Precipitation


Everyone knows what a cloud looks like, but not everyone knows that this white, puffy formation is actually a mass of tiny water droplets and ice crystals suspended above the earth by air currents. Clouds are formed when warm, moist air rises and cools. During this cooling process the water vapor (water not in a liquid state) in the rising air condenses (changes into a liquid state) to form water droplets, or it sublimates (changes into a solid state without becoming liquid first) to form ice crystals. From these water droplets and ice crystals come all forms of precipitation, including rain, drizzle, sleet, snow, and hail.

Air currents within the cloud keep the water droplets moving about, and when they collide, the larger ones absorb the smaller ones. This process is called coalescence, which means “growing together into one body.” When they grow large enough—one to three millimeters in diameter—gravity causes them to fall as raindrops. To give you an idea of just how small the cloud droplets are, the average raindrop contains about a million of them.

Ideal cloud temperatures for the formation of snow range from ten degrees above zero to four degrees below. However, for a beautiful snow scene to occur, air and ground temperatures must also be low. Otherwise, the snowflakes that form in the cloud will melt either as they fall through the air or when they touch the ground.

Cloud temperatures may range from seventy-five degrees above to forty degrees below zero. In the colder air, when water vapor comes in contact with a microscopic, airborne particle of dust, soil, rock, or volcanic ash, it crystallizes and forms a single tiny ice crystal. As more water vapor freezes around it, the original crystal forms branches and slowly grows into a small, flat, six-sided snowflake. Very cold air contains too little moisture to produce big snowflakes, so large flakes occur only when the temperature is just a few degrees below freezing. Ideal temperatures for the formation of snow range from ten degrees above zero to four degrees below.

Once a snowflake has reached the proper size, it begins falling. If the air near the ground is warm, the snowflake melts on the way down and becomes a raindrop. However, if the air temperature is cold, the snowflake falls without melting, and we are able to see it float to the earth. Whether the snowflake melts when it touches the earth or remains a frozen crystal depends on the ground temperature. Because the ground is solid and cools much more slowly than air, it is possible for the air temperature to be cold enough for snowflakes to fall before the ground is cold enough for them to remain frozen when they land. When the ground is cold enough, the snowflakes quickly cover it.

Sleet is another cold-weather product, but it forms when the air near the ground is freezing or below and the air above that is warm enough to allow raindrops to pass through. When the raindrops strike the cold air, they freeze into beads of cloudy-white ice that fall to the ground. These little ice pellets are usually about one-eighth of an inch or less in diameter.

Sleet often referred to as freezing rain, but true freezing rain is known as glaze. Glaze occurs when rain or drizzle (very fine, light rain droplets 0.1 to 0.25 millimeters in diameter) is supercooled but not frozen as it falls through the air. When it comes in contact with a cold surface, it freezes immediately, forming a clear ice coating on everything it touches. These ice storms can be beautiful, but as the ice builds up, extensive damage may occur. Tree limbs break under the weight of the ice, and telephone and electrical lines often are pulled down or broken.

Another damaging type of precipitation is hail. Although there are three basic kinds of hail—soft hail, small hail, and true hail—only true hail grows large enough to break windows and windshields, dent automobiles and metal buildings, tear roofs to pieces, destroy crops and vegetation, and sometimes kill small animals and livestock. Millions of dollars’ worth of damage can result from a single fifteen-minute storm containing true hail.

Soft hail is a white, snowlike pellet about one-fourth of an inch in diameter. It is easily crushed and usually falls apart when it bounces on the ground. It occurs at temperatures above the ground freezing point and appears before or with snow.

Although a hailstorm may last only a few minutes, in that short time these lumps of ice can beat down crops and other vegetation and cause tremendous financial loss to farmers in the area.

Small hail usually has a center of soft hail with a thin layer of ice formed around it. This gives it a glazed, semitransparent appearance. Small hail occurs at temperatures above freezing and often falls with rain. It does not fall apart when it hits the ground, but, like soft hail, it is too small to be destructive.

True hail is a hard pellet of ice which may vary in size from one-fourth inch to five inches or more in diameter. Since it is formed only in the extremely cold air found in a thunderstorm cloud, hailstones and violent thunderstorms occur together.

The hailstone usually starts out as a frozen raindrop. As it collides with supercooled water droplets, the water is spread over the frozen raindrop’s surface and quickly freezes into a layer of clear ice. When ice crystals and snow come in contact with the surface, they freeze into a cloudy layer of ice. Cutting a hailstone in half will reveal these growth layers, which resemble the rings in an onion slice.

There are two theories as to how the hailstone acquires these growth layers. The first and oldest theory is that strong updrafts of air lift the hailstone into the area of freezing temperatures, where it quickly gathers a coating of snow and ice crystals. Then the hailstone falls back into the area of water droplets, where it gathers a layer of water which quickly freezes to its surface. It is then forced up by another updraft to gather another layer of snow and ice crystals. The size of the hailstone depends on the strength of the upward air currents and the number of times the hailstone travels up and down before falling to the earth.

The second theory assumes that the hailstone’s growth takes place in one continuous drop through the cloud, during which it captures supercooled droplets that lie in its path through the subfreezing layers of air. Although the fall is slowed and even stopped at times by updrafts, the hailstone is not lifted by air currents and always maintains a downward direction. The layers result from differences between freezing rate and water accumulation rate.

Whichever theory is correct, the hailstone grows until it becomes too large to be supported by the air currents, and then it falls to earth. It is easy to see that the larger the hailstone the more damage it can cause.

Hailstones are not always round. They also may be pyramid-shaped, saucer-shaped, or covered with spikes. The pyramid shape is caused when the hailstone falls without tumbling. As it passes through water droplets and ice crystals, they collect on the bottom, forming a teardrop or pyramid-shaped pellet. If this type of hailstone is cut in half, you will be able to see that the rings grow downward from the pointed end. The saucer shape results from a tumbling, rotating action within the cloud.

When the frozen raindrop center is shaped similar to a top, the hailstone turns as it falls through the cloud. This turning action causes the surface water to move away from the hailstone the way water and mud are thrown from a spinning bicycle tire. If the rotation is slow enough, the water freezes before flying off the hailstone and forms bumps. Additional water follows the same pattern and the bumps slowly grow into iciclelike spikes. Sometimes an irregular mass of ice is mistaken for a single hailstone when it is really two or more frozen together. The only way to be sure the large mass is a true hailstone is to cut it in half and look for the single pattern of expanding growth rings.

The largest hailstone officially recorded fell in September 1970 at Coffeyville, Kansas. It measured 17 ½ inches around the outside and weighed 1.67 pounds.

The first recorded hailstorm in America was reported in the journals of the Coronado Expedition, 1540-42. It is believed that Coronado was probably somewhere in Texas at the time of this recording. “While the army was resting in this ravine, as we have related, a tempest came up one afternoon with a very high wind and hail, and in a very short space of time a great quantity of hailstones, as big as bowls, or bigger, fell as thick as raindrops, so that in places they covered the ground to a depth of two or three spans or more deep. [A span is nine inches] … The hail broke many tents, and battered many helmets, and wounded many of the horses, and broke all the crockery of the army, and the gourds, which was no small loss, because they do not have any crockery in this region.”

Damaging as it may be, hail is just one of the forms of precipitation that is with us all year long. Precipitation can occur only when there are clouds in the sky, so watch those clouds as they build and grow and be ready for their gifts of moisture, whatever form they may take.


Additional Information:

Ilo Hiller
1983 Precipitation. Young Naturalist. The Louise Lindsey Merrick Texas Environment Series, No. 6, pp. 143-147. Texas A&M University Press, College Station.

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