Читать книгу Know Your Clouds - Tim Harris - Страница 6

A sunset-lit cumulus cloud over the Guatemalan coastline

Quite apart from their aesthetic value, human life would not exist were it not for clouds. They make all terrestrial life possible. Clouds are the freshwater delivery mechanism that transports water from the oceans to the land and makes possible the growth of plants and animals.

A cloud is a visible concentration of minute water droplets, ice particles, or a mixture of both, floating in the atmosphere. The vast majority of them are in the lowest layer of the atmosphere, which is called the troposphere. On average, each water droplet has a diameter of about 0.01 mm, but droplets may be much smaller or larger. There are hundreds in each cubic centimetre, millions in every cubic metre. Surface tension keeps them spherical, and currents keep them airborne. Because they are so small, the droplets can remain in liquid form at temperatures as low as -30°C. When still liquid at sub-zero temperatures, they are described as supercooled. At higher levels in the troposphere, clouds are made of ice crystals, which come in many shapes and sizes.

An English amateur meteorologist and artist called Luke Howard named the clouds in the early nineteenth century. He proposed three principle categories—cumulus, stratus, and cirrus—and their modifiers. There have been a few additions, but his names pretty much stuck. When trying to identify a cloud, the key pointers to look for are whether it is heaped (a type of cumulus), flat (a type of stratus), or wispy (a type of cirrus); how low its base is; and whether rain or snow is falling from it. Clouds are named from a combination of their appearance, altitude, and ability to produce precipitation. The names are derived from five Latin words: Alto – rain; Cirrus – curled; Cumulus – heaped; Nimbus – rain; Stratus – layered. Additionally, lenticularis is Latin for lens-shaped; mammatus means breast-shaped; and pyro means fire.