some other way, as we'll discuss in class). But even if you don't ADD to the wikis, you'll want to be able to ACCESS them, because they will be a good source of notes and information before each test.

Blueline has the ability to make wikis, but they don't work very well, so we are going to be using a different piece of software called "Moodle". Our "Moodle Server" is called "stratocumul.us". If you navigate to http://stratocumul.us, you'll be able to create an account there. Play-by-play instructions about how to create this account are available by watching this video.

Online Participation. A total of 5% of your grade comes from your "online participation" in the course. There are a number of ways to participate online. Probably the best way for you to learn about this is to watch this video.

Okay, I hope that I haven't "overwhelmed" you! I think that you'll find that this stuff turns out to be a lot of fun. See you on the 10th!