Geoscience Reference
In-Depth Information
dependent on the type of pressure system. High-pressure
areas are generally associated with descent and warming
of deep layers of air, hence decreasing the temperature
gradient and frequently causing temperature inversions
in the lower troposphere. In contrast, low-pressure
systems are associated with rising air, which cools
upon expansion and increases the vertical temperature
gradient. Moisture is an additional complicating factor
(see Chapter 3E), but it remains true that the middle
and upper troposphere is relatively cold above a surface
low-pressure area, leading to a steeper temperature
gradient.
The overall vertical decrease of temperature, or
lapse
rate
, in the troposphere is about 6.5°C/km. However,
this is by no means constant with height, season or
location. Average global values calculated by C. E. P.
Brooks for July show increasing lapse rate with height:
about 5°C/km in the lowest 2 km, 6°C/km between 4
and 5km, and 7°C/km between 6 and 8 km. The seasonal
regime is very pronounced in continental regions with
cold winters. Winter lapse rates are generally small and,
in areas such as central Canada or eastern Siberia, may
even be negative (i.e. temperatures increase with height
in the lowest layer) as a result of excessive radiational
Figure 3.19
Direct solar radiation as a function of altitude
observed in the European Alps. The absorbing effects of water
vapour and dust, particularly below about 3000 m, are shown by
comparison with a theoretical curve for an ideal atmosphere.
Source
: After Albetti, Kastrov, Kimball and Pope; from Barry (1992).
air. The various factors interact in a highly complex
manner. The energy terms are the release of latent
heat by condensation, radiative cooling of the air and
sensible heat transfer from the ground. Horizontal
temperature advection, by the motion of cold and warm
airmasses, may also be important. Vertical motion is
Figure 3.20
Average direct beam solar
radiation (W m
-2
) incident at the surface
under cloudless skies at Trier, West
Germany, and Tucson, Arizona, as a
function of slope, aspect, time of day and
season of year.
Source
: After Geiger (1965) and Sellers
(1965).
