Geoscience Reference
In-Depth Information
has a very gentle slope, of the order of 0.5-1°, so
the cloud systems associated with the upper
portion of the front herald its approach some 12
hours or more before the arrival of the surface
front. The ana-warm front, with rising warm air,
has multi-layered cloud that steadily thickens and
lowers towards the surface position of the front.
The first clouds are thin, wispy cirrus, followed by
sheets of cirrus and cirrostratus, and altostratus
(
Figure 9.11A
). The sun is obscured as the
altostratus layer thickens and drizzle or rain begins
to fall. The cloud often extends through most of
the troposphere and, with continuous precipita-
E FRONTAL CHARACTERISTICS
The character of frontal weather depends upon
the vertical motion in the air masses. If the air in
the warm sector is rising relative to the frontal
zone the fronts are usually very active and are
termed
ana-fronts
, whereas sinking of the warm
air relative to the cold air masses gives rise to less
intense
kata-fronts
(
Figure 9.11
).
1 The warm front
The warm front represents the leading edge of the
warm sector in the wave. The frontal boundary
(A)
Movement of system
Height (m)
Ci
Cs
Ci
10000
Cs
J
U
WARM AIR
As
COLD AIR
COLD AIR
5000
convective
layer top
Ns
Sc
J
L
Cu
Cu
St
0
Dry
descent
2000
1500
1000
500
kilometers
0
Rain belt
Rain belt
(B)
Movement of system
Height (m)
T
10000
WARM AIR
COLD AIR
COLD AIR
5000
Base of subsidence inversion
Thick
Sc
Thick
Sc and Cu
Cu
Sc
Sc
St
0
2000
1500
Rain belt
1000
500
kilometers
0
Belt of light rain
or drizzle
Figure 9.11
A: Cross-sectional model of a depression with ana-fronts, where the air is rising relative to
each frontal surface. Note that an ana-warm front may occur with a kata-cold front and vice versa. JU and
JL show the locations of the upper and lower jet streams, respectively. B: Model of a depression with kata-
fronts, where the air is sinking relative to each frontal surface.
Sources: After Pedgley, A Course in Elementary Meteorology, and Bennetts et al. (1988). (Crown copyright ©), reproduced by
permission of the Controller of Her Majesty's Stationery Office.
