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 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 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 46 Chapter Six 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 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 not reach the surface, are called virga or fallstreaks CLASSIFICATION OF RAIN There are three main types: 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 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 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 47 Chapter Six the ice crystals not unite to form snow flakes, but may 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 to kg have been reported 48 Chapter Six 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 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 49 Chapter Six 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 50