Last Thursday came with a BANG! as storms rolled through the area. In fact, State College was even under a Severe Thunderstorm Warning Thursday afternoon. Those storms were a result of two clashing air masses…the first air mass was the hot and humid air mass in place much of last week. And the second air mass was the cool and dry air that moved in last weekend that continues to dominate our weather. The boundary between two contrasting air masses is called a FRONT. In this specific case, the front that moved through was a COLD FRONT because the front brought an end to warm weather and ushered in cooler, drier air. Many of you have probably heard the terms “front”, “cold front”, or “warm front” used when talking about the weather, but you may not be familiar with how fronts work. Let’s take a look at why we can get storms when a cold front moves through.

Side-view of a cold front cutting in to a warm air mass

In the above schematic, you can see the cold front cutting through the middle of the picture. Cold fronts are generally represented by a blue line with triangular points. This picture views the atmosphere from the side. This allows us to see what is happening at the surface and aloft in the atmosphere. So how do cold fronts create storms? Cold fronts are very well-defined boundaries whose leading edges are at the surface. As you know, warm air is more buoyant than cold air, so as the cold air behind the cold front intersects warm air ahead of the front, the pocket of cold air “cuts” underneath the warm air and forces the warm air to rise. When warm air rises, it expands and cools until the air becomes saturated, at which point clouds begin to form. If conditions aloft in the atmosphere are favorable, the air will continue to rise, and this will allow the clouds to grow vertically upward. I won’t go in to specifics, but generally the taller the cloud, the more severe conditions the cloud will be able to produce. In some cases, these clouds, called cumulonimbus clouds, can grow to heights of near 50,000 feet!

An example of a well-developed cumulonimbus cloud. You can see how the cloud reaches a certain height and then flattens out at the top. This top layer is called the anvil (for obvious reasons) and it marks the maximum height to which the cloud can rise. This cloud may be producing rain, high winds, hail, and even tornadoes at the surface.

You may have even observed this process without knowing it. Think of a recent hot, muggy day when skies in the morning were clear. Chances are, a few puffy cumulus clouds began to develop around mid-day…and next thing you know, it’s pouring rain! If you were to observe those initial clouds over time, you’d see that they’d grow taller and taller and taller until they resembled the picture above or blotted out the whole sky, at which point you might have figured out that rain is coming.  So that’s a general overview of how cold fronts create storms! It is the contrast between air masses that provides the lift for storms to occur. Thus, the greater the contrast (i.e. the stronger the cold front), the more likely storms are to develop, and the more likely they are to be strong.