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Instability: The Key To Growing Thunderstorms

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No matter whether you're flying a Cessna 172 or a Boeing 777, thunderstorms can quickly ground you or send you hundreds of miles off your planned route. And, out of all of the weather you study in training, thunderstorms are the most emphasized - and probably the most exciting.

You learn that every thunderstorm requires three ingredients: moisture, instability and a lifting action. But what does that mean - and how does that affect your average afternoon storm?

Airmass storms may be the most common storm you'll encounter. In the summer, they form almost daily all over the United States, and often last less than an hour. It's fun to watch them form - a towering cumulus shooting upward, a rain shaft dropping down and an anvil forming. But how does it get started?

Instability Does The Trick

Instability is the key to a thunderstorm's growth. When you're talking about the atmosphere, instability describes what happens to the air once you raise it. If you pushed up air from the surface several thousand feet, would it continue rising, stay level, or drop back down? If it continues rising, it's unstable.

What causes the air to continue rising? Density. Colder air is more dense than warm air. And if you had a bubble of warm air surrounded by colder air, it would float up - just like a hot air balloon.

The Lapse Rate And Instability

As you climb, the air temperature cools. On a standard day, the temperature cools 3 degrees Celsius for every thousand feet you climb. But, on an actual day, that rate varies. It may decrease faster than 3 degrees per thousand feet. Or, instead of getting colder, it could get warmer - which is called a temperature inversion.

Take the day below - the air at the surface is 31 degrees Celsius (88 degrees Fahrenheit). But, as you climb, the air temperature decreases 4 degrees Celsius per thousand feet - faster than the standard lapse rate.

lapse-rate-fast

Now, take some air from the surface and lift it up 1000'. Lets say the air hasn't reached the saturation point yet - meaning the air's relative humidity is still under 100% and the moisture hasn't condensed into a cloud. As you lift this parcel of air, it will cool at a rate of 3 degrees per thousand feet - the standard dry adiabatic lapse rate.

air-lifting

At 1000 feet, the air you've raised is warmer than the air around it. So, what will happen? It will keep rising. In fact, it will keep rising until it's the same temperature as the air around it.

When the temperature in the atmosphere decreases faster than 3 degrees Celsius per thousand feet, the atmosphere's absolutely unstable. This means that anytime you lift up air from the surface, it will be warmer than the air around it, and it will continue to rise.

What could cause the air to rise? On a hot summer day, surface heating from the Sun. You're already familiar with the phenomenon as thermals - the light to moderate turbulence you feel when flying on a summer afternoon.

Adding Moisture

In the last example, you saw what happens when you lift dry air in an unstable atmosphere. But, to create a thunderstorm, the air needs to have moisture. So, what happens if you lift air that has moisture?

Let's look at the air again. As you lift some air from the surface, it cools. The temperature keeps dropping and approaching the air's dew point. Once it hits the dew point, moisture starts to condense out of the air into clouds. This altitude is the convective condensation level - it's the lowest altitude that condensation occurs because of convection from surface heating.

You can see this altitude when you look at cumulous clouds - notice how all of the bases are flat, and start about the same altitude? That's approximately the convective condensation level.

flat-cloud-bases

As the moisture condenses out of the air, it releases energy. (It takes energy to turn water into a gas, and that energy releases as heat as the gas condenses back into water.) Now your parcel of air is actually cooling less than 3 degrees Celsius per thousand feet.

How fast does it cool? That depends on the air's temperature. Warmer air holds more moisture than cooler air, and the amount of water the air can hold decreases exponentially as the temperature decreases.

water-vapor

As your saturated air cools, the saturated adiabatic lapse rate gets closer to the dry adiabatic lapse rate - because less water and heat are released as the air ascends and cools.

Now that moisture's condensing out of your lifted air, it's much warmer than the surrounding air. As it rises, that temperature gap grows and the air continues to accelerate upward - forming a strong updraft. This creates a towering cumulus cloud, or TCU, and you have the developing stage of a thunderstorm.

How High Will It Go?

The updraft will continue to rise until its temperature matches the temperature of the air around it. This is known as the equilibrium level. In fact, it will climb a little higher due to the inertia from its upward speed.

In an unstable atmosphere, the updraft may climb up through the troposphere into the tropopause or stratosphere - where the air temperature starts to increase with altitude. As the updraft slows down, it flattens out - resulting in the anvil and flat top you see on a mature or dissipating thunderstorm. And that, as they say, is how thunderstorms form.

Aleks Udris

Aleks is a Boldmethod co-founder and technical director. He's worked in safety and operations in the airline industry, and was a flight instructor and course manager for the University of North Dakota. You can reach him at aleks@boldmethod.com.

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