Environmental Engineering Reference
In-Depth Information
650 cm s
−1
. The momentum of the droplet when it reaches the ground is known as the
terminal velocity, and, with the mass of the drop, determines its kinetic energy.
The total kinetic energy of a storm depends upon the number of raindrops reaching the
ground. This is a measure of the rainfall intensity. Rainfall intensity varies considerably
both within an individual storm and between storms. Rainfall from thick cumulus-type
clouds is particularly variable, owing to spatial differences in cloud
Table 5.1 Types of precipitation
Type
Characteristics
Typical amount
Dew
Deposited on surfaces, especially vegetation; hoar
frost when frozen
0·1-1·0 mm per night
Fog-drip
Deposited on vegetation and other obstacles from fog;
rime when frozen
Up to 4 mm per night
Drizzle
Droplets under 0·5 mm in diameter
0·1-0·5 mm per hour
Rain
Drops over 0·5 mm in diameter, usually 1-2 mm
Light, under 2 mm per
hour; heavy, over 7
mm per hour
Hail
Roughly spherical lumps of ice 5-50 mm or more in
diameter, often showing a layered structure of opaque
and clear ice in cross-section
Highly variable
Snowflakes
Clusters of ice crystals up to several centimetres
across
Variable
Granular snow
Very small flat opaque grains of ice; solid equivalent
of drizzle
Light, under 1 mm per
hour
Snow pellets
(graupel or soft
hail)
Opaque pellets of ice 2-5 mm in diameter falling in
showers
Variable
Ice pellets
Clear ice encasing a snowflake or snow pellet
Sleet (UK)
Mixture of partly melted snow and rain
Sleet (US)
Frozen rain or drizzle drops
thickness and updraught strength, but intensities may be as high as 200 mm hr
−1
or more
for short periods. Precipitation from stratiform clouds is less variable and intensities are
usually low - less than 5 mm hr
−1
. This is why tropical rainstorms can have more
dramatic effects on soil erosion than temperate rainfall.
SNOW
In most areas of the world, rainfall is by far the most important input to the surface
hydrological system. In some areas significant inputs can fall in the form of snow. Snow
occurs mainly in winter, and, despite its thickness and persistence, the quantities of
moisture involved are relatively small (Table 5.2). In general 120 mm of freshly fallen
