PRECIPITATION - Fundamentals of Hydrology - page 16

Another mechanism leading to increased water
droplet size is the so-called Bergeron process . The
pressure exerted within the parcel of air, by having
the water vapour present within it, is called the
vapour pressure . The more water vapour present
the greater the vapour pressure. Because there is a
maximum amount of water vapour that can be held
by the parcel of air there is also a maximum vapour
pressure, the so-called saturation vapour pres-
sure . The saturation vapour pressure is greater over
a water droplet than an ice droplet because it is
easier for water molecules to escape from the surface
of a liquid than a solid. This creates a water vapour
gradient between water droplets and ice crystals so
that water vapour moves from the water droplets to
the ice crystals, thereby increasing the size of the ice
crystals. Because clouds are usually a mixture of
water vapour, water droplets and ice crystals, the
Bergeron process may be a significant factor in
making water droplets large enough to become rain
drops (or ice/snow crystals) that overcome gravity
and fall out of the clouds.
The mechanisms of droplet formation within
a cloud are not completely understood. The relative
proportion of condensation-formed, collision-
formed, and Bergeron-process-formed droplets
depends very much on the individual cloud circum-
stances and can vary considerably. As a droplet is
moved around a cloud it may freeze and thaw several
times, leading to different types of precipitation (see
Table 2.1).

Dewfall

The same process of condensation occurs in dewfall ,
only in this case the water vapour condenses into
liquid water after coming into contact with a cold
surface. In humid-temperate countries dew is a
common occurrence in autumn when the air at
night is still warm but vegetation and other surfaces
have cooled to the point where water vapour coming
into contact with them condenses onto the leaves
and forms dew. Dew is not normally a major part
of the hydrological cycle but is another form of
precipitation.

PRECIPITATION DISTRIBUTION

The amount of precipitation falling over a location
varies both spatially and temporally (with time).
The different influences on the precipitation can
be divided into static and dynamic influences. Static
influences are those such as altitude, aspect and
slope; they do not vary between storm events.
Dynamic influences are those that do change and are
by and large caused by variations in the weather. At
the global scale the influences on precipitation
distribution are mainly dynamic being caused
by differing weather patterns, but there are static
factors such as topography that can also cause major
variations through a rain shadow effect (see case
study on pp. 18-19). At the continental scale large
differences in rainfall can be attributed to a mixture

Table 2.1 Classes of precipitation used by the UK Meteorological Office

Class Definition

Rain Liquid water droplets between 0.5 and 7 mm in diameter

Drizzle A subset of rain with droplets less than 0.5 mm

Sleet Freezing raindrops; a combination of snow and rain

Snow Complex ice crystals agglomerated

Hail Balls of ice between 5 and 125 mm in diameter