Geoscience Reference
In-Depth Information
Moist
adiabatic
lapse rate
Moist
adiabatic
lapse rate
Environmental
lapse rate
Environmental
lapse rate
Environmental
lapse rate
Lifting
condensation
level
Dry
adiabatic
lapse rate
Dry
adiabatic
lapse rate
Dry
adiabatic
lapse rate
Virtual temperature
Virtual temperature
Virtual temperature
Case 3
Case 1
Case 2
Figure 10.4
The mechanism of ascent and cloud formation for the three different cases described in the text.
lapse rate until it reaches the
lifting condensation level
, i.e., the height at which the
temperature of the parcel has cooled enough for it to saturate. At this level cloud
formation begins.
In Cases 2 and 3 the parcel then continues to rise, but now being saturated
inside the cloud, it cools at the moist adiabatic rate. The rate of cooling is therefore
less, but the cooling rate gradually increases as the air temperature decreases with
height inside the cloud. These two cases are different because the environmental
lapse rates into which the buoyant moist air is seeded are different. In Case 2 the
moist adiabatic lapse rate intersects the environmental lapse rate before the tropo-
pause so ascent and cloud formation stops at this level. However, in Case 3 the
environmental lapse rate is such that the moist adiabatic lapse rate does not inter-
sect the environmental lapse rate before the tropopause. In Case 2 cloud develop-
ment is inhibited by loss of buoyancy, but in Case 3 tall cumulus tower clouds can
develop with a consequently greater potential to generate stronger uplift inside the
cloud and heavier precipitation.
Foehn effect
When a moist, unsaturated air mass moving horizontally impinges on a topo-
graphic barrier, the air is forced to ascend (Fig. 10.5a). If ascent is sufficiently
rapid, the moist air first cools at approximately the dry adiabatic lapse rate
