Clouds

High clouds are found from between 16,500 and 45,000 feet (5,000 to 13,700 meters) and include (from highest to lowest) cirrus, cirrocumulus, and cirrostratus. A cirrus cloud appears in delicate, feather-like bands, sometimes in tufts, and is usually white. Cirrocumulus clouds look like very small round balls or flakes. Sometimes cirrocumulus clouds form the pattern of what is called a "buttermilk" or "mackerel" sky. Cirrostratus clouds sometimes form tangled webs or thin whitish sheets. When cirrostratus clouds cover the sky, a large ring or halo is sometimes seen around the sun or moon. This is caused by the bending of rays of sunlight or moonlight as they pass through the ice particles which make up the clouds.

Middle layer clouds are found from between 6,500 and 23,000 feet (2,000 to 7,000 meters) and include (from highest to lowest) altocumulus, altostratus, and nimbostratus. Altocumulus clouds are rounded puffs of cloud larger than cirrocumulus. Altostratus clouds cover the sky with a greyish veil through which the sun or moon may shine as a spot of pale light. Nimbostratus clouds are thick, dark, and shapeless and usually bring rain or snow.

The altocumulus cloud may immediately precede bad weather.	A fair-weather cumulus cloud sometimes turns into an anvil-shaped cumulonimbus cloud.

Low clouds are found from between ground level and 6,500 feet (2,000 meters) and include (from highest to lowest) stratocumulus, stratus, cumulus, and cumulonimbus. Stratocumulus clouds are large and lumpy, round or rolled-looking, and often cover the entire sky. Stratus clouds are generally dark and appear as streaks across the sky or as a grey layer hanging above the Earth. Stratus clouds are generally shapeless.

Like altocumulus clouds the low-hanging stratus clouds may send a warning of turbulent weather ahead.

Cumulus clouds range in size from the small puffball-like forms to huge dome-topped thick piles of "woolpack" that often develop into thunderclouds. These storm clouds, cumulonimbus, may range in thickness from about 1 to 3 miles (1.6 to 4.8 kilometres) in Great Britain to 8 or 9 miles (12.9 or 14.5 kilometres) near the equator.

The anvil-shaped cumulonimbus is a type of cloud formation that often trails streamers of rain.

Rain Clouds (see also Rain)

When a large storm approaches, warm, moist air rises and spreads out at high altitudes far ahead of the storm centre. As this air cools, its moisture freezes into tiny ice particles, producing cirrus clouds. The first sign of a storm is the appearance of feathery cirrus clouds. Later, as the storm comes closer, these clouds thicken into milky white cirrostratus clouds or small cirrocumulus clouds. If the sun or moon is in the sky, a halo may be seen. This is a fairly reliable sign that it will rain or snow within 24 hours.

Later, the clouds become still thicker. They form at lower levels and, instead of ice particles, the clouds are made of water droplets. These are altostratus and altocumulus clouds. In northern countries, such as Great Britain, winds will then usually shift to the south and blow harder. By this time, rain or snow is usually about six to eight hours away. The south wind becomes steadily stronger, and dark stratocumulus and nimbostratus clouds finally appear. Rain or snow usually begins very soon after these clouds are seen.

In the summer, small white cumulus clouds that appear in the early morning often turn into dark cumulonimbus clouds during the day. Such clouds bring heavy summer showers, with thunder, lightning, and strong, gusty winds. Sometimes, if the clouds push high enough into the very cold upper air, hail will fall out of cumulonimbus clouds.

Rain occurs at temperatures above freezing, with drop sizes ranging in diameter from 0.004 inch (0.01 centimetre) in a light drizzle to 0.4 inch (1 centimetre) in a heavy downpour. Snow will fall at temperatures slightly below freezing.

Hail consists of balls of ice with a diameter of 0.1 to 4 inches (about 0.3 to 10 centimetres). Hail most often occurs in the spring, and it falls mainly from cumulonimbus clouds during thunderstorms. It begins its fall as rain droplets within a cloud, but these droplets may be carried upward repeatedly by strong wind currents to colder regions where they develop several coatings of ice, creating hailstones with diameters as large as 5 inches (12.7 centimetres).

Sleet, which consists of small ice pellets with a diameter less than 0.2 inch (0.5 centimetre), also begins as rain, but updrafts carry the drops into colder levels of the cloud, causing them to become frozen. In Great Britain and parts of the United States a mixture of rain and snow is called sleet.