Static Stability, Cloud Formation & The Environmental Lapse Rate

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  • 0:01 Stability and Clouds
  • 0:49 Lapse Rates
  • 2:26 Measuring Stability
  • 4:17 Lesson Summary
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Lesson Transcript
Instructor: Meredith Mikell
Here, we explore how the static stability of air in the atmosphere influences the vertical movement of air's likelihood of cloud formation. At the end, you can test your knowledge with a brief quiz.

Stability and Clouds

If you've ever witnessed the arrival of a big thunderstorm, you have likely seen tall, billowing, cumulonimbus clouds high up in the sky. The presence of these clouds indicate significant atmospheric instability, wherein warm air rises and any water vapor in it condenses. That condensation can lead to heavy precipitation: a rain storm!

Cloud formation depends upon this instability and the rising of a warm air parcel, air that has a temperature and/or pressure different from the surrounding air. Think of a parcel of air like a balloon rising in the atmosphere - minus the rubber! The likelihood of cloud formation can be estimated by assessing the temperature of a rising parcel as compared to the surrounding air, both of which change in temperature with altitude.

Lapse Rates

It may not look like it when you look up in the sky on a clear day, but Earth's atmosphere does not have an even temperature or pressure throughout. Pressure is highest at sea level, and temperature gradually decreases with altitude. There are some exceptions to Earth's temperature change as we get higher in the atmosphere, but in dealing with cloud formation, we are sticking pretty close to the ground.

This gradual decrease in temperature with altitude is the atmosphere's environmental lapse rate. This decrease has two main causes: the radiating warmth of the surface and how the atmosphere gets thinner and thinner with altitude. Thinner air means less heat absorbed, and the heat reflected off of the Earth's surface means rising warm air in the lowest part of the atmosphere.

But, the temperature of the parcel is not the only factor involved. The humidity of the air can also have an impact on the stability of the atmosphere. While the environmental lapse rate is fairly constant, the adiabatic lapse rate describes the rate of temperature change of a parcel of air not exchanging heat with the surrounding air. This rate can vary depending on how much water vapor is in the air.

When that water vapor condenses, it releases latent heat, delaying the cooling of the parcel. Under the dry adiabatic rate, the parcel of air cools at a rate of 10 degrees Celsius per 1,000 meters, whereas under the wet adiabatic rate, the parcel cools at a rate of 6 degrees Celsius per 1,000 meters. So, rising humid air is going to remain unstable in the atmosphere longer than rising dry air.

Measuring Stability

Rising warm air is very important for weather formations. A parcel of warm air rises due to convection, and often times, that parcel has water vapor in it. The parcel is considered to be buoyant, less dense than the surrounding air. As it climbs into the atmosphere, where the pressure is decreasing, the parcel of air expands and cools. This cooling causes condensation and cloud formation and indicates an unstable atmosphere.

To illustrate this, let's say that we send up a parcel of air - picture a balloon - that gets to a certain point in the sky and stops rising. It is perfectly neutral, floating in the air, not rising or sinking. If we want to know how buoyant it is, we can test its static stability, how buoyant it is in the surrounding air.

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