5.1: Rising Air

Adiabatic Processes

Have you ever used a hairspray, deodorant spray, or any kind of aerosol spray and wondered why it feels so cold, even though the spray can is at room temperature? If you haven’t done that before, find a spray can and briefly spray some of it onto your forearm. This happens because it's an adiabatic process, similar to what happens to air as it rises and sinks in the atmosphere. Before we examine how changes in altitude and pressure affect air temperature, it is important to state a few assumptions and rules that guide this analysis:

1. Rather than thinking of the atmosphere as a single, large blob of air, meteorologists treat it as trillions of tiny “bubbles” of air called air parcels. The air inside a spray can behaves similarly.
2. Air parcels are completely isolated from the surrounding environment. They neither exchange heat nor gases with other parcels. We'll call this the “Las Vegas Rule: what happens in the parcel, stays in the parcel". 

Ok, now that we have stated the assumptions and rules, let's examine what happens to the air parcel as it rises or sinks in the atmosphere: 

1. When an air parcel rises into the atmosphere, it enters a region of lower pressure. This causes the parcel to expand. The same thing happens to air parcels that escape the spray can's compression.
2. Expansion requires the parcel to do work on the environment, which requires energy. The energy used in the expansion is taken from the kinetic energy (heat) of the air parcel molecules. Therefore, as the parcel expands, it cools.
3. On the other hand, when a parcel sinks, it enters a higher-pressure environment. This causes the parcel to compress.
4. Compression adds energy to the parcel because the environment does work on it, forcing the parcel molecules together and increasing their kinetic energy (heat). Therefore, as the parcel compresses, it heats up.

Figure \(\PageIndex{1}\) sums these concepts up succinctly – As air rises, it expands and cools, and as air sinks, it compresses and warms.

Figure \(\PageIndex{1}\): Rising air expands and cools, while sinking air contracts and warms. (CC BY 4.0; Alicia Mullens)

Because these parcels follow the “Las Vegas Rule,” they exchange no heat with their surroundings. This is what an adiabatic (a = without; diabatic = two or more heat sources) process is: a process in which no heat is exchanged with its surroundings. This makes life easier for meteorologists because instead of accounting for all the possible heat sources like advection from other regions, radiation emitted from the sun, ground, surroundings, convection by contact with surrounding parcels, etc., we only need to focus on expansion and contraction. Because of this, meteorologists have a straightforward method for determining the temperature of a parcel as it rises or sinks in the atmosphere. They can use a 'Lapse Rate' to determine the parcel’s temperature at a given height if they know the parcel's surface temperature. A lapse rate is the rate at which temperature changes with height in the atmosphere. There are two lapse rates, depending on whether the parcel is dry or saturated with water vapor:

• Dry Adiabatic Lapse Rate: An unsaturated parcel of air cools at a rate of 10°C/1000 m. For every 1000 meters a dry parcel rises, its temperature decreases by 10°C.
• Moist Adiabatic Lapse Rate: A saturated parcel of air cools at a rate of 6°C/1000 m. For every 1000 meters it rises, a moist parcel loses 6 °C in temperature.

Based on this information, let's answer the following questions.

1.  Consider a parcel of unsaturated (dry) air at the Earth's surface with a temperature of 15°C. If the parcel rises to 1000 meters, it will have a temperature of:
   1. 9°C
   2. 25°C
   3. 15°C
   4. 5°C

Typically, a parcel of air begins its ascent as an unsaturated parcel. However, as the parcel rises, it eventually cools to its dew point and becomes saturated with water vapor.  

2. Suppose the parcel from question 1 became saturated at 1000 m, but continued to rise. Its temperature at 2000 m would be:
   1. 6°C
   2. -1°C
   3. 11°C
   4. 5°C

We can also use the same lapse rates mentioned previously to track sinking parcels. The only difference is that instead of subtracting 10°C (dry) or 6°C (moist) for every 1000 meters, you would add the corresponding amount to account for sinking parcels. 

3.  For example, suppose an unsaturated parcel of air at 6°C and 1000 meters sinks back to the ground. At the surface, it would have a temperature of:
   1. 16°C
   2. 0°C
   3. 12°C
   4. -4°C

4. Now, suppose the same parcel in question 3 were moist (saturated) instead of dry, but still at 6°C at 1000 meters. If it sinks to the surface, it will have a temperature of:
   1. 16°C
   2. 0°C
   3. 12°C
   4. -4°C