Formation Of Precipitation

The puzzle of raindrop formation has already been noted. The simple growth of cloud droplets by condensation is apparently an inadequate mechanism and more complex processes have to be envisaged.

Various early theories of raindrop growth can be discounted. Proposals were that differently charged droplets could coalesce by electrical attraction, but it appears that distances between drops are too great and the difference between the electrical charges too small for this to happen. It was suggested that large drops

Figure 5.11 Average zonal distribution of total cloud amount (per cent), derived from surface observations over the total global surface (i.e. land plus water) for the months of December to February and June to August during the period 1971 to 1981.

Source: From London et al. (1989).

Figure 5.11 Average zonal distribution of total cloud amount (per cent), derived from surface observations over the total global surface (i.e. land plus water) for the months of December to February and June to August during the period 1971 to 1981.

Source: From London et al. (1989).

Figure S.I2 Mean annual net cloud forcing (W m-2) observed by the Nimbus-7 ERB satellite for the period June 1979 to May 1980. Source: Kyle et al. (1993) From Bulletin of the American Meteorological Society, by permission of the American Meteorological Society.

Figure S.I2 Mean annual net cloud forcing (W m-2) observed by the Nimbus-7 ERB satellite for the period June 1979 to May 1980. Source: Kyle et al. (1993) From Bulletin of the American Meteorological Society, by permission of the American Meteorological Society.

might grow at the expense of small ones. However, observations show that the distribution of droplet size in a cloud tends to maintain a regular pattern; the average radius is between 10 and 15 ^m, and few are larger than 40 ^m. A further idea was that atmospheric turbulence might bring warm and cold cloud droplets into close conjunction. The supersaturation of the air with reference to the cold droplets and the under-saturation with reference to the warm ones would cause the latter to evaporate and cold droplets to develop at their expense. However, except perhaps in some tropical clouds, the temperature of cloud droplets is too low for this differential mechanism to operate. Figure 2.14 shows that, below about -10°C, the slope of the saturation vapour pressure curve is low. Another theory was that raindrops grow around exceptionally large condensation nuclei (observed in some tropical storms). Large nuclei do experience a more rapid rate of initial condensation, but after this stage they are subject to the same limiting rates of growth that apply to all cloud drops.

Current theories for the rapid growth of raindrops involve either the growth of ice crystals at the expense of water drops, or the coalescence of small droplets by the sweeping action of falling drops.

1 Bergeron-Findeisen theory

This widely accepted theory is based on the fact that at subzero temperatures the atmospheric vapour pressure decreases more rapidly over an ice surface than over water (Figure 2.14). The saturation vapour pressure over water becomes greater than over ice, especially between temperatures of -5 and -25°C, where the difference exceeds 0.2 mb. If ice crystals and supercooled water droplets exist together in a cloud, the drops tend to evaporate and direct deposition takes place from the vapour on to the ice crystals.

Freezing nuclei are necessary before ice particles can form - usually at temperatures of about -15 to -25°C. Small water droplets can, in fact, be supercooled in pure air to -40°C before spontaneous freezing occurs. But ice crystals generally predominate in clouds where temperatures are below about -22°C. Freezing nuclei are far less numerous than condensation nuclei; there may be as few as 10 per litre at -30°C and probably rarely more than 1000. However, some become active at higher temperatures. Kaolinite, a common clay mineral, initially becomes active at -9°C and on subsequent occasions at -4°C. The origin of freezing nuclei has been a subject of much debate but it is generally considered that very fine soil particles are a major source. Biogenic aerosols emitted by decaying plant litter, in the form of complex chemical compounds, also serve as freezing nuclei. In the presence of certain associated bacteria, ice nucleation can take place at only -2 to -5°C.

Tiny ice crystals grow readily by deposition from vapour, with different hexagonal forms (Plate 10) developing at different temperature ranges. The number of ice crystals also tends to increase progressively because small splinters become detached by air currents during growth and act as fresh nuclei. The freezing of supercooled water drops may also produce ice splinters (see F, this chapter). Figure 5.13 shows that a low density of ice particles is capable of rapid growth in an environment of cloud water droplets. This results in a slower decrease in the average size of the much larger number of cloud droplets although this still takes place on a time scale of 101 minutes. Ice crystals readily aggregate upon collision, due to their branched (dendritic) shape, and groups of ten crystals may form a single snowflake. Temperatures between about 0 and -5°C are particularly favourable to aggregation, because fine films of water on the crystal surfaces freeze when two crystals touch, binding them together. When the fall speed of the growing ice mass exceeds the existing velocities of the air upcurrents the snowflake falls, melting into a raindrop if it falls about 250 m below the freezing level.

This theory can account for most precipitation in middle and higher latitudes, yet it is not completely satisfactory. Cumulus clouds over tropical oceans can give rain when they are only some 2000 m deep and the cloud-top temperature is 5°C or more. In mid-latitudes in summer, precipitation may fall from cumuli that have no subfreezing layer (warm clouds). A suggested mechanism in such cases is that of 'droplet coalescence', discussed below.

Practical rainmaking has been based on the Bergeron theory with some success. The basis of such experiments is the freezing nucleus. Supercooled (water) clouds between -5 and -15°C are seeded with especially effective materials, such as silver iodide or 'dry ice' (CO2) from aircraft or ground-based silver iodide generators, promoting the growth of ice crystals and encouraging precipitation. The seeding of some cumulus clouds at these temperatures probably produces a mean increase of precipitation of 10 to 15 per cent from clouds that are already precipitating or are 'about to precipitate'. Increases of up to 10 per cent have resulted from seeding winter orographic storms. However, it appears likely that clouds with an abundance of natural ice crystals, or with above-freezing temperatures throughout, are not susceptible to rain-making. Premature release of precipitation may destroy the updrafts and cause dissipation of the cloud. This explains why some seeding experiments have actually decreased the rainfall! In other instances, cloud growth and precipitation have been achieved by such methods in Australia and the United States. Programmes aimed at increasing winter snowfall on the western slopes of the Sierra Nevada and Rocky Mountains by seeding cyclonic storms have been carried out for a number of years with mixed results. Their success depends on the presence of suitable supercooled clouds. When several cloud layers are present in the atmosphere, natural seeding may be important. For example, if ice crystals fall from high-level cirrostratus or altostratus (a 'releaser' cloud) into nimbostratus (a 'spender' cloud)

Figure 5.13 The effect of a small proportion of initially frozen droplets on the relative increase/decrease in the sizes of cloud ice and water particles. The initial droplets were at a temperature of -I0°C and at water saturation. (A) A density of 100 drops per cc, I per cent of which were assumed to be frozen. (B) A density of 1000 drops per cc, 0.1 per cent of which were assumed to be frozen.

Figure 5.13 The effect of a small proportion of initially frozen droplets on the relative increase/decrease in the sizes of cloud ice and water particles. The initial droplets were at a temperature of -I0°C and at water saturation. (A) A density of 100 drops per cc, I per cent of which were assumed to be frozen. (B) A density of 1000 drops per cc, 0.1 per cent of which were assumed to be frozen.

Source: Jonas (1994). Reprinted from Weather, by permission of the Royal Meteorological Society. Crown copyright ©.

composed of supercooled water droplets, the latter can grow rapidly by the Bergeron process and such situations may lead to extensive and prolonged precipitation. This is a frequent occurrence in cyclonic systems in winter and is important in orographic precipitation (see E3, this chapter).

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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