Describe the atmospheric conditions that produce rain, snow, sleet, freezing rain, and hail.

Atmospheric conditions vary greatly both geographically and seasonally, resulting in several different types of precipitation (Figure 5.18). Rain and snow are the most common and familiar forms, but others, listed in Table 5.5, are important as well. Sleet, freezing rain (glaze), and hail often produce hazardous weather and occasionally inflict considerable damage.

Rain

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. Most rain originates in either nimbostratus clouds or in towering cumulonimbus clouds that are capable of producing unusually heavy rainfalls known as cloudbursts.

As rain enters the unsaturated air below the cloud, it begins to evaporate. Depending on the humidity of the air and the size of the drops, rain may completely evaporate before reaching the ground. This phenomenon produces virga, which appear as streaks of precipitation falling from a cloud that extend toward Earth’s surface without reaching it. Similar to virga, ice crystals may sublimate when they enter the dry air below. These wisps of ice particles are called fallstreaks.

Fine, uniform droplets of water with diameters less than 0.5 millimeters are called drizzle. Drizzle and small raindrops generally are produced in stratus or nimbostratus clouds, from which precipitation may be continuous for several hours or, on rare occasions, for days.

Precipitation containing the very smallest droplets able to reach the ground is called mist. Mist can be so fine that the tiny droplets appear to float, and their impact is almost imperceptible. Mist closely resembles fog. Meteorologists use the word fog when the visibility is less than 1 kilometer (0.6 miles) and mist when the visibility is greater than 1 kilometer.

Snow

Snow is winter precipitation in the form of ice crystals, or aggregates of ice crystals. The size, shape, and concentration of snowflakes depend to a great extent on the temperature profile of the atmosphere.

Recall that at very low temperatures, the moisture content of air is low. The result is the generation of very light and fluffy snow made up of individual six-sided ice crystals (Figure 5.19). This is the “powder” that downhill skiers covet. By contrast, at temperatures warmer than about −5°C (23°F), the ice crystals may join together into larger clumps consisting of tangled aggregates of crystals. Snowfalls consisting of these composite snowflakes are generally heavy and have a high moisture content, which makes them ideal for making snowballs.

Snow can reach Earth’s surface when the temperature of a thin layer of air near the ground is above freezing. In this situation, the snow does not have enough time to melt before reaching the ground. This type of snow is usually quite wet.

Under certain atmospheric conditions, falling snow crystals grow as they intercept tiny supercooled cloud droplets that freeze on them. The resulting snowflakes are described as being rimed. If riming continues and makes the shape of the original six-sided snow crystal no longer identifiable, the soft ice pellet is called graupel. Usually oblong in shape and fragile enough that it will fall apart when touched, graupel is also known as soft hail or snow pellets.

Sleet and Freezing Rain

Sleet, a wintertime phenomenon, 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, also called glaze, in contrast, falls as supercooled raindrops that freeze on contact with roads, power lines, and other structures.

As shown in Figure 5.20, both sleet and freezing rain occur mainly in the winter and early spring. 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 rain as it falls through the layer of warm air below the frontal boundary. When the newly formed raindrops encounter a shallow warm layer that overlies a thick cold layer of air, sleet results (Figure 5.20). In this setting, the snowflakes mostly melt, except for tiny ice crystals that remain in the drop. As these raindrops fall through the thick layer of subfreezing air, they refreeze and reach the ground as small pellets of ice roughly the size of the raindrops from which they formed.

By comparison, if the warm layer is thick and the drop melts completely, then the shallow 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—hence the term freezing rain (Figure 5.20). The result is thick coating of ice that has sufficient weight to break tree limbs, down power lines, and make walking and driving extremely hazardous.

