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!