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

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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Chapter Six

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


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