precipitation (weather)

precipitation in meteorology, condensed moisture that falls to the surface of the earth in the form of rain , sleet , snow , hail , frost , or dew .

precipitation Precipitation—water falling to the ground—is one of the single most obvious elements of the weather. The presence of precipitation is quickly felt once it occurs, and its total duration in time at any location may consequently seem to be far greater than it actually is. However, even in the wettest parts of the tropics, or in exposed maritime areas in the temperate zones, there is at least as much ‘dry’ time as there is ‘wet’, and frequently much more. In truth, very few clouds precipitate.

Precipitation may take a variety of different forms (rain, snow, sleet, etc.); it may show considerable variation in magnitude (intensity), from the heaviest in thunderstorms or tropical cyclones to the lightest drizzle of windward coasts and hills; and it may be fleeting in showers, be continuous, or be intermittent. ‘Clouds’ at ground level (fog or mist) can also yield precipitation when the wind drives water droplets on to solid obstacles, such as buildings or trees. These general characteristics of precipitation are used in its classification. They also often give vital clues to the processes behind its generation. These processes are still imperfectly understood, and in all of them there is the major physical problem of generating particles, large enough to fall to the ground, from the very tenuous collection of small particles of ice or water which make up clouds. Surviving the fall to the surface is a further problem in the unsaturated air beneath the cloud.

Precipitation is conventionally measured using rain gauges which conform to a standard national pattern. These are read daily, generally at 09.00hours local time, to the nearest 0.1mm depth. In many countries such records extend well back into the nineteenth century. Some gauges are also capable of measuring precipitation automatically at much shorter intervals (e.g. every hour). The gauges themselves have to be carefully sited away from obstructions to obtain a ‘realistic’ measure of amount, but they are subject to error, and each represents only a minute ‘sample’ of an often huge surrounding area. Precipitation is highly variable at the microscale in both space and time. The reliance on gauge measurement has until recently restricted precipitation measurement to populated land areas, but during the 1980s and 1990s rapid advances were made in the field of radar and satellite ‘sensing’ of precipitation. Many nations now operate a network of radar installations, enabling good estimates to be made of precipitation occurrence and movement every 15minutes, and the newest satellite technology provides good estimates of precipitation amount over the two-thirds of the Earth's surface covered by water.

At lower levels, in the tropical latitudes, and during the warmer parts of the year in the temperate zone, most precipitation is in the liquid form—rain or drizzle—once it hits the ground. The distinction between the two is mainly one of size: drizzle drops have diameters of less than 0.5 mm and are close together. Raindrops are equal to, or greater than, 0.5mm. Intensities greater than 4.0 millimetres per hour (mm h⁻¹) indicate heavy rain, and intensities less than 0.5mm h⁻¹ slight rain; moderate rain fills the gap between these limits. When the rain falls as showers, a different range of intensities is used, because of the nature of ‘short, sharp showers’: ‘violent’, greater than 50mmh⁻¹; heavy, 10−50mmh⁻¹; moderate, 2−10mmh⁻¹; slight, less than 2mmh⁻¹. The definitions for drizzle intensity are more descriptive: ‘thick’ drizzle ‘definitely impairs visibility and accumulates at a rate of up to 1 mm h⁻¹’; moderate drizzle ‘causes windows and road surfaces to stream with moisture’; slight drizzle is ‘readily detected on face, but produces very little runoff’.

At higher elevations, in high latitudes, and also during the colder part of the year in temperate areas, solid precipitation is more common. Typically this falls as snow, but there is a wide variety of other terms for solid precipitation based on the size and structure of the individual particles. Snowflakes comprise the familiar loose aggregates of ice crystals which often adopt a hexagonal and branched form; there are also snow and ice pellets, snow grains, granular snow (graupel), and ice prisms. The distinction between snow and ice is that snow particles are always opaque. The intensity ranges for snow are ten times those for rain: 10 cm of freshly fallen snow is equivalent to 10 mm of rain. At times, formerly solid precipitation will have melted only partially by the time it reaches the ground surface, or snow and rain can fall simultaneously, that is, as sleet.

Another important category of solid precipitation is that of hail. Hailstones are generated by a unique set of processes that operate in cumulonimbus clouds, which contain rapidly rising thermals of warm air and strong down draughts of cold air, through a considerable depth of atmosphere, and are sometimes associated with thunder, lightning, and tornadoes. When looked at in section, hailstones show a series of alternately opaque and clear ice rings. Each opaque layer represents passage through a part of the cloud where freezing was very rapid, trapping air bubbles. Each clear layer represents times when air bubbles were able to escape when freezing was slower. Together, pairs of rings mark the completion of a cycle of vertical movement within the cloud, during which the hailstone is alternately carried rapidly to high levels in the cloud on top of rising thermals, and then downwards within equally strong down draughts. Freezing is very rapid in the upper layers of the cloud and much slower at lower levels.

Precipitation develops within clouds in which there is notable and sustained uplift. Under such conditions the clouds become thick enough for one or more of a variety of processes to operate and cause the precipitation. Mechanisms for uplift are present at the local scale within shower and thunderstorm clouds (cumulus and cumulonimbus), where air is forced upwards over hills and mountain ranges or along locally developed zones of convergence, such as sea-breeze fronts (‘forced uplift’ or ‘forced convection’), and at the larger scale, within weather systems such as temperate-latitude frontal depressions and tropical cyclones. These synoptic weather features can also incorporate local-scale uplift.

