How Does Air Change When It Rises and Sinks?

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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?
Air at higher
altitudes has
less pressure
Note that in
reality, the air
molecules only
get farther apart
from each
other—they don’t
actually get
bigger!
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?
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)
X 100
0.8
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 = 2.8°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.
21.25°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 = -15.7°C = air temp. at 3250m

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