How Does Air Change When It Rises and Sinks?

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Transcript How Does Air Change When It Rises and Sinks?

How Does Air Change When It Rises and Sinks?

Suppose a parcel of air rises up… ?

Where is the pressure higher? Air near the surface has more pressure Earth’s Surface

So, What happens to the air when it rises into air that has less pressure?

Note that in reality, the air molecules only get farther apart from each other —they don’t actually get bigger!

Air at higher altitudes has less pressure Air near the surface has more pressure Earth’s Surface

So why does this impact temperature?

• When air expands it ‘uses’ energy to move the surrounding air molecules —thus the overall random kinetic energy (speed) of the air molecules is LESS.

• This is called

Adiabatic Cooling


Check out this example of adiabatic cooling!

• Why did this happen?

• The gas is under high pressure in the can— but when it moves into the lower pressure atmosphere it expands (thus exerting energy on its surroundings) resulting in cooling. • [Note: it also experienced a phase change b/c it came out as a liquid —but went to a gas —and we know evaporation is a cooling process —this magnified the change]

Air that sinks from a high altitude down to the surface will…

1) Move into air that has more pressure and thus expand.

2) Move into air that has less pressure and thus expand.

3) Move into air that has more pressure and thus contract.

4) Move into air that has less pressure and thus contract.

If air sinks down toward the surface… Will the air…?

1) Get warmer 2) Get cooler 3) Remain the same temperature

Air will get warmer!

• Adiabatic Warming When the air sinks it moves into higher surrounding pressure which will compress it into a smaller space. This means that the surrounding air gives energy to this parcel of air —thus making the molecules move faster.

How else is Adiabatic Temperature Change used in the real world?

• Refrigerators • Air Conditioners • Heat pumps HOW?

How much does pressure affect the temperature of air in the atmosphere?

• In DRY air… Rising air cools 1 °C for every 100m of altitude In humid air (100% RH)… Rising air cools 0.6

°C for every 100m of altitude WHY IS IT DIFFERENT IN DRY vs. HUMID air?


• When you cool air that has 100% RH, condensation will occur.

• Condensation happens when hydrogen bonds FORM between water vapor molecules.

• This means that energy is GIVEN to the environment (thus it doesn’t cool as much)

OK —So does anything else change as air rises?

• DEW POINT gets lower too. WHY?

• Since air is at a lower pressure at higher altitudes it is also easier for water molecules to break their hydrogen bonds —thus you must achieve an even cooler temperature in order to get condensation.

So how do I use this to find out the altitude where condensation will occur (A.K.A. condensation level)?

CL = (surface temp. – surface dew pt) 0.8

X 100 Your answer will be the altitude (in meters) above the surface.

Practice Problem #1

• A group of students is going to hike up a mountain that is 2200m tall. When they start at sea level, the air is 22 °C and the dew point is 10 °C. a. If the wind is pushing air up the mountain slope, will students encounter a cloud (fog) before they reach the top of the mountain?

b. What temperature should they expect when they reach an altitude of 1000 m?

c. What temperature will it be at the mountain top?

1a. solution:

At what elevation does a cloud form?

(22 °C - 10°C) / 0.8 * 100 = CL in m 12/0.8 = 15 15 * 100 = 1500 meters of elevation is when the cloud will form.

1b. solution

• What will the temperature be at 1000 m?

– Since the cloud will not form until 1500 m, you only need to use the dry adiabatic lapse rate.

Air cools at 1 °C per 100m.

1000m * 1 °C/100m = 10°C of temperature drop, so the temperature at 1000m = 12 °C

1c. solution

• Temperature at the top of the mountain: – Since a cloud forms at 1500m you need to apply 2 different rates of adiabatic temperature change. Use the dry rate up until the cloud forms, then use the humid rate once the cloud is forming.

Temp @ 1500m = 15 °C cooler, so 7°C.

2200m – 1500m = 700 m of elevation change using the humid rate.

700m * 0.6

°C/100m = 4.2°C more cooling 7 °C - 4.2°C =


°C at the mountain top

Practice Problem #2

• Air is rising over Connecticut and leading to cloud formation. An airplane is currently 3250m above sea level and coming in for a landing at Bradley airport —but can’t see the runway due to clouds. On the ground at the airport (50m above sea level), a dry bulb thermometer reads 12 °C while a wet bulb reads 5 °C. • At what altitude will they begin to see the runway?

• What is the temperature of the air around the plane @ 3250m above sea level?

Problem #2 Solution

Total elevation change = 3250m-50m = 3200m According to the RH chart, RH = 29% 12 °C has a capacity of 8.9 g/kg 0.29 * 8.9 g/kg = 2.581 g/kg = SH Using the table, this makes the dew point = - 5 °C (12 °C - - 5°C) / 0.8 * 100 = 2125m = CL  runway becomes visible at 2175m above sea level.


°C cooling occurs  -9.25

°C @CL 3200m-2125m = 1075m of elevation change to go 1075 * 0.6

°C/100m = 6.45°C of additional cooling -9.25 – 6.45°C =


°C = air temp. at 3250m