When water vapor has condensed and is now falling back to earth as rain sleet hail or snow

Any liquid or solid particle of water that falls from the atmosphere to the ground is a form of precipitation. Precipitation is part of a complex water cycle in which water is continuously recycled from the Earth's surface into the atmosphere and back to the surface again. Rain, snow, sleet, and hail are all forms of precipitation.

In addition to precipitation, the water cycle consists of three other basic processes: evaporation, condensation, and accumulation. Moisture gets into the atmosphere by evaporation, a process in which a liquid changes into a gas. Water, like other liquids, is made up of tiny particles called molecules. These water molecules are continually changing into water vapor—a gas that mixes with air and rises into the atmosphere. The rate of evaporation depends on such factors as the dryness and warmth of the air, the amount of wind, and the warmth of the water. The drier and warmer the air, the greater the evaporation. This is also true when it is windier and when the water is warmer. Everyone has experienced evaporation in various situations. For example, as you lie in the sun or wind after swimming, the moisture on your body disappears and your bathing suit becomes dry. This occurs as a result of evaporation. Evaporation also causes puddles to disappear and wet clothes to dry while they are hanging on a clothesline.

Condensation is the process in which water vapor changes back into a liquid. Condensation occurs when the air becomes saturated with water vapor. When this happens, some of the water vapor changes into water droplets. For example, after you take a hot shower, you will notice that water droplets have formed on the mirror in the bathroom. This occurs because the air in the bathroom has become saturated with water vapor. The warmer the air, the more water vapor it can hold. Therefore, condensation is more likely to occur when air cools. You can observe this on a hot, humid day when water droplets form on the outside of a glass of ice water. This condensation occurs because the air around the glass is cooled by the ice inside it.

When water droplets condense in the atmosphere, they form clouds. As the droplets become larger and heavier, they may start to fall from these clouds and drop toward the Earth. Sometimes they evaporate before reaching the ground. At other times, the droplets fall to the ground as precipitation. The process of accumulation occurs as this precipitation builds up in puddles, rivers, lakes, oceans, and other bodies of water. Precipitation is also stored in glaciers and polar ice caps. As the water that accumulates begins to evaporate, the water cycle begins again.

Rain

The surface waters of the Earth are constantly evaporating, changing into water vapor that mixes with air and is carried high into the atmosphere by winds and air currents. The temperature and pressure of the atmosphere decrease with altitude, so the air cools as it rises. Eventually, this cooling causes the water vapor in the air to condense. As it condenses, tiny droplets of water form around tiny airborne particles of matter called condensation nuclei, which include dust, solid particles in smoke, tiny crystals of sea salt, and volcanic ash.

Clouds are collections of enormous numbers of tiny water droplets. The droplets are so small and light that rising currents of air often keep many of them aloft. When small droplets fall toward the surface of the Earth, many of them evaporate before they can reach the ground. The droplets that remain suspended in the atmosphere because of air currents grow larger and heavier as a result of coalescence. This is a process in which water droplets collide with one another and in the process form larger droplets. When enough water droplets form and grow too heavy for rising air currents to keep them aloft, they fall to the ground as rain. The falling droplets of rain often collide with each other as well to form even larger raindrops. Depending on such factors as temperature, winds, and the thickness of clouds, raindrops can occur in various sizes. Small raindrops less than 2/100 inch (0.05 centimeter) in diameter that fall close together are called drizzle.

Under certain conditions, water droplets in clouds freeze because of low temperatures. However, if the temperature of the air at the bottom of a cloud or beneath a cloud is above the freezing point of 32°F (0°C), the frozen particles melt into raindrops as they fall to the Earth.

Snow

Like rain, snow begins as water vapor in a cloud. Depending on the temperature in the cloud and the types of condensation nuclei there, this water vapor changes either to all ice crystals or to a mixture of ice crystals and water droplets. Even at temperatures as low as −40°F (−40°C), liquid water can be present in clouds. This water is said to be supercooled. Supercooled droplets remain liquid until they bump into ice crystals, at which point they freeze.

When a cloud is very cold, water vapor may change directly from a gas into solid crystals of ice rather than into droplets of water that freeze. This process in which a gas changes directly into a solid is called sublimation. At temperatures below freezing, ice crystals may form around tiny particles of matter, just as water droplets are formed. Sometimes, ice crystals and water droplets form together. When this happens, the ice crystals quickly grow larger because water vapor is changed more easily to ice than to water. This is due to differences in the pressure of water vapor when it is over water or ice.

As ice crystals fall and move around, they collide with each other and stick together in a process known as aggregation. This is the process that forms large snowflakes. Each snowflake is made up of a number of ice crystals. The shapes of these ice crystals depend on the temperature in the cloud where they are formed. The shapes include hollow columns, thin plates, needles, six-pointed stars, and branchlike forms known as dendrites. Since snow often falls from different parts of a cloud and from areas with different temperatures, it is not unusual for the cloud to have a mixture of various types of ice crystals.

As snow nears the ground, it sometimes melts and falls as rain. This generally occurs when the air temperature is above freezing.

Sleet

Sometimes falling rain passes through a layer of air in which the temperature is below freezing. When this happens, the rain freezes into small, round pieces of ice known as sleet. Sleet can fall and reach the ground even if the temperature at ground level is slightly above freezing. Although sleet typically has a diameter of only 2/100 inch (0.05 centimeter) or less, it can sometimes accumulate to depths of several inches on the ground.

