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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
radiational cooling at the snow/ice surface which
cools the air layer above up to a height of about
1km; in summer they are the result of the surface
cooling, by conduction, of warmer air that is
advected (transported horizontally) over the ice
surfaces of the polar regions. The inversions
persist owing to the prevailing high pressure
situations that prevent the spread of cloud cover
associated with storm systems. The tropical and
subtropical deserts have very steep lapse rates in
summer causing considerable heat transfer from
the surface and generally ascending motion;
subsidence associated with high pressure cells
is predominant in the desert zones in winter.
Over the subtropical oceans, sinking air leads to
warming and a subsidence inversion near the
surface (see Chapter 13).
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 2km, 6
°
C TERRESTRIAL INFRARED
RADIATION AND THE
GREENHOUSE EFFECT
Radiation from the sun is predominantly short-
wave, whereas that leaving the earth is longwave,
or infrared, radiation (see Figure 3.1 ). The infrared
emission from the surface is slightly less than that
from a black body at the same temperature and,
accordingly, Stefan's equation (see p. 41) is
modified by an emissivity coefficient (
C/km between 4 and 5km, and
7°C/km between 6 and 8km. 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 cooling over a snow
surface. A similar situation occurs when dense,
cold air accumulates in mountain basins on
calm, clear nights. On such occasions, mountain
summits may be many degrees warmer than the
valley floor below (see Chapter 5C.1). For this
reason, the adjustment of average temperature of
upland stations to mean sea level may produce
misleading results. Observations in Colorado at
Pike's Peak (4301m) and Colorado Springs
(1859m) show the mean lapse rate to be 4.1
°
), which is
generally between 0.90 and 0.95, i.e., F = T 4 .
Figure 3.1 shows that the atmosphere is highly
absorbent to infrared radiation (due to the effects
of water vapour, carbon dioxide and other trace
gases), except between about 8.5 and 13.0μm - the
'atmospheric window'. The opaqueness of the
atmosphere to infrared radiation, relative to its
transparency to shortwave radiation, is commonly
referred to as the greenhouse effect . However, in the
case of an actual greenhouse, the effect of the glass
roof is probably as significant in reducing cooling
by restricting the turbulent heat loss as it is in
retaining the infrared radiation.
The total 'greenhouse' effect results from the
net infrared absorption capacity of water vapor,
carbon dioxide and other trace gases - methane
°
C/km
in winter and 6.2
C/km in summer. It should be
noted that such topographic lapse rates may bear
little relation to free air lapse rates in nocturnal
radiation conditions, and the two must be
carefully distinguished.
In the Arctic and over Antarctica, surface
temperature inversions persist for much of the
year. In winter, these inversions are due to intense
°
 
 
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