In January 1998 an ice storm of historic proportions caused enormous damage in New England and southeastern Canada. Five days of freezing rain deposited a heavy layer of ice on exposed surfaces from eastern Ontario to the Atlantic coast. The 8 centimeters (3 inches) of precipitation caused trees, power lines, and high-voltage towers to collapse, leaving over 1 million households without power—many for nearly a month following the storm (Figure 5.21). At least 40 deaths were blamed on the storm, which caused damages in excess of $3 billion. Much of the damage was to the electrical grid, which one Canadian climatologist summed up this way: “What it took human beings a half-century to construct, took nature a matter of hours to knock down.”

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 Figure 5.22A, 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 the hailstone grows too heavy to be supported by the updraft or encounters a downdraft and falls to the surface.

It was once believed that hailstones traveled up and down many times through a cloud to form a large hailstone composed of spherical clear and milky layers. Recent research, however, indicates that there are two methods by which large hailstones develop, wet growth and dry growth (Figure 5.22B). Clear ice is produced by wet growth in regions of the cloud that contain abundant moisture, where colliding droplets coat the surface of the hailstones. The latent heat released keeps the outside of the stone wet. As these droplets slowly freeze, any air bubbles in the water escape—producing relatively bubble-free, clear ice. By contrast, in regions that have less moisture, the growth rate is slower, and less latent heat is released. The supercooled cloud droplets immediately freeze as they collide with the growing hailstone. The air bubbles are “frozen” in place, leaving milky ice—also referred to as rime ice.

Most hailstones have diameters between 1 centimeter (pea size) and 5 centimeters (golf ball size), although some can be as big as softballs. Occasionally, hailstones weighing 1 pound or more have been reported; most of these are composites of several stones frozen together. These large hailstones have fall velocities that exceed 160 kilometers (100 miles) per hour.

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 900 grams (2 pounds). The stone that held the previous record of 766 grams (1.69 pounds) fell in Coffeyville, Kansas, in 1970 (Figure 5.22B). 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 reportedly been recorded in Bangladesh, where a 1987 hailstorm killed more than 90 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 (Figure 5.23). In the United States, hail damage each year can run into the hundreds of millions of dollars. One of the costliest hailstorms to occur in North America took place June 11, 1990, in Denver, Colorado, with total damage estimated to exceed $625 million. Figure 5.24 shows the average number of hail occurrences per year over a 10-year period.

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 (Figure 5.25). In these situations, objects such as pine needles act as freezing nuclei, causing the supercooled droplets to freeze on contact. On occasions when the wind is blowing, only the windward surfaces of objects will accumulate the layer of rime.

• drizzle: Precipitation from stratus clouds consisting of tiny droplets.

• fallstreaks: Precipitation falling from clouds that evaporates before it reaches the ground, also called virga.

• freezing rain: A coating of ice on objects formed when supercooled rain freezes on contact. A storm that produces glaze is termed an “icing storm" or "ice storm."

• graupel: “Soft hail” that forms as rime collects on snow crystals to produce irregular masses of “soft” ice.

• hail: Precipitation in the form of hard, round pellets or irregular lumps of ice that may have concentric shells formed by the successive freezing of layers of water.

• mist: Droplets small enough that they are barely felt, typically associated with stratus clouds.

• rain: Liquid drops falling from nimbostratus or cumulonimbus clouds.

• rime: A delicate accumulation of ice crystals formed when supercooled fog or cloud droplets freeze on contact with objects.

• sleet: Frozen or semifrozen rain formed when raindrops pass through a subfreezing layer of air.

• snow: Precipitation in the form of white or translucent ice crystals, chiefly in complex branched hexagonal form and often clustered into snowflakes.

• virga: Wisps or streaks of water or ice particles that fall out of a cloud and evaporate before reaching Earth’s surface.

• The two most common and familiar forms of precipitation are rain and snow.

• Sleet forms when raindrops freeze while falling through a thick layer of subfreezing air. Freezing rain results when supercooled raindrops freeze upon contact with cold objects.

• Hail is produced in towering cumulonimbus clouds.