Download dry adiabatic rate

CLOUDS AND
STABILITY
Key Points
• A rising parcel of unsaturated air will cool at the
dry adiabatic rate, which is about 10°C per 1000
m (5.5°F per 1000 ft); similarly, a descending
parcel of unsaturated air will warm at the same
rate.
• A rising parcel of saturated air will cool at the
moist adiabatic rate, which varies, but an
average of 6°C per 1000 m (3.3°F per 1000 ft) is
often used; similarly, a descending parcel of
saturated air will warm at the same rate.
Key Points
• A parcel of air in stable equilibrium will tend to
return to its original position, while a parcel of air in
unstable equilibrium will tend to move away from its
original position.
• The atmosphere is absolutely stable when the air at
the surface is either cooler than the air aloft (an
inversion), or the temperature difference between
the warmer surface air and the air aloft is not very
great, I.e., the environmental lapse rate is less than
the moist adiabatic rate.
• The atmosphere can be made more stable by
cooling the surface air, warming the air aloft, or by
causeing air to sink (subside) over a vast area.
Key Points
• The atmosphere is absolutely unstable when the
surface air is much warmer than the air aloft, I.e.,
the environmental lapse rate is greater than the dry
adiabatic rate.
• The atmosphere can be made more unstable by
warming the surface air, cooling the air aloft, or by
lifting a layer of air.
• The atmosphere is conditionally unstable when
unsaturated air can be lifted to a point where
condensation occurs and the rising air becomes
warmer than the air around it. This takes lace when
the environmental lapse rate lies between the moist
adiabatic rate and the dry adiabatic rate.
• The majority of clouds form due to surface heating,
the convergence of surface air, and forced uplift
along topographic barriers and weather fronts.
Key Points
• The atmosphere is conditionally unstable when
unsaturated air can be lifted to a point where
condensation occurs and the rising air becomes
warmer than the air around it. This takes lace when
the environmental lapse rate lies between the moist
adiabatic rate and the dry adiabatic rate.
• The majority of clouds form due to surface heating,
the convergence of surface air, and forced uplift
along topographic barriers and weather fronts.
Key Points
• The atmosphere can be made more stable by
cooling the surface air, warming the air aloft, or
by causing air to sink (subside) over a vast area.
• The atmosphere can be made more unstable
by warming the surface air, cooling the air aloft,
or by lifting a layer of air.
Key Points
• The majority of clouds form due to surface heating,
the convergence of surface air, and forced uplift
along topographic barriers and weather fronts.
Rising Air
Temperature
drops at moist
adiabatic lapse
rate (~6°C/km)
Dew point
temperature
drops at moist
adiabatic lapse
rate (~6°C/km)
Saturated
Unsaturated
Dew point
Temperature drops at
temperature
dry adiabatic lapse rate
drops at 2°C/km
(~10°C/km)
Subsiding Air
Temperature
rises at moist
adiabatic lapse
rate (~6°C/km)
Dew point
temperature rises
at moist adiabatic
lapse rate
(~6°C/km)
Saturated
Unsaturated
Temperature rises at
dry adiabatic lapse rate
(~10°C/km)
Dew point
temperature rises
at 2°C/km
Stratocumulus (Sc)
Lenticular
Flow over a Mountain
T = -6°C
Td = -6°C
T = -3°C
Td = -3°C
T = 0°C
Td = 0°C
T = 3°C
Td = 3°C
T = 8°C
Td = 4°C
T = 13°C
Td = 5°C
T = 18°C
Td = 6°C
0m
2500m
2000m
1500m
1000m
500m
3000m
Dry adiabatic rate =
10°C/km
Moist adiabatic rate
= 6°C/km
Flow over a Mountain
Dry adiabatic rate =
10°C/km
Moist adiabatic rate
= 6°C/km
3000m
T = -6°C
Td = -6°C
T = -1°C
Td = -5°C
2500m
T = +4°C
Td = -4°C
2000m
1500m
1000m
T = +9°C
Td = -3°C
T = +14°C
Td = -2°C
500m
T = 18°C
Td = 6°C
0m
T = =19°C
Td = -1°C
0m
T = +24°C
Td = 0°C