Geoscience Reference
In-Depth Information
Air mass cools as it rises
because pressure decreases
and molecules spread out
Air mass warms as it sinks
because pressure increases
and molecules are compressed
High
5
1
5
1
4
2
4
2
3
3
3
3
Altitude
2
4
2
4
1
5
1
5
Low
Warmer
Cooler
Temperature
Figure 7.15 Adiabatic heating and cooling.
Air cools as it rises because the pressure drops and air molecules disperse. When air
descends, it warms adiabatically because the air molecules are compressed by higher pressure.
can change with time or place. In contrast, the DAR is always
constant and is applied to a parcel of rising air according to
physical laws. The DAR has a value of about 10°C/1000 m
(5.5°F/1000 ft) of vertical lift and is applied
only
when the air
mass is not saturated.
This is a very important rule to remember
.
You can see this rate of change if you examine Figure 7.16 and
focus on the area at the bottom of the graph, which indicates
altitudes of less than 1000 m.
The DAR is relevant not only to lifting air masses that are
not saturated but also to descending ones. In the case of a de-
scending air mass, the air
warms
at the DAR. That is, for every
1000 m that the air descends, it warms 10°C (5.5°F/1000 ft) due
to compression of the air under pressure.
Temperature (
°
F)
40
50
60
70
2500
8000
Wet adiabatic
lapse rate
5
°
C/1000 m
(2.7
°
F/1000 ft)
7000
2000
0
0
6000
5000
1500
00
Latent heat
begins to be
released
4000
Condensation level
Dry adiabatic
lapse rate
10
°
C/1000 m
(5.5
°
F/1000 ft)
1000
00
3000
2000
Rising air
parcel
500
Figure 7.16 Adiabatic cooling in a hypothetical air parcel.
The air
cools at the dry adiabatic lapse rate (DAR) as it rises until it reaches the
level of condensation. At that point, the rate of temperature change
switches to the wet adiabatic lapse rate (WAR) due to the release of
latent heat.
1000
0
25
0
5
10
15
20
Temperature (
°
C)
