Educational - Cap
Back to: Weather Q & A
In order for thunderstorms to develop, the air near the surface must be buoyant. Just like air bubbles in water. Since air is less dense then water, any air bubbles released in water float to the surface, they are buoyant.
So, think of thunderstorm clouds as being almost like those air bubbles. When the air near the surface becomes buoyant, it bubbles up through the atmosphere. What makes air buoyant? It has to be less dense then the atmosphere surrounding it. Air is less dense than water, so air is buoyant in water. Warm, moist air is less dense then cool air. So, the air near the surface needs to be warm and moist, and the air above needs to be cool and dry. As the air rises, the pressure of the atmosphere surrounding it decreases (there is less atmosphere above pressing down), this causes our warm moist air to cool. So we have another concept to remember: when air goes into an area of lower pressure, it cools. When air is compressed (into an area of higher pressure), it warms. This described by the "Ideal Gas Law".
Okay, back to the air rising from the surface. As the buoyant air rises, it cools. This cooling causes the water vapor (gas) in the air to turn to clouds (the water vapor becomes small water droplets). These water droplets are what we see as cloud.
That's part one. We understand that thunderstorms form when air near the surface becomes buoyant, rises, cools, and forms clouds.
The Gulf Coast area has a VERY special weather situation in Spring and Summer that creates the CAP you are speaking of. Often there is moist air flowing from the Gulf of Mexico over the whole region, and this air is relatively cool, especially if it's already cloudy. At the same time, air higher in the atmosphere is flowing from the West (from the desert) as part of a high pressure system. This dry desert air is being forced to flow downward toward the surface by the high pressure. As the warm air descends, it gets compressed. Remember from above, when air is compressed, it warms further.
This forms the "CAP". It's called a cap because it acts just like a bottle cap. The air near the surface is cool and moist, and this dry desert air above is warm. So, the air at the surface ISN'T buoyant. It's cool and dense, while the air above is warm and less dense.
So throughout a typical severe thunderstorm day, we start will cool, cloudy air near the surface and warm air aloft. The sun slowly warms the surface air, and erodes the clouds. Our cool, denser surface air is becoming less and less dense as it warms. Eventually, there will be a spot, or several spots over the region where the this moist surface air suddenly becomes warm enough to be buoyant!
These spots of buoyant air explode into thunderstorms!
Since the air near the surface can only poke through these small holes where the air is buoyant, you tend to get big/severe storms. All of the energy of the warming sun is concentrated into a few places! I think that's really cool.
So, next time your area is forecasted to have a cap, look outside from time to time. Watch for the places where the cap breaks first. You'll see thunderstorm clouds suddenly form.