The vertical movement of air is a crucial factor in cloud formation and weather phenomena.
We’ll investigate how atmospheric instability influences the rising of air and the resulting weather. At the end, you can test yourself with a brief quiz.
If you’ve flown on airplanes frequently, you’ve probably experienced turbulence. The seatbelt light goes on, the cabin shakes, drinks spill, and maybe it’s even a little scary.
Turbulence is a result of instability in the atmosphere surrounding the aircraft. Unstable air is a parcel of air that is warmer than the air around it, causing it to rise. It is helpful to think of a parcel of air like a balloon containing gas that differs from the surrounding air in terms of temperature and humidity.
Types of Lifting
Several factors can cause a parcel of air to rise in the atmosphere. As we just mentioned, temperature is a common reason because convection occurs when warm air rises and cool air sinks.But the topography of land can also cause a parcel of air to rise.
If, for example, wind blows humid air up the side of a mountain to a higher altitude, that air may find itself surrounded by cooler air. The water vapor in that parcel of air condenses, forming clouds. This is called orographic lifting, wherein the movement of air up the side of a mountain causes condensation and precipitation.The lateral movement of air can also cause lifting to occur. When a warm air mass collides with a cooler air mass, as is the case in a warm front, the less-dense air will rise above the more-dense air.
Imagine two plates, upside-down on a table: one glass dinner plate and one paper plate. Now imagine pushing them so they collide. Which plate will slide on top of the other? Naturally, the paper plate. Air works the same way. Warm air rises over cooler air when they collide during frontal lifting, which also causes condensation of humid air and subsequent precipitation.
Stability in the atmosphere is determined by how much warmer a parcel of air is than the surrounding air.
It’s all relative. The atmosphere itself decreases in temperature with increased altitude due to the environmental lapse rate. A rising parcel of air will also change temperature with altitude because the outside air pressure is also decreasing. Less pressure means that the parcel of air gradually expands, which also impacts its temperature. So basically, the temperature of the air parcel relative to the surrounding air determines stability, and both temperatures are changing with altitude.When no heat is exchanged between the air parcel and the surrounding air, a parcel’s temperature change with altitude can be measured as its adiabatic lapse rate.
To make matters just a little more complicated, the humidity of the parcel of air affects its lapse rate. As a humid parcel expands and cools, condensation occurs, which causes some heat to be released.So, there is a difference between the dry adiabatic rate, wherein a parcel of air cools at a rate of about 10 degrees Celsius per 1000 meters, and the wet adiabatic rate, wherein a parcel of air cools at a rate of about 6 degrees Celsius per 1000 meters.
The less the air parcel cools with altitude, the greater the difference between the parcel’s temperature and the surrounding air temperature, and the greater the atmospheric instability.
Because Earth’s surface radiates much of the solar heat that it absorbs (land more so than water), the rising warm air and the instability it causes in the atmosphere is what generates all weather experienced on Earth. The troposphere, the lowest layer of the atmosphere, from the ground up to about 11 miles high, is the site of all of Earth’s weather and most of the atmospheric instability.The stratosphere is the second layer of the atmosphere, located between the troposphere and the mesosphere, and contains relatively stable air. This layer puts an invisible lid on much of the unstable air lifting that goes on in the troposphere. In addition, a parcel of air, even a very warm and unstable one, will rise until it has expanded and cooled to match the surrounding temperature, and its lifting will stop.
Unstable air is warm air that rises, causing atmospheric instability and weather phenomena to occur. The heat reflected off of Earth’s surface generates much of this instability, either through convection, as warm air rises and cool air sinks; orographic lifting, where warmer or humid air rises up the side of a mountain and condenses; or frontal lifting, when warm and cool air collide, causing warm air to rise and condense.Instability is determined by the relative difference in a rising parcel of air and the atmosphere around it, the latter of which naturally cools with altitude due to its environmental lapse rate. Air parcels cool with altitude at a dry adiabatic rate or a wet adiabatic rate, depending on the humidity of the parcel.
When a parcel leaves the troposphere, the lowest part of Earth’s atmosphere, and enters the stratosphere, the second layer of Earths’ atmosphere, it will likely have cooled enough to match the surrounding air, and is no longer unstable or lifting. This dynamic air movement is the crux of our weather systems and the transport of heat within the atmosphere!
When you are done, you should be able to:
- State what unstable air is
- Explain what can cause a parcel of air to rise
- Describe how stability in the atmosphere is determined
- Differentiate between the wet adiabatic rate and the dry adiabatic rate
- Identify the troposphere and stratosphere and understand how the stratosphere acts as a sort of lid for the unstable troposphere