Keeping A Lid On Storms
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At times in the summer months when showers and thunderstorms are absent, weather forecasters often blame something called a cap. Hi I'm Dave Thurlow for the Weather Notebook.

What does a cap have to do with storms? Well if you think of a cap as more of a lid really, it's something that prevents clouds from growing. Here's how it works. On hot summer days, the air near the ground is much warmer than it is higher up. Warm air rises and it does so by bubbling up from the ground, just like the bubbles in a pot of boiling water. If the air has enough moisture in it, these bubbles form cumulus clouds, which often continue to grow into giant thunderheads or cumulonimbus clouds as cloud chart says.

But on certain summer days, you may notice that the cumulus clouds never quite turn into thunderstorms. Instead, they bubble up, but just sort of flatten out on the top. They simply stop growing. Because of the cap. A cap in this sense is slice of very warm, dry air that sits just a mile or two above ground. And it is huge. While only a few hundred meters thick it can cover half of the country. It often forms in the desert Southwest and then sweeps eastward.

When the bubbling cumulus clouds reach a warm cap, the bubbles are actually cooler than the surroundings. This takes away their buoyancy, so the air stops rising and the clouds stop growing. But sometimes-there's always a sometimes--a cap is just enough to keep a lid on things for a few hours. For a rapidly growing cloud just might blow the lid right off. All that pent up energy explodes right through the cap and creates a storm that's much more intense than it would be had there been no cap at all.