1 Forms of precipitation
Strictly, precipitation refers to all liquid and frozen forms of water. The primary ones are:
Rain Falling water drops with a diameter of at least 0.5 mm and typically 2 mm; droplets of less than 0.5 mm are termed drizzle. Rainfall has an accumulation rate of >1 mm/hour. Rain (or drizzle) that falls on a surface at subzero temperature forms a glazed ice layer and is termed freezing rain. During the protracted 'ice storm' of 5-9 January 1998 in the northeastern United States and eastern Canada, some areas received up to 100 mm of freezing rain.
Snow Ice crystals falling in branched clusters as snowflakes. Wet snow has crystals bonded by liquid water in interior pores and crevices. Individual crystals have a hexagonal form (needles or platelets, see Plate 10). At low temperatures (-40°C), crystals may float in the air, forming 'diamond dust'.
Hail Hard pellets, balls or irregular lumps of ice, at least 5 mm across, formed of alternating shells of opaque and clear ice. The core of a hailstone is a frozen water drop (ice pellet) or an ice particle (graupel).
Graupel Snow pellets, opaque conical or rounded ice particles 2 to 5 mm in diameter formed by aggregation.
Sleet Refers in the UK to a rain-snow mixture;
in North America, to small translucent ice pellets (frozen raindrops) or snowflakes that have melted and refrozen. Dew Condensation droplets on the ground surface or grass, deposited when the surface temperature is below the air's dew-point temperature. Hoar-frost is the frozen form, when ice crystals are deposited on a surface. Rime Clear crystalline or granular ice deposited when supercooled fog or cloud droplets encounter a vertical structure, trees or suspended cable. The rime deposit grows into the wind in a triangular form related to the wind speed. It is common in cold, maritime climates and on mid-latitude mountains in winter.
In general, only rain and snow make significant contributions to precipitation totals. In many parts of the world, the term rainfall may be used interchangeably with precipitation. Precipitation is measured in a rain gauge, a cylindrical container capped by a funnel to reduce evaporative losses, which most commonly stands on the ground. Its height is about 60 cm and its diameter about 20 cm. More than fifty types of rain gauge are in use by national meteorological services around the world. In windy and snowy regions they are often equipped with a windshield to increase the catch efficiency. It must be emphasized that precipitation records are only estimates. Factors of gauge location, its height above ground, large- and small-scale turbulence in the airflow, splash-in and evaporation all introduce errors in the catch. Gauge design differences affect the airflow over the gauge aperture and the evaporation losses from the container. Falling snow is particularly subject to wind effects, which can result in under-representation of the true amount by 50 per cent or more. It has been shown that a double snow fence erected around the gauge installation greatly improves the measured catch. Corrections to gauge data need to take account of the proportion of precipitation falling in liquid and solid form, wind speeds during precipitation events, and precipitation intensity. Studies in Switzerland suggest that observed totals underestimate the true amounts by 7 per cent in summer and 11 per cent in winter below 2000 m, but by as much as 15 per cent in summer and 35 per cent in winter in the Alps between 2000 and 3000 m.
The density of gauge networks limits the accuracy of areal precipitation estimates. The number of gauges per 10,000-km2 area ranges from 245 gauges in Britain to ten in the United States and only three in Canada and Asia. The coverage is particularly sparse in mountain and polar regions. In many land areas, weather radar provides unique information on storm systems and quantitative estimates of area-averaged precipitation (see Box 4.1). Ocean data come from island stations and ship observations of precipitation frequency and relative intensity. Satellite remote sensing, using infra-red and passive microwave data, provides independent estimates of large-scale ocean rainfall.
The climatological characteristics of precipitation may be described in terms of mean annual precipitation, annual variability and year-to-year trends. However, hydrologists are interested in the properties of individual rainstorms. Weather observations usually indicate the amount, duration and frequency of precipitation, and these enable other derived characteristics to be determined. Three of these are discussed below.
a Rainfall intensity
The intensity (= amount/duration) of rainfall during an individual storm, or a still shorter period, is of vital interest to hydrologists and water engineers concerned with flood forecasting and prevention, as well as to conservationists dealing with soil erosion. Chart records of the rate of rainfall (hyetograms) are necessary to assess intensity, which varies markedly with the time interval selected. Average intensities are greatest for short periods (thunderstorm-type downpours) as Figure 4.10 illustrates for Milwaukee, USA.
In the case of extreme rates at different points over the earth (Figure 4.11), the record intensity over ten minutes is approximately three times that for 100 minutes, and the latter exceeds by as much again the record intensity over 1000 minutes (i.e. 16.5 hours). Note that many of the records for events with a duration greater than a day are from the tropics. High-intensity rain is associated with increased drop size rather than an increased number of drops. For example, with precipitation intensities of 1, 13 and 100 mm/hr (or 0.05, 0.5 and 4.0 in/hr), the most frequent raindrop diameters are 1, 2 and 3 mm, respectively. Figure 4.12 shows
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