Geoscience Reference
In-Depth Information
Fig. 3.1
Cross section of a typical
cold front. (Vertical scale
is exaggerated.)
Warm air
Cold air
Fig. 3.2
Cross section of a
typical warm front.
Warm air
Cold air
of the cumuliform type and the precipitation convective (see Section 3.2.2), if the warm
air is conditionally unstable (see Figure 3.1). In this case, scattered thunderstorms and
showers may develop, and in extreme cases the front may evolve into a continuous line
of thunderstorms, called a
squall line
.
Warm fronts are usually not as steep, with slopes that are on average of the order
of 0.01; they also move more slowly than cold fronts, and are not as well defined. As
the warm air moves over the cold air a broad band of clouds develops (Figure 3.2),
extending up to several hundreds of kilometers ahead of the front's position on the
ground. Also in the case of a warm front, it is the stability of the warm air that determines
the type of precipitation produced by the front. When the warm air in the approaching
frontal air mass is moist and stable, the sequence of cloud types is cirrus, cirrostratus,
altostratus, and nimbostratus, and the precipitation increases gradually. When the air is
moist and conditionally unstable, the same sequence may occur, but altocumulus and
cumulonimbus, often with thunderstorms, will also be observed.
The interface between contrasting air masses tends to be unstable and it often evolves
further, through the rotation of the Earth, into a spiraling stream called a
cyclone
.A
cyclone is a large low-pressure zone, and it is usually accompanied by cloud systems
and precipitation. An anti-cyclone is the opposite case, namely a high-pressure zone,
which usually brings fair weather; it is also normally characterized by
subsidence
,a
slow downward air motion resulting from the horizontal divergence of the air away from
the pressure high.
Although there is an infinite variability in the occurrence of cyclones, they have certain
features in common; a typical life cycle of a cyclone is sketched in Figures 3.3, 3.4
