Geoscience Reference
In-Depth Information
Snow pellets
(also called
granular snow
or
graupel
) are a form of precipitation
consisting of white, opaque, small grains with diameters between roughly
0.5 and 5 mm.
Small hail
is precipitation consisting of white, semitransparent or translucent grains
with diameters ranging from about 2 to 5 mm. These grains are mostly
round, and sometimes conical in shape, and they have a glazed appearance.
Small hail falls usually accompanied by rain, when the temperature is above
freezing.
Soft hail
consists of round, opaque grains in the same size range as small hail, but
they are softer in appearance and tend to disintegrate more easily.
Hail
consists of balls or irregular chunks of ice with diameters between 5 and
50 mm, or even larger. These lumps of ice can be transparent or they can
consist of concentric layers of clear and opaque ice; such layered structure
is the result of the alternating rising and falling movements during the hail
formation. Hail usually falls during violent and prolonged convective storms
under above-freezing temperature conditions near the ground; it can cause
severe damage.
Dew
consists of moisture in the form of liquid drops on the ground surface and on
the vegetation and other surface elements, as a result of direct condensation
of atmospheric water vapor. It typically occurs at night on surfaces that have
been cooled by outgoing long-wave radiation.
Hoar frost
forms in the same way as dew, but the water vapor condenses directly
into ice. These ice crystals can assume a wide variety of shapes.
3.2
MAJOR PRECIPITATION WEATHER SYSTEMS
3.2.1
Extratropical cyclones and fronts
These types of systems normally result from the interaction of two contrasting air masses.
An air mass can be defined as a body of air with approximately uniform physical char-
acteristics such as (potential) temperature and humidity. The interface between two
different air masses is called a
cold front
when relatively colder air displaces and moves
beneath relatively warmer air, and a
warm front
in the opposite case. Cold fronts tend
to be relatively steep, with average slopes on the order of 0.015; in the Northern Hemi-
sphere they are often oriented from the southwest to the northeast and move toward the
east and southeast. An approaching cold front is usually announced by increasing wind
speeds and the appearance of altocumulus clouds (Figure 3.1). All the while the pressure
decreases, and lower clouds, mainly of the cumulonimbus type, move in with the onset
of precipitation. As the front comes closer the precipitation intensity increases. After the
passage of the front the pressure rapidly rises and the temperature falls sharply; the wind
direction changes, typically from a southerly or southwesterly direction to a more west-
erly or northerly direction. Cold fronts are often followed by drier and cooler weather.
The stability of the warm air mass determines the type of precipitation generated by a
cold front. If the warm air is stable the clouds are of the stratiform type. The clouds are