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Lecture 7
Air Pressure
Pressure =
- In an air filled container, pressure is due at
the molecular level to molecules pushingthe sides outward by rebounding off them.
- This also applies to ‘parcel’ of air sur-
rounded by more air; now molecules createpressure by rebounding off air moleculessurrounding parcel.
- At any point, pressure is same in all directions, but...pressure varies from
one point to another.
- Higher density (more molecules pushing) or temperature (faster molecules)
produce more pressure.
Force------------Area
Recoil
force (pressure)
Pressure supports air column
weight
- Higher up, the weight of overlying
air molecules is smaller, so pres-sure is also smaller.
- High pressure squeezes air most at
bottom, so air is densest there.
Gravity (due to weightof molecules) pulls downAir pressure pushes up
Mercury barometers
- Weight of mercury in tube balanced by pres-
sure
- A water barometer at sea level would have to
be read from a ladder 30 ft high, since wateris only 10% as dense as mercury.
Gravity Pressure
EOM 6.
Aneroid Barometers
- Aneroid is partly evacuated and sealed. Pressure increase compresses aner-
oid.
- Similar barometers in miniature are used in altimeters and altimeter
watches.
(EOM)
Lapse Rate, Compressibility and Stratification
- The troposphere is 50 C hotter at the bottom than at the top.• We know that in convection, hot air rises and cold air sinks.Question:• Why don’t we have convection everywhere, moving the warmer air from the
surface to the upper troposphere?
Temperature (C)
Height (km)
5 0 10
0
20
40
Pressure (mb)
1000 250 500 700 850
Warm
Cold
Tropopause
Stratosphere Troposphere
Lapse Rate =
Temperature decrease
Height Increase
=
65 C / 10 km = 6.5 C/km
Temperature changes as air rises and sinks
- As air rises, pressure of surrounding air drops.• The air ‘parcel’ expands, pushing outward on surrounding air until its pres-
sure drops to ambient.
- Pushing outward against a force (‘work’) consumes energy, which is taken
from the random motions of the molecules, reducing temperature
- Thus dry air cools ‘adiabatically’ (without addition of heat) at about 10
C/
km -- the
dry-adiabatic lapse rate
- Thus, as one hikes up the mountainside above, the temperature will be 10
C
lower for each 1 km ascended.
EOM, p. 31
reduced to 10 C/km.
- In clouds, latent heating of rising air due to condensation of water vapor into
clouds may permit ‘moist’ convection to occur even in ‘stable stratification’(lapse rate less than 10 C/km). Discuss this later.
1 0
Temperature (C)
10
20
heat Rising surface
air
Ambient
Convection willoccur in this layerwhere rising air iswarmer than ambient
