Thursday, August 22, 2013

All About Hail

Yesterday while I was working the museum and showing several people some of my favorite rocks, a hail storm hit.  The sound of the hailstones hitting the roof of the museum's gift shop was so loud that I had to stop talking.  That gave me an opportunity to snap a couple of photos.

Hail is a form of solid precipitation. It consists of balls or irregular lumps of ice, each of which is called a hailstone. Hailstones consist mostly of water ice and measure between 5 millimeters (0.20 in) and 15 centimeters (6 in) in diameter. Hail is possible within most thunderstorms as it is produced by cumulonimbi, and within 2 nautical miles (3.7 km) of the parent storm.

Hail formation requires strong, upward motion of air with the parent thunderstorm (similar to tornadoes) and lowered heights of the freezing level. Unlike ice pellets, hailstones are layered and can be irregular and clumped together. Hail is composed of transparent ice or alternating layers of transparent and translucent ice at least 1 millimeter (0.039 in) thick, which are deposited upon the hailstone as it travels through the cloud, suspended aloft by air with strong upward motion until its weight overcomes the updraft and falls to the ground. Although the diameter of hail is varied, in the United States, the average observation of damaging hail is between 2.5 cm (1 in) and golf ball-sized (1.75 in).  In the mid-latitudes, hail forms near the interiors of continents, while in the tropics, it tends to be confined to high elevations.

Like other precipitation in cumulonimbus clouds hail begins as water droplets. As the droplets rise and the temperature goes below freezing, they become super cooled water and will freeze on contact with condensation nuclei. A cross-section through a large hailstone shows an onion-like structure. This means the hailstone is made of thick and translucent layers, alternating with layers that are thin, white and opaque.  A couple of my customers rescued some of the hailstones yesterday so I could get some photos.  Notice the "agate like" layers.

Former theory suggested that the concentric rings in hailstones form when hailstones are subjected to multiple descents and ascents, falling into a zone of humidity and refreezing as they were uplifted. New research, based on theory as well as field study, has shown this is not necessarily true.

The storm's updraft, with upwardly directed wind speeds as high as 110 miles per hour (180 km/h), blow the forming hailstones up the cloud. As the hailstone ascends it passes into areas of the cloud where the concentration of humidity and super cooled water droplets varies. The hailstone’s growth rate changes depending on the variation in humidity and super cooled water droplets that it encounters. The accretion rate of these water droplets is another factor in the hailstone’s growth. When the hailstone moves into an area with a high concentration of water droplets, it captures the latter and acquires a translucent layer. Should the hailstone move into an area where mostly water vapor is available, it acquires a layer of opaque white ice.

There are methods available to detect hail-producing thunderstorms using weather satellites and weather radar imagery. Hailstones generally fall at higher speeds as they grow in size, though complicating factors such as melting, friction with air, wind, and interaction with rain and other hailstones can slow their descent through Earth's atmosphere. Severe weather warnings are issued for hail when the stones reach a damaging size, as it can cause serious damage to human-made structures and, most commonly, farmers' crops.

The National Climate Extremes Committee, which is responsible for validating national records, formally accepted the measurements in 2003: seven inches in diameter (17.8 centimeters) and a circumference of 18.75 inches (47.6 centimeters).  The photo below is not the largest, but is quite big.

According to NOAA, large hailstones can fall at speeds faster than 100 miles (160 kilometers) per hour. They can sometimes contain foreign matter, such as pebbles, leaves, twigs, nuts, and insects.
Hail causes nearly one billion dollars (U.S.) in damage to property and crops annually. The costliest United States hailstorm: Denver, Colorado, July 11, 1990. Total damage was 625 million dollars (U.S.).

After the storm passed, there were still some clouds -- just enough to light up the sky at sunset.


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