Types of Precipitation
Precipitation consists of liquid water drops and solid crystals that fall from the atmosphere and reach the ground. This precipitation can take several different forms.
Rain is precipitation that reaches the ground as liquid water. Raindrops can form in warm clouds as liquid water, which through collisions can coalesce with other drops and grow large enough to fall to Earth. They can form through other processes as well. For example, solid ice, in the form of snow or hail, can also produce rain by melting if it falls through a layer of warm air.
To fall from the sky and reach the ground, raindrops usually have to grow larger than 0.2 mm (0.008 in.). At these sizes, we refer to the drops as mist or drizzle. Once the drops reach 0.5 mm (0.02 in.), they are termed raindrops. Typically, raindrops can only have a maximum size of 5–8 mm (0.2–0.3 in.); drops any larger than that become unstable and break into smaller drops as they fall through the atmosphere.
Snow forms as individual water vapor molecules are deposited on existing ice crystals. If these ice crystals are formed entirely by deposition, they take the shape of snowflakes with their characteristic intricate crystal structure. However, most particles of snow have endured collisions and coalesce with each other and with supercooled water drops. As they do so, they lose their shape and can become simple lumps of ice.
Eventually, whether they are intricate snowflakes or accumulations of ice and supercooled water drops, these ice crystals become heavy enough to fall from the cloud. By the time this precipitation reaches the ground, it may have changed form. Snow produced in cold clouds reaches the ground as a solid form of precipitation if the underlying air layer is below freezing. Otherwise, the snow melts and arrives as rain.
SLEET AND FREEZING RAIN
Perhaps you have experienced an ice storm. Ice storms occur when the ground is frozen and the lowest air layer is also below freezing. Actually, ice storms are more accurately named “icing” storms because it is not ice that is falling but supercooled rain. Rain falling through the cold air layer is chilled and freezes onto ground surfaces as a clear, slippery glaze, making roads and sidewalks extremely hazardous. Ice storms cause great damage, especially to telephone and power lines and to tree limbs pulled down by the weight of the ice.
Hailstones are formed by the accumulation of ice layers on ice pellets that are suspended in the strong updrafts of thunderstorms. As these ice pellets—called graupel—move through subfreezing regions of the atmosphere, they come into contact with supercooled liquid water droplets, which subsequently freeze to the pellets in a thin sheet. This process, called accretion, results in a buildup of concentric layers of ice around each pellet, giving it its typical ball-like shape.
With each new layer, the ball of ice—now called hail—gets larger and heavier. When it becomes too heavy for the updraft to support, it falls to Earth. When the updrafts are extremely strong, the hail remains aloft, slowly accumulating more mass and getting larger. In that case, hailstones can reach diameters of 3–5 cm (1.2–2.0 in.).
Precipitation is recorded as a depth that falls during a certain time—for example, as millimeters or inches per hour or per day. A millimeter of rainfall would cover the ground to a depth of 1 mm if the water did not run off or sink into the soil.
Rainfall is measured with a rain gauge. This simple meteorological instrument is constructed from a narrow cylinder with a wide funnel at the top. The funnel gathers rain from a wider area than the mouth of the cylinder, so the cylinder fills more quickly. The water level gives the amount of precipitation, which is read on a graduated scale.
For meteorological records, snowfall is measured by the amount of liquid water it yields when melted. We can also measure snowfall by depth in millimeters or inches. Ordinarily, a 10-mm (or 10-in.) layer of snow is assumed to be equivalent to 1 mm (or 1 in.) of rainfall, but this ratio may range from 30 to 1 in very loose snow to 2 to 1 in old, partly melted snow.
Precipitation as water droplets or snow particles can be identified using radar, a technology in which a beam of radio waves is sent out from a transmitter, strikes the water particles, and then returns to a receiver. By measuring the intensity of the return beam and observing the time it takes for the beam to travel and return, it is possible to determine the intensity of precipitation and its location. Images obtained by scanning weather radars are widely used and familiar to anyone who routinely watches weather forecasts on television.