Atmospheric conditions vary greatly both geographically and seasonally, resulting in several different types of precipitation. Rain and snow are the most common and familiar forms, but others, listed in the table below, are important as well. Sleet, freezing rain (glaze), and hail often produce hazardous weather and occasionally inflict considerable damage.

Rain, Drizzle, and Mist

In meteorology, the term rain is restricted to drops of water that fall from a cloud and have a diameter of at least 0.5 millimeter (0.02 inch). Most rain originates either in nimbostratus clouds or in towering cumulonimbus clouds; the latter are capable of producing unusually heavy rainfalls known as cloudbursts. As mentioned in the previous section, raindrops rarely exceed about 5 millimeters (0.2 inch) in diameter because the surface tension that holds the drops together is exceeded by the frictional drag of the air.

Fine, uniform drops of water less than 0.5 millimeter (0.02 inch) in diameter are called drizzle and require about 10 minutes to fall from a cloud 1000 meters (3300 feet) overhead. Drizzle can be so fine that the tiny drops appear to float, and their impact is almost imperceptible. Precipitation containing the very smallest droplets able to reach the ground is called mist.

Snow

Snow is precipitation in the form of ice crystals (snowflakes) or, more often, aggregates of crystals. The size, shape, and concentration of snowflakes depend to a great extent on the temperature at which they form.

Recall that, at very low temperatures, the moisture content of air is low. The result is the formation of very light, fluffy snow made up of individual six-sided ice crystals. This is the “powder” that downhill skiers love so much. By contrast, at temperatures warmer than about -5°C (23°F), the ice crystals join together into larger clumps consisting of tangled aggregates of crystals. Snowfalls composed of these composite snowflakes are generally heavy and have high moisture content, which makes them ideal for making snowballs.

Sleet and Freezing Rain (Glaze)

Sleet consists of clear to translucent ice pellets. Depending on intensity and duration, sleet can cover the ground much like a thin blanket of snow. Freezing rain, or glaze, on the other hand, falls as supercooled raindrops that freeze on contact with roads, power lines, and other surfaces.

As shown in ▼, both sleet and freezing rain occur in the winter, and they most often form along a warm front where a mass of relatively warm air is forced over a layer of subfreezing air near the ground. Both begin as snow, which melts to form raindrops as it falls through the layer of warm air below. When the newly formed raindrops encounter a thick cold layer of air below the frontal boundary, sleet results. In this setting, as raindrops fall through the subfreezing air, they refreeze and reach the ground as small pellets of ice roughly the size of the raindrops from which they formed.

If, however, the layer of cold air near the ground is not thick enough to cause the raindrops to refreeze, they instead become supercooled—that is, they remain liquid at temperatures below freezing (▲). Upon striking subfreezing objects on Earth’s surface, these supercooled raindrops instantly turn to ice, creating a coating of freezing rain. Freezing rain makes walking and driving extremely hazardous, and when the ice grows thick, it can break tree limbs and down power lines (▼).

Hail

Hail is precipitation in the form of hard, rounded pellets or irregular lumps of ice with diameters of 5 millimeters (0.20 inches) or more. Hail is produced in the middle to upper reaches of tall cumulonimbus clouds, where updrafts can sometimes exceed speeds of 160 kilometers (100 miles) per hour and where the air temperature is below freezing. Hailstones begin as small embryonic ice pellets or graupel that coexist with supercooled droplets. The ice pellets grow by collecting supercooled water droplets and, sometimes, other small pieces of hail as they are lifted by updrafts within the cloud.

Cumulonimbus clouds that produce hail have a complex system of updrafts and downdrafts. As shown in ▼A, a region of intense updrafts suspends rain and hail aloft, producing a rain-free region surrounded by an area of downdrafts and heavy precipitation. The largest hailstones are generated around the core of the most intense zone of updraft, where they rise slowly enough to collect appreciable amounts of supercooled water. The process continues until a hailstone grows too heavy to be supported by the updraft or encounters a downdraft and falls to the surface.

Large hailstones often show alternating layers of clear and milky ice (▲B). These layers reflect two different processes by which a hailstone can grow, termed wet growth and dry growth. Wet growth occurs when hail collects supercooled droplets quickly, causing the surface of the hailstone to remain wet. As this wet layer gradually freezes, it produces clear, bubble-free ice. Dry growth, on the other hand, occurs when the accumulation of droplets occurs at a much slower rate. The supercooled droplets freeze immediately on contact, trapping tiny air bubbles that result in milky-looking ice.

The record for the largest hailstone ever found in the United States was set on July 23, 2010, in Vivian, South Dakota. The stone was over 20 centimeters (8 inches) in diameter and weighed nearly  grams ( pounds). The stone that held the previous weight record of 766 grams (1.69 pounds) fell in Coffeyville, Kansas, in 1970 (▲B). The diameter of the stone found in South Dakota also surpassed the previous record of a 17.8-centimeter (7-inch) stone that fell in Aurora, Nebraska, in 2003. Even larger hailstones have been reported; in the Gopalganj district of Bangladesh, in 1986, a storm with hail of 1.02 kilograms (2.25 pounds) killed 92 people.

The destructive effects of large hailstones are well known, especially to farmers whose crops have been devastated in a few minutes and to people whose windows, roofs, and cars have been damaged (▼). In the United States, annual hail damage in 2021 totaled just over 1 billion dollars.

Rime

Rime is a deposit of ice crystals formed by the freezing of supercooled fog or cloud droplets on objects whose surface temperature is below freezing. When rime forms on trees, it adorns them with its characteristic ice feathers, which can be spectacular to observe (▼). In these situations, objects, such as pine needles, act as ice nuclei, causing the supercooled droplets to freeze on contact. When a wind is blowing, the windward surfaces of objects tend to accumulate rime.

• The two most common and familiar forms of precipitation are rain and snow. Rain can form in either warm or cold clouds. When it falls from cold clouds, it begins as snow that melts before reaching the ground.

• Sleet consists of spherical to lumpy ice particles that form when raindrops freeze while falling through a thick layer of subfreezing air. Freezing rain results when supercooled raindrops freeze upon contact with cold objects. Rime consists of delicate frostlike accumulations that form as supercooled fog droplets encounter objects and freeze on contact. Hail consists of hard, rounded pellets or irregular lumps of ice produced in towering cumulonimbus clouds, where frozen ice particles and supercooled water coexist.

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