How Are Thunderstorms Made?

    Three things are needed to make a thunderstorm:

1) Thunderstorms need moist air. Even the driest spots on earth have tiny invisible drops of water in the air, called water vapor. Humid days, which mean that air has lots of these invisible water drops floating around, are best for thunderstorms.

2) Thunderstorms need the temperature high in the air (thousands of feet above the ground) to be a lot cooler than the temperature of the air around us on the ground.

3) Thunderstorms need something to lift the moist air from near the ground up to where the cooler air is. When a front pushes through a place, air ahead of the front will be pushed up from the ground up into the sky.
 

    Heating from sunshine is another way that air is lifted. When the ground gets heated by sunshine, like a hot parking lot on a summer day, then the air near the ground is warmer than the air above it. Warmer air is lighter than cool air so the warm air will start to rise, even though you can't see it. Just think of a helium balloon lifting through air when you let it go (that happens because helium is lighter than air).
 

    The blanket of air that covers the Earth is called the atmosphere. Currents of air move through the atmosphere like water currents in a river. Currents of air, called wind, can move across the earth, such as to the east or to the west. They can also be pushed up or down through the air, moving toward the sky or down from high in the air toward the ground.

When blobs of warm air from near the ground flow toward the sky and move into colder temperatures high in the atmosphere, they will be warmer than the air around them. This means that the blob of warm air will be lighter than the air around it and it will start to rise on its own, the same way that a hot air ballon lifts into the air. As the air rises, it will cool very slowly. The tiny invisible drops of water, called water vapor, will start to clump together when it gets cold and soon you have enough water droplets to make a cloud.

You can see the same sort of thing at home when you leave a cold drink on the table on a warm day. Drops of water will pop up on the outside of the glass. That water didn't leak through the glass. It actually came from the air. Water vapor in the warm air turns into bigger water drops when the air is cooled. If the air is lifted high enough and long enough, a huge thunderstorm cloud, or cumulonimbus cloud, will form.
 
 
 
 

Thunderstorms can bring many kinds of dangerous weather:
 
 
 

  Lightning starts many fires around the world each year. It also hurts or kills many people when they are struck.

  Heavy rain from thunderstorms can cause flooding, changing small creeks into dangerous rivers of water in a matter of minutes.

  Hail, often as big as baseballs, damages cars and windows and can kill animals caught out in the open.

Strong thunderstorm winds called Downbursts (up to more than 120 mph) can knock down large areas of trees and power lines and damage buildings.

  Tornadoes (with winds up to about 300 mph) can destroy homes and buildings and anything else in their path.
 
 
 
 

Your National Weather Service calls a thunderstorm severe if:
It has hail the size of a penny or larger  or wind speeds 58 mph or higher or tornadoes.

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