Precipitation thus dominantly occurs in parts of the world that are subject to the development and passage of major synoptic weather features within the temperate westerly belts in both hemispheres, in some parts of the tropics where cyclones can form, and in areas prone to sustained daytime surface heating, generating strong thermals, or the windward slopes of higher ground. Such processes may also typically show marked seasonal or diurnal variation, with the more intense thunderstorm or shower-type precipitation generally occurring in mid- to late afternoon, in the summer months in temperate zones, and within the humid tropics. The larger-scale development of temperate depressions favours the winter half of the year, whereas tropical cyclone activity peaks during the hottest months.

These weather systems are efficient generators of precipitation at varying spatial scales. More generally though, the range of conditions that must be satisfied to generate precipitation within a cloud is very exacting. The concentration of water droplets or ice particles within clouds is very low, and their size is extremely small. There is thus far more ‘space’ between adjacent particles than there is particle volume. This has important consequences for the potential generation of precipitation. Given their spacing, the opportunities for collision between particles are remote. Their size is such that many are more likely to be carried upwards by thermals, rather than to fall under the influence of gravity; and even if they were to fall from the cloud base they would rapidly evaporate once in the unsaturated air between cloud and ground. Some mechanisms are required that will significantly increase their size (and therefore their mass).

Early theories of precipitation assumed that as a cloud developed and grew in volume, the cloud droplets would simply continue to grow until some of them became large enough to fall as precipitation. For spherical cloud droplets the surface area increases very rapidly for a comparatively modest increase in radius. An increased surface area means that even if copious amounts of water vapour are available in the surrounding air, encouraging initial condensation and droplet growth, the supply is rapidly exhausted. Although the rate of growth for small droplets may be rapid, for large droplets to grow to raindrop size may in theory take days, which is more than the lifetime of the cloud.

There are two basic processes that enable cloud particles to mature into fully fledged precipitation. The first entails collision between cloud particles. This is afforded by the considerable differential horizontal and, particularly, vertical motion within clouds, and the varying rates of fall (or rise) of particles within the cloud according to their mass (i.e. size). Small particles in static air will fall only very slowly towards the ground under gravity. Initially they will accelerate, but they will soon reach a constant terminal velocity when the air resistance around them offsets their downward acceleration. For larger particles this terminal velocity will be reached later, and will be higher. The terminal velocity for a droplet of 0.05 mm radius will be 0.25 metres per second (m s⁻¹), and that for a 0.001 mm droplet will be 0.0001 m s⁻¹. For a 2.5 mm raindrop it is 9.1 m s⁻¹. Smaller particles will also be more easily carried aloft by updraughts; larger ones, less so. Larger particles fall more rapidly towards the surface, the smaller less so. The opportunity for collision between particles is thus considerably increased, with larger particles sweeping up smaller particles in the downward direction and colliding with upward-moving ones. Where collision occurs between liquid droplets, the process is referred to as coalescence. Collision between solid particles, is called aggregation; between liquid and solid it is known as accretion.

The second process that generates precipitation within a cloud is commonly known as the Bergeron–Findeisen process, after the two meteorologists who independently evolved the theory in the 1930s. The process centres on the ability of liquid and solid cloud particles to coexist at temperatures between 0 °C and −40 °C. All cloud particles above 0 °C will be liquid, and all below −40 °C will be solid. The number of liquid (supercooled) droplets decreases below 0°C. Between −10 and −30°C there is commonly a mixture of both solid and supercooled liquid particles. Many clouds (particularly those with considerable vertical extent such as conventional clouds) contain a deep layer between these two temperatures. In thunderstorm clouds, the upper layers are below −40°C, while the lower layers are frequently above freezing point. When significant quantities of supercooled droplets are available, nearby solid ice particles will grow at the expense of the liquid droplets. This occurs because air which is saturated with respect to ice is unsaturated with respect to water. Water will initially evaporate from a droplet and its size will be reduced. Because the air around the ice is now supersaturated, deposition occurs on to the surface of the ice particle, increasing its bulk. The air near the droplet is now once again unsaturated, and the process continues, allowing ice particles to grow to considerable size at the expense of water droplets.

Within typical clouds a combination of collision and Bergeron–Findeisen processes can occur. For ‘warm’ clouds (exclusively above 0 °C) only coalescence operates. Outside the tropics, warm clouds are shallow, so that only slight rain or drizzle will result. For ‘cold’ clouds, however, all processes can operate, and the considerable vertical motion within cumulonimbus clouds, with their tops at temperatures below −40 °C, and their lower layers perhaps above 0 °C, affords the opportunity for particles to circulate within them many times over, growing continually, sometimes by collision, sometimes accreting droplets, sometimes by aggregation, and often producing hailstones of a significant size. In addition, the upper parts of the cloud may continually be ‘seeding’ the higher-temperature parts of the cloud beneath with ice particles, which ‘feed’ on this raw material. At the same time, new liquid water may be swept up into the same layer, perpetuating the growth of particles by the Bergeron–Findeisen process.

Graham Sumner

Bibliography

Meteorological Office (1972) Observers' handbook. HMSO Publications, London.
Sumner, G. (1988) Precipitation: process and analysis. John Wiley and Sons, Chichester.

precipitation In meteorology, all forms of water particles, whether liquid or solid, that fall from the atmosphere to the ground. Distinguished from cloud, fog, dew, and frost, precipitation includes rain, drizzle, snow and hail. Measured by rain and snow gauges, the amount of precipitation is expressed in millimetres or inches of liquid water depth. In chemistry, the formation of an insoluble solid in a liquid by a reaction in the liquid between two or more soluble substances. This is the opposite of dissolving. Precipitation is used to create insoluble salts; precipitation reactions are employed to recognize certain ions.