Ah, spring! We’re finally getting back into seasonable temperatures, moving frontal zones (as opposed to the stalled system of last week), and the chances for a little thunder with our showers. It is many meteorologists favorite time of year, including myself. The excitement of thunderstorms is what drew me to study the weather in the first place. Imagine my surprise when I learned that one of the main triggers for a convective storm is warm sunshine! Let me explain why.
In order for a good thunderstorm to get going, the atmosphere must have some instability, which means that there needs to be enough energy present above us that when a specific surface temperature is reached, clouds will grow tall. We know that warm air rises and will continue to rise as long as it’s warmer than the atmosphere around it. Sometimes, it reaches a temperature inversion - a layer of warm air higher up in the atmosphere that stops a rising air parcel’s progress. This warmer layer is called the capping inversion, or cap for short. If you’ve ever heard the phrase “break the cap” when someone is talking about a forecast for storms, this inversion is what they’re talking about.
The temperature at the surface must get warm enough for the air to rise above and beyond the cap, or the inversion must be broken down by some outside influence in that level of the atmosphere. The most common way for the surface temperature to rise is for the sun to do its thing and warm the earth. When the day starts out with clouds blocking the sun, there is a chance the temperature won’t reach the magical number (which could be different every day depending on a number of factors), and the cap won’t break.
If the cap breaks, the warmed parcel of air can continue rising until it reaches the tropopause, where the air stops cooling with height and becomes almost completely dry. When you’re looking at the flattened portion of an anvil topped cumulonimbus cloud, you’re actually seeing where the rising air has hit the tropopause. Cooh, huh? When the rising air has enough energy behind it, the parcel can actually punch through the tropopause, and that’s when you get the overshooting top of a thunderstorm.
Photo courtesy of NSSL Photo library .