Meredith holds a B.S. in marine science with a minor in philosophy, as well as a master's of aeronautical science with a space science emphasis. She has taught subjects including marine science, biology, astronomy, math, and reading to students from kindergarten through high school.
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.
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. Latent heat is the energy absorbed or released by a substance as it changes between solid, liquid, or gas states. 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.
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.
If we lightly push the balloon upward, and it continues to rise, then we know the parcel is warmer than the surrounding air. The atmosphere is considered to be absolutely unstable under these conditions. If we lightly push the balloon downward, and it continues to sink, then we know the parcel is colder than the surrounding air. The atmosphere is considered to be absolutely stable under these conditions. If the balloon returns to a neutral position and does not rise or sink after being pushed, the atmospheric stability is thought to be neutral.
But as we've already seen, the humidity of the air is a big factor in its stability, not just the temperature. If a parcel of air is humid but neutrally buoyant in the atmosphere, it has great potential to become unstable. Under these conditions, the atmosphere is said to be conditionally unstable, where the environmental lapse rate is greater than the dry adiabatic rate but less than the wet adiabatic rate. Earth's atmosphere is conditionally unstable most of the time, except in some of the driest parts of the world.
Cloud formation can be attributed to atmospheric instability, wherein a humid parcel of air rises, cools, and condenses. Earth's atmosphere naturally cools with altitude due to the environmental lapse rate, and the upward movement of air parcels also change temperature based upon their humidity, either at the dry adiabatic rate or wet adiabatic rate.
A parcel of air is considered buoyant when it floats upward in the atmosphere, a condition that is associated with atmospheric instability. We can understand the stability of the atmosphere and thus the likelihood of cloud formation by assessing the static stability of an air parcel, whether it moves upward, downward, or remains neutral in the sky. If that parcel rises, it is absolutely unstable. If the parcel sinks, it is absolutely stable. If the parcel is neutral but is high in humidity, it is considered to be conditionally unstable, with a high potential for instability. This dynamic movement of air, changes in temperature, and changes in humidity are responsible for most of the intense weather phenomena we experience!
By the end of this lesson, you should feel confident in completing these tasks:
- Describe what causes atmospheric instability
- Recall what lapse rate is
- Differentiate between dry adiabatic rate and wet adiabatic rate
- Explain the concepts of buoyancy and static stability as they relate to atmospheric instability
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