Rain explained

By Storm Dunlop

It is an interesting juxtaposition that World Meteorological Day should come immediately after World Water Day. World Water Day has been an event in the United Nations calendar since 1993, and the involvement of the international organisation and its topic for 2013 (‘Water Cooperation’) evokes the thought of ‘water wars’: arguments between nations over the use of this precious resource, efforts to actually map water resources, and of the extreme strife that can arise, even within developed nations such as the United States, over access to water.

For 2013, the World Meteorological Organisation has chosen their topic as ‘Watching the weather to protect life and property’ and is also celebrating 50 years of World Weather Watch, the coordinated, world-wide system that provides access to weather data to meteorological services around the world, so essential for monitoring weather systems and, in this context, rainfall, or as it is known to meteorologists in a broader context: precipitation.

Rainfall in excessive quantities or in an unusual location may give rise to flooding – as we have seen only too frequently in Britain in the past year – but quite apart from such problems and its many other uses, water is absolutely essential for agriculture – particularly in tropical countries where the onset and progress of the monsoon is anxiously awaited, and in regions where agriculture is utterly dependent on precipitation brought by the less predictable tropical cyclones – known as ‘cyclones’, ‘hurricanes’, or ‘typhoons’, depending on their location around the world.

Yet the source of the world’s water may be expressed in a single word: ‘Rain’. (To be pedantic, we should really use two words: ‘Rain’ and ‘Snow’.) It is a persistant urban myth that the Inuit have a vast vocabulary of different words for ‘snow’, and there may various sayings about the intensity of rain: ‘soft rain’; ‘raining cats and dogs’; ‘raining pokers’; and ‘raining stair-rods’ – I wonder how many people nowadays are familiar with stair-rods? – but ‘rain’ is just simply ‘rain’. However, meteologists do sometimes, slightly light-heartedly, refer to two types of rain ‘warm rain’ and ‘cold rain’. These have nothing to do with the actual temperature of the eventual raindrops, but are a form of shorthand for the way in which the rain originates.

The technical terms for these two processes are coalescence, or ‘collision-coalescence’ responsible for warm rain and the Bergeron process (the ‘Bergeron-Findeisen process’ or ‘ice-crystal theory’) that produces cold rain. Raindrops have a typical range of 0.1—9 millimetres in diameter, yet the cloud droplets from which many form are extremely tiny, with typical diameters of about 1–100μm (1μm – a micron – being one thousandth of a millimetre). Vast numbers of cloud droplets are therefore required to form a single raindrop. In ‘warm rain’ this growth occurs simply through collisions and the coalescence of two droplets. Such collisions occur only when clouds are the site of extremely vigorous convection and turbulence and are not subject to freezing (glaciation) in their upper levels. (We will come to glaciation shortly.) Convective clouds of this sort are very deep cumulus congestus and cumulonimbus, and such clouds, and the warm rain that they generate, occur all year round in the tropics. In Britain, the vigorous convection and other conditions required to create these deep clouds tend to be confined to the summer.

‘Cold rain’ by contrast, does involve freezing. But this is not completely straightforward. In the absence of any suitable nuclei on which to freeze, water may exist in a liquid state at temperatures well below 0°C. This condition is known as supercooling and such droplets may survive at temperatures as low as -40°C, before they freeze spontaneously. (Clouds, such as altocumulus that often consist of supercooled droplets form a major icing hazard for aircraft, because the droplets freeze instantly on contact with a solid surface.) When a cloud is in such a state, any ice crystals that may have formed on suitably shaped solid dust or other nuclei grow rapidly at the expense of supercooled water droplets. Eventually the crystals become so large that they begin to fall towards the ground, and may subsequently melt into raindrops, or else, if temperatures are sufficiently low, be deposited as snow.

Glaciation occurs in many different forms of cloud, with the ice crystals turning into raindrops on their descent, but is most clearly seen in operation in the tops of cumulonimbus clouds. Here the cloud towers turn from vigorously growing cells, like those seen in cumulus congestus to a form known as cumulonimbus calvus, with slightly softer outlines – a slight misnomer, because ‘calvus’ actually means ‘bald’ – before going on to become cumulonimbus capillatus, where striations are clearly visible. Both states are signs that freezing is taking place. If the cloud towers reach the tropopause they may flatten into an ‘anvil’ shape, known as cumulonimbus incus. Depending on the wind shear at altitude, such anvils may grow, often explosively, to cover large areas of the sky.

Ice crystals may form in various ways and in different sizes, the largest of which may be regarded as the hail pellets that are created by repeated passage through freezing layers, with the pellets being carried up to higher layers in strong updraughts, before eventually becoming so heavy that they fall out of the cloud. The very largest raindrops result from the melting of individual hail pellets. Small droplets are approximately spherical in shape, but the larger ones become flattened as they descend, somewhat resembling buns in shape. The very largest recorded reached diameters of 10 millimetres, but large drops (over about 5 mm in diameter) normally fragment into smaller droplets during their descent.

Trails of precipitation that do not reach the ground (known as virga) may often be seen below a number of different types of cloud. When the precipitation (of whatever type) does reach the ground, it is known technically as praecipitatio.

Storm Dunlop is the author of A Dictionary of Weather, which is also available online as part of Oxford Reference. He is a fellow of both the Royal Astronomical Society and the Royal Meteorological Society.

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