Geoscience Reference
In-Depth Information
2 The cold front
The weather conditions observed at cold fronts are
equally variable, depending upon the stability of the
warm sector air and the vertical motion relative to
the frontal zone. The classical cold front model is of
the ana-type, and the cloud is usually cumulonimbus.
Figure 9.15 illustrates the warm conveyor belt asso-
ciated with such a frontal zone and the line convection.
Over the British Isles, air in the warm sector is rarely
unstable, so that nimbostratus occurs more frequently at
the cold front (see Figure 9.11A). With the kata-cold
front the cloud is generally stratocumulus (see Figure
9.11B) and precipitation is light. With ana-cold fronts
there are usually brief, heavy downpours, sometimes
accompanied by thunder. The steep slope of the cold
front (approximately 2°) means that the bad weather is
of shorter duration than at the warm front. With the
passage of the cold front, the wind veers sharply,
pressure begins to rise and temperature falls. The sky
may clear abruptly, even before the passage of the
surface cold front in some cases, although with kata-
cold fronts the changes are more gradual. Forward-
tilting cold fronts are sometimes observed, either due
to surface friction slowing the low-level motion of
the front, or as a result of a cold front aloft (see Figure
9.10).
Figure 9.13
Fronts and associated rain bands typical of a mature
depression. The broken line X-Y shows the location of the cross-
section given in Figure 9.14.
Source
: After Hobbs; from Houze and Hobbs (1982), copyright ©
Academic Press. Reproduced by permission.
Figure 9.14
Cross-section along the line X-Y in Figure 9.13 showing cloud structures and rain bands. The vertical hatching represents
rainfall location and intensity. Raindrop and ice particle regions are shown, as are ice particle concentrations and cloud liquid water
content. Numbered belts refer to those shown in Figure 9.13. Scales are approximate.
Source
: After Hobbs and Matejka
et al
.; from Houze and Hobbs (1982), copyright © Academic Press. Reproduced by permission.
