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Clouds Over Mount Shasta

Summer Thunderstorms

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How do Convective Thunderstorms Form?

First of all, what does convective mean? Convection refers to the movement of a liquid or a gas due to heat rising and cold (air, in this case) sinking. At the equator, where it is warm because the sun shines overhead all year long, the hot (moist) air rises. As the air gets higher up in the atmosphere, the water vapor cools and condenses forming clouds. When condensation occurs, heat is released and so the air rises even more. Eventually, the air becomes saturated (full of water) and so it rains. As the rain falls toward the earth, it drags some of the air with it. Also, as the rising air cools, it too sinks back to the earth.

Why do Convective Thunderstorms Occur in the Summer in Siskiyou County?

Near the equator there are convective thunderstorms almost every day. Here in Siskiyou County, however, we usually have convective thunderstorms in the summer, especially in late July and August. Why do they tend to occur at the end of summer? During the summer in the northern hemisphere, the sun is over the Tropic of Cancer and so the sun shines more directly at us and for a longer time in our summer. At the end of summer, when the heat has had time to warm things up all summer long, there is finally enough heat to produce thunderstorms.

The Development of Thunderstorms

If you look at the figure at the top of the page, you can see how thunderstorms develop. First of all (#1), the hot air rises. As the water vapor in the air cools, it condenses and forms puffy cumulus clouds. If you can see these puffy clouds (that look similar to cauliflower) in the morningtime, then there just might be thunderstorms that afternoon or evening.

In the second stage (#2), the clouds rise higher and higher because heat is released as the water vapor in the air condenses. Eventually, the water droplets in the clouds become too heavy and so they fall down (it rains!). You can see in the picture that at this point the warm air continues to rise and heat is still being released due to condensation, so the clouds can still get bigger even if it is raining. Sometimes the clouds get so big we call them thunderheads. Thunderheads often have an anvil-shaped top. The rain (or hail) continues to fall dragging the air down with it.

In the third stage (#3), more air is sinking than rising and so the storm dissipates. The air is sinking because the rain is dragging the air down, the rising air is cooling off, and often it is getting on toward evening and so it is generally getting cooler.

What Causes Hail?

Usually we get at least one thunderstorm at the end of summer that produces hail. Why is there hail when it is so hot outside? If you look at these towering cumulonimbus clouds, you can see that they reach way up into the sky. It gets pretty cold up there. What happens is that the water droplets in the clouds are so light at first that they are carried up with the rising air. When these water droplets get high enough they freeze. Eventually these frozen drops start to fall and more water collects on their surface. If the updraft (rising air) is strong enough, it can lift that frozen little drop with water on the outside right back up again. Then the water on the outside freezes and eventually it, too, drops and the whole process can be repeated many times. Of course, everytime more water is added to the outside of the hail and it freezes again, the hail gets bigger. The updrafts have to be quite strong for large-sized hail to form. If you could cut a hailstone in half, you would see concentric layers. This is because the water is frozen to the outside one layer at a time.

If you have ever been in a thunderstorm or a hailstorm you know how exciting they can be! Of course, you need to be careful if there is lightning. Seek shelter inside houses but stay away from metal objects (especially the wood stove!) and don't talk on the phone (unless it is cordless). If you are outside, don't seek shelter under trees and stay low to the ground. Never hold anything metal during a thunderstorm. Even though thunderstorms can be dangerous, if you take precautions you should be just fine. Find a safe place and enjoy the storm!


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