If rain falls through a thin layer of freezing air near the ground, or if the ground or objects there are below freezing, the rain can freeze on contact. This is called freezing rain, or glaze. If drizzle is falling and freezes, it is called freezing drizzle. Sleet, glaze, and freezing drizzle can cause very dangerous conditions. For example, driving is often extremely hazardous, and significant amounts of freezing rain can damage trees and power lines.

Hail

Sometimes small balls of ice, called graupel, form in the atmosphere when supercooled water freezes upon contact with ice crystals in clouds. As additional water freezes on the graupel, they may grow into larger balls of ice known as hail or hailstones.

Hail generally forms in large, violent thunderstorms with strong currents of rapidly rising air. These currents of air carry graupel high into the atmosphere, where layers of water freeze on them to produce hailstones, which may then fall thousands of feet to the ground or be carried up again on strong air currents. If the hailstones are carried up and down several times, they may collect several layers of frozen water. They can also remain suspended within a cloud, allowing layers of water to collect and freeze on them.

When a large hailstone is sliced open, it will often contain several cloudy and clear layers. These different layers are believed to be the result of variations in temperature and the amount of water present while the hailstone was being formed. For example, water that freezes quickly tends to be cloudy, while water that freezes slowly is clear.

Hailstones come in many shapes and sizes. Small hailstones usually have round or flattened ends. Large hailstones often have spikes or are oddly shaped, which is probably due to the rapid changes the hail experiences while being blown about in the atmosphere.

While an average hailstone is about the size of a pea, hail can grow to the size of a baseball or even larger. The speeds of the rising air currents in a thunderstorm are among the factors that determine the size of hailstones. Air currents moving at about 22 miles (35 kilometers) per hour produce hailstones about 1/2 inch (1.3 centimeters) in diameter. At a speed of about 37 miles (60 kilometers) per hour, hailstones about 3/4 inch (2 centimeters) are formed. The size increases to about 1 3/4 inches (4.5 centimeters) at 56 miles (90 kilometers) per hour and to about 3 inches (7.6 centimeters) at 100 miles (160 kilometers) per hour. One of the largest hailstones ever recorded fell in Coffeyville, Kansas, in September 1970. It measured more than 5 1/2 inches (14 centimeters) in diameter.

Measuring Precipitation

Rain is measured with a rain gauge, which is often simply a hollow tube with a vertical measuring scale on its side. The size of the tube and the length of the vertical scale may vary, allowing the measurement of widely differing amounts of rain. Many rain gauges have a partial cover to help lessen the amount of evaporation that may occur between the time of rainfall and the time of measurement.

Snow can be measured by using a ruler and sticking it through the snow until it touches the ground. However, since wind causes snow to drift to varying depths, it is often difficult to get an accurate measure with this method. Several measurements often must be taken within an area, and the numbers are then averaged.

The liquid equivalent of snow can be determined by filling a rain gauge with snow. When the column of snow in the gauge melts, the amount of water can then be measured. The ratio of snow to liquid varies depending on the consistency of the snow. Typically, 10 inches (25.4 centimeters) of snow will equal about 1 inch (2.54 centimeters) of water. However, with a dry, powdery snow, it takes about 12 inches (30 centimeters) of snow to equal 1 inch (2.54 centimeters) of water. The wetter the snow, the lower the snow-to-liquid ratio.

Global Precipitation Patterns

The amount of precipitation that falls in a particular place depends on a number of factors, including temperature, wind patterns, proximity to oceans or other large bodies of water, and the presence of mountains. Some parts of the Earth receive more rain than others. For example, many areas along and near the equator have very heavy rainfall, while the polar regions and certain areas near the tropics of Cancer and Capricorn typically receive very little rain. Along the equator, currents of hot air absorb large amounts of water vapor that evaporates from the surface of the oceans. As this moisture-laden air rises to colder areas of the atmosphere, much of the water vapor condenses and falls to the surface. The polar regions receive little rainfall because the air is so cold that it cannot hold much water vapor. In many desert regions, the air is so hot and dry that rain rarely ever forms.

In addition to large-scale precipitation patterns, variations on a smaller scale can occur over the course of many years or in particular places. The most dramatic small-scale variations occur in and near mountain ranges. A great deal of rain typically falls on the windward side of coastal mountains, which is the side facing oncoming winds. As moisture-laden air blows inland from the ocean, it sweeps up the slopes of these mountains, cooling as it rises. The cooling of the moist air causes the water vapor in the air to condense and fall either as rain or as snow. Meanwhile, the leeward side of the mountains, which is the side facing away from the wind, receives little rain or snow because most of it has already fallen on the other side. In 1982, winter storms along the coast of central California dropped up to 24 inches (610 millimeters) of rain on the windward slopes of the coastal mountains there. The leeward side of the mountains received only a few inches.

Similar variations in precipitation sometimes occur near large lakes. For example, the so-called lake effect often results in snowfalls of several feet within a short period of time around the Great Lakes during the winter. The snow generally falls only in narrow strips along the lakes. Locations that are only a few miles away may not receive any snow at all.

H. Michael MogilMeteorologist

How the Weatherworks

SOURCE: The New Book of Knowledge

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