Name the different kinds of precipitation and explain how each forms.

One of the most important aspects of clouds is their ability to bring water to Earth’s continents. Without this water, life would exist only in the oceans. How does rain form and fall out of clouds?

The rain-making process starts with condensation nuclei: small particles, roughly 0.2 μm in diameter, that provide a surface on which water vapor can condense. Tiny bits of dust, bacteria, salt, smoke, pollen, volcanic ash, industrial pollution—all of these aerosols can act as condensation nuclei. In saturated air, water vapor condenses onto condensation nuclei to form cloud droplets.

Collision and coalescence, the process by which cloud droplets merge to make raindrops (Figure 3.26), occurs in clouds at air temperatures of about –15°C (5°F) and warmer. When the raindrops become heavy enough, they fall to Earth. Referring back to Figure 3.5, without water’s cohesive property, cloud droplets would not form or merge, and there would be no rainfall.

Normally, water freezes at 0°C (32°F), but the microscopic size of a cloud droplet allows it to enter a supercooled state rather than freezing at 0°C. In a supercooled state, water freezes quickly as it comes into contact with a solid object on the ground. Below about –10°C (14°F), a mixture of ice crystals and supercooled droplets occurs. Below about –40°C (or –40°F), all cloud droplets freeze. Supercooled cloud droplets are essential in the process of forming snow.

The ice-crystal process (or Bergeron process) forms snow in clouds in which the temperature is 0°C or below. Most winter precipitation at middle and high latitudes, where clouds are typically well below freezing, results from the ice-crystal process.

The ice-crystal process is somewhat more complex and less well understood than the collision-and-coalescence process that forms raindrops. In general, there are two ways in which ice crystals grow to form snow. First, snow forms from water vapor as it is deposited directly on ice nuclei, as shown in Figure 3.27. Ice nuclei differ from condensation nuclei in that they are platelike or angular in shape. Salt crystals, some bacteria, and clay minerals are among the particles that can act as ice nuclei. In cold clouds, the saturation vapor pressure is slightly lower around ice crystals than it is around cloud droplets. Therefore, water vapor is deposited on an ice crystal more readily than it condenses on a cloud droplet. The second way snow forms is through contact freezing, shown in Figure 3.28.

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In warm clouds, the temperature is above 0°C, and the collision-and-coalescence process dominates. In cold clouds, the temperature is below 0°C throughout, and the ice-crystal process dominates. Clouds with a range of temperatures, and in which both processes occur, are called mixed clouds (Figure 3.29).

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With the exception of hail, all types of precipitation are produced either through the collision-and-coalescence process or the ice-crystal process. Whether precipitation takes the form of rain, snow, sleet, or freezing rain depends on the temperature profile of the atmosphere—that is, the pattern of temperature change with altitude, as illustrated in Figure 3.30.

The world record for greatest snowfall in a single season was set on Mount Baker, in Washington State, which received 28.96 m (95 ft) in the 1998–1999 season. How was that snow depth measured? There is an established protocol for measuring snow and rain. Some of the techniques used are outlined in Figure 3.31.

Hail is formed in cumulonimbus clouds with strong vertical airflow. Hail consists of hard, rounded pellets of ice, called hailstones, that precipitate out of cumulonimbus clouds. Hailstones start as small pellets of ice called graupel, which form as supercooled droplets of water freeze directly on snow crystals. As graupel is transported through the upper parts of a cumulonimbus cloud, where the temperature is below freezing. Raindrops and cloud droplets freeze onto it and add to its mass. Updrafts keep hailstones suspended in the cloud. The longer the growing hailstone remains aloft in the cloud, the larger and heavier it will grow. Therefore, the size of hailstones is related to the strength of updrafts in the cumulonimbus cloud (Figure 3.32).

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Most hailstones are less than 2 cm (0.75 in) in diameter. Sometimes, however, they are much larger. In the late afternoon of July 23, 2010, in Vivian, South Dakota, the world’s largest officially recorded hailstone fell to Earth in a farm field (Figure 3.33). As recorded by a National Oceanic and Atmospheric Administration (NOAA) official, it measured 21.9 cm (8.63 in) in diameter—about the size of a bowling ball—and weighed 0.88 kg (1.94 lb).

Hail destroys crops and property, costing taxpayers more than a billion dollars per year. Most of this damage comes as hail pits fruits and disfigures them, making them worthless on the market, or damages young plants. Large hailstones can be deadly. Hailstones the size of tennis balls or larger fall to Earth at about 160 km/h (100 mph).

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