Rain: Encyclopedia II - Rain - Rain in nature

Rain - Rain in nature

Rain plays a major role in the hydrologic cycle in which moisture from the oceans evaporates, condenses into clouds, precipitates back to earth, and eventually returns to the ocean via streams and rivers to repeat the cycle again. There is also a small amount of water vapor that respires from plants and evaporates to join other water molecules in condensing into clouds.

The amount of rainfall is measured using a rain gauge. It is expressed as the depth of water that collects on a flat surface, and can be measured to the nearest 0.25 mm or 0.01 in. It is sometimes expressed in litres per square metre (1 L/m² = 1 mm). Falling raindrops are often depicted in cartoons or anime as "tear-shaped", round at the bottom and narrowing towards the top, but this is incorrect (only drops of water dripping from some sources are tear-shaped at the moment of formation). Small raindrops are nearly spherical. Larger ones become increasingly flattened, like hamburger buns; very large ones are shaped like parachutes. [1] On average, raindrops are 1 to 2 mm in diameter. The biggest raindrops on Earth were recorded over Brazil and the Marshall Islands in 2004 - some of them were as large as 10 mm. The large size is explained by condensation on large smoke particles or by collisions between drops in small regions with particularly high content of liquid water. Raindrops impact at their terminal velocity, which is greater for larger drops. At sea level and without wind, 0.5 mm drizzle impacts at about 2 m/s, while large 5 mm drops impact at around 9mm/s [2].

Generally, rain has a pH slightly under 6. This is because atmospheric carbon dioxide dissolves in the droplet to form minute quantities of carbonic acid, which then partially dissociates, lowering the pH. In some desert areas, airborne dust contains enough calcium carbonate to counter the natural acidity of precipitation, and rainfall can be neutral or even alkaline. Rain below pH 5.6 is considered acid rain.

Rain is said to be heavier immediately after a bolt of lightning. The cause of this phenomenon is traceable to the bipolar aspect of the water molecule. The intense electric and magnetic field generated by a lightning bolt forces many of the water molecules in the air surrounding the stroke to line up. These molecules then spontaneously create localized chains of water (similar to nylon or other 'poly' molecules). These chains then form water droplets when the electric/magnetic field is removed. These drops then fall as intensified rain.

Adapted from the Wikipedia article "Rain in nature", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki