BÀI GIẢNG KHÍ TƯỢNG LÝ THUYẾT CHƯƠNG 6

Chapter Six

CHAPTER SIX

Precipitation
INTRODUCTION
In meteorology "precipitation" is a generic word embracing most forms of
water deposit which are derived from the condensation of water vapour in the
atmosphere. It includes rain, drizzle, snow. sleet and hail which, together, are
the more common concept of the term: but it also indudes dew. hoar frost, rime
and glazed frost which are, more often than not, regarded by mariners as "not
strictly precipitation". Meteorologists refer to all of these phenomena as
hydrometeors.
Cloud, fog and mist are not classed as precipitation but are hydrometeors. The
difference between rain and drizzle is only that the drops in drizzle are
relatively very small (diameter between 0.2 and 0.5 millimetres) and light.
They fall slowly and gently from low based Stratus cloud. Unless the relative
humidity is high beneath the cloud base the drops are likely to evaporate before
reaching the surface.
RAIN AND DRIZZLE
Formation
1. Raindrops vary in size but they are all larger than the tiny droplets or ice
particles of which clouds are composed; to turn these into rain appreciable
convection (vertical movement) within the cloud is necessary. When
convection is active within cloud the water droplets are carried up to greater
heights and the process of cooling and condensation continues. A proportion of
the droplets will increase in size due to either:
(a) Collision and coalescence of very small droplets with larger ones, and/ or
(b) Growth of ice crystals at the expense of water droplets, in clouds where ice
crystals and water droplets initially co-exist.
Whatever the formation process rain is nearly always created in clouds of

appreciable vertical extent. The greater the vertical thickness of the cloud the
larger the rain drops. Thus drizzle may fall from quite shallow cloud.
2. When the droplets are large and heavy enough to overcome the upward
motion of air they will commence to fall. During descent through cloud they
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will continue to increase in size due to collision with the rising cloud droplets,
until they fall as rain from the base of the cloud.
3. Some evaporation takes place in the warmer unsaturated air below the cloud
base; if the failing drops are large enough in both size and number they will
reach the surface.
Virga. The dark vertical or trailing streaks of precipitation seen falling from
the base of a cloud. and which do not reach the surface, are called virga or
fallstreaks.
CLASSIFICATION OF RAIN
There are three main types:
1. Convectional rain. Associated with unstable atmosphere, high relative
humidity, and a large lapse rate in the lower levels due to strong solar heating
of a land surface, particularly during the hottest hours of the day.
Sea surface temperatures undergo very little change in temperature during
the course of a day (see Chapter 2). but relatively cool moistureladen air
moving over a relatively very warm sea surface will often produce
convectional rain, usually in the form of isolated showers, sometimes heavy
with hail and thunder, especially in tropical regions.
2. Orographic rain occurs when a moisture-laden airstream encounters a
range of hills or mountains. and is thus forced to rise to heights well above the
condensation level. It is usually heaviest on the weather slopes and may be

very light or negligible on the leeward side. (See Fohn in Appendix 1.)
This type of rain can be exceptionally heavy and persistent if given suitable
conditions. For example, the Western Ghats of India (height about 1900 m).
during the south-west monsoon the very heavy rain is almost continuous for
three months but is comparatively slight on the leeward side.
When sea winds cross the coast, surface friction on forested land is
considerable and forms a barrier of air over which the oncoming air is forced
to rise and sometimes causes precipitation.
3. Frontal rain is associated mainly with depressions of the temperate
latitudes. Details are given in Chapter 13.
SNOW AND SLEET
Formation. When water vapour condenses at temperatures well below
freezing point it forms minute ice crystals which, during their very slow fall
through cloud, build up a growth of feathery crystals forming snow flakes.
The size of snow flakes depends on temperature. In very low temperatures
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the ice crystals do not unite to form snow flakes, but may do so on reaching
lower levels of the cloud where the temperatures are less cold. Thus the lower
the temperature the smaller the snow flakes which reach the surface.
For snow to reach the ground, air temperature near the surface must be
lower than 3.5°C (38°F). Above about 3°C (37°F) it will fall as sleet which is
a mixture of snow and rain or of melting snow. Whether the snow lies or not
depends mainly on the temperature of the surface on which it falls.
In very cold weather heavy snowfall can adversely affect a ship's stability.
Heavy snow can also seriously affect visibility.
HAIL

Hail falls from cumulonimbus cloud in the form of hard ice pellets of varying
shapes and is often associated with thunderstorms.
Formation. Vigorous convection currents may carry supercooled (see Appendix
1) water drops up to a height where ice crystals are present and are supported by
strong updrafts. The ice particles grow in size by collision and coalescence with
the supercooled water drops which freeze instantly on impact thereby forming
pellets of white opaque ice (called soft hail). When the pellets become large
enough they will commence to fall and continue to grow.
On entering the lower levels of the cloud where the temperature may be a little
above 0oC (32°F) they may encounter water drops which are not supercooled and
which freeze slowly on to the freezing hailstones surrounding them with a coating
of hard clear ice before they fall below the cloud base.
Due to the strong turbulent eddies, which are a characteristic of cumulonimbus
clouds. some hailstones make several upward and downward journeys between the
upper and lower levels of the cloud before finally falling to earth. This would
account for the concentric structure of very large hailstones which when cut in
half may be seen to be made up of alternate layers of opaque and clear ice.
In winter when the freezing level is well below the cloud base, above which all
water drops must be supercooled, there will be no coating of clear ice on the
hailstones.
The size of hailstones on reaching the surface depends mainly on the vertical
extent of the cloud in which they are formed and the strength of the upcurrents
within it. Usually they measure only a few millimetres in diameter. In some hot,
moist regions of the world hailstones larger than cricket balls and weighing 1 to 2
kg have been reported.

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GLAZED FROST
This, as the name suggests, is a layer of ice which looks like glass. It occurs when
surface temperatures are below 0oC (3Z°F) often at the end of a severe "cold
spell".
Rain or drizzle falling from the cloud associated with a warm front will freeze
immediately on contact with the cold surface and other cold objects, coating
everything with smooth clear ice.
This form of ice can also be produced by fog droplets freezing onto cold objects.
The term black ice is also used to describe a thin coating of this ice on a road
surface, the temperature of which is below 0oC. It is occasionally confused with
black frost (see Appendix).
SEA SPRAY
The most dangerous form of icing encountered at
sea is produced by sea spray freezing onto the
vessel. Ice from this source can accumulate very
rapidly and can pose a severe threat to the stability
of the vessel. The added weight will reduce a
vessel's freeboard and make her “top heavy” in
addition to problems with life saving appliances.
antennae and other equipment becoming frozen.
Sea water freezes at about -2oC (28.5°F). If the
air temperature is below this, sea spray landing on
the superstructure will freeze. producing a coating
of ice. Significant amounts of spray are not
generally present until wind speed reaches Force 5
and the rate of icing increases with increasing
wind speed above this Force.
DEW
A deposit of water formed by condensation on surfaces which have been

cooled by radiation to a temperature below that of the dew-point of the air.
Favourable conditions are - a calm night with a clear sky and high relative
humidity.
HOAR FROST
A deposit of thin ice crystals or frozen dew upon surface whose
temperatures have fallen below both dew-point and O°C.
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RIME
When suspected water droplets of fog strike solid objects such as trees.
telephone wires, ship's masts. rigging and superstructure. at temperatures
below 0° they freeze on impact, forming a deposit of white ice crystals. The
deposit is rough in appearance and grows to windward of the object.

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