ST. LOUIS — We tend to only think of what’s going on down here on the ground.  But in reality, almost everything we experience as weather down here on the ground begins up over our heads in the atmosphere above us.  One big feature in that atmosphere that becomes very important this time of year is what is known as the atmospheric cap. 

Before we can talk about the cap, we need to remember a simple fact about the atmosphere.  That fact is that on an average day, the temperature gets colder as you go higher up in the sky. On average, it gets 3.5 degrees colder for every 1000 feet you go up.  That’s why it is so much colder at the top of a tall mountain like Pikes Peak than it is down at the base of the mountain.  And that brings us to the atmospheric cap. 

The sun heats up the ground, which heats up the air directly above it.  That air becomes a lot warmer than the air above.  So, like a hot air balloon, that air starts to rise in bubbles or currents that we call thermals.  And as long as the air above the bubble remains colder than the air inside the bubble itself, it will keep rising.  

Eventually, moisture inside that bubble will condense and we get clouds.  More often than not, there is a layer of warmer air that spreads east off the hot, dry deserts of the southwestern United States at around 10,000 to 15,000 feet.  That layer of warm air is the cap.  

When the thermals from below hit that cap, it stops the rising currents, which keeps thunderstorms from developing.  So, a strong cap usually means we get some stubby looking clouds, but no thunderstorms. 

But sometimes, the heating from the sun causes the air near the ground to get really warm, and those thermals rise with a lot more force.  One after another, those bubbles of warm air pound away at the underside of the cap, using it like a punching bag, and eventually the cap will break.  When that happens, you get explosive thunderstorm development as all that energy from those thermals explodes upwards through the atmosphere.   It’s a lot like popping a balloon.