Precipitation

10.1 General 192

10.2 Rainfall Intensity 193

10.3 Spatial Differences 195

10.4 Variations of Rainfall 199

10.5 Water Balances 205

10.6 Floods 208

10.7 Droughts 209

10.8 Snow 215

10.1 GENERAL

The importance of rain is obvious in the natural world, and as regards our water supplies, crop growth and so on (Note 10.A). The pattern of rainfall and the temperature are often taken as a concise description of the climate of a place.

Kinds of precipitation include drizzle (Table9.1 ), snow (Section 10.8) and hail (Section 9.8). Dew was considered in Section 4.7. Symbols for them were shown in Figure 9-4.

Rain may be either continuous or showery. Showers result from convective activity and therefore are more common in spring and summer over land. They may be described as 'isolated' (affecting less than 10 per cent of the area), 'scattered' if they occur over 10-50 per cent of the area, or 'widespread'.

Measuring rainfall is easy to do (but hard to do properly) by means of either rain gauges (Note 10.B) or some indirect technique, perhaps involving satellites or radar (Note 10.C). One indirect record of seasonal rainfall in times past is given by the width of tree rings in dry climates (Note 10.D).

Acid Rain

A particular aspect of rainfall is its acidity, arising from gases dissolved in the drops. The acidity is described in terms of the 'pH', which can range from unity (i.e. extreme acidity—lemon juice has a pH of 2.2) to 14 (i.e. extreme alkalinity) (Note 10.E). Droplets in clouds over remote oceans have a pH of about 6.2, not far short of 'neutrality' (which would mean neither acid nor alkaline) represented by a pH of 7. Rain at Cape Grim at the north-east tip of Tasmania in the path of the clean oceanic westerlies, has a pH of 6. 'Pure' rainwater has a value of about 5.6 on account of dissolved carbon dioxide (Note 10.E). Values of 5 have been obtained outside Newcastle, an industrial city in New South Wales, and 4.7 at Katherine, a rural site at 14°S in northern Australia. The pH of summer rain in Sydney is commonly around 4.4, chiefly because of sulphuric acid formed by the dissolving of sulphur dioxide and nitrogen oxides from the air pollution of vehicles and the burning of coal. However, the problem of acid rain is more serious at some places in the northern hemisphere.

Plate 10.1 The various kinds of cloud responsible for rainfall. This image from the Japanese geostationary satellite GMS on 4 February 1995 shows a line of frontal cloud (yielding frontal rainfall) over South Australia and Western Australia, with isolated convective clouds (producing heavy showers) near the equator, and orographic cloud over New Zealand.

Plate 10.1 The various kinds of cloud responsible for rainfall. This image from the Japanese geostationary satellite GMS on 4 February 1995 shows a line of frontal cloud (yielding frontal rainfall) over South Australia and Western Australia, with isolated convective clouds (producing heavy showers) near the equator, and orographic cloud over New Zealand.

Heavy rain has less acidity, presumably because of dilution by more water. But fog in industrial areas can be extremely acid.

A different contamination of rainwater is found near the coast, where rain is made corrosive by aerosols of salt, mainly sodium chloride from the sea. The concentration of chlorides in rain at the coast at Perth or in Victoria is reduced by a factor of about 13 at places 100 km inland.

Was this article helpful?

0 0
Solar Power

Solar Power

Start Saving On Your Electricity Bills Using The Power of the Sun And Other Natural Resources!

Get My Free Ebook


Post a comment