Monday, July 26, 2010

Hail Storms - A perfect lesson in weather

What makes giant sized hailstones? Do they hurt more when hitting you on the head? (YES!) but we didn't need a book to tell us that.

A perfect hail storm came our way today and in the process we had all the neighborhood kids out learning about how they are formed, as well as learning to measure circumference and diameter.

Compared to many places our hail stones are very small, but for our area it's the biggest we have ever seen.

Here is what we found out :


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Large hailstones up to 5 centimetres (2 in) in diameter with concentric rings
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Hail is a form of solid precipitation which consists of balls or irregular lumps of ice, that are individually called hail stones. Hail stones on Earth consist mostly of water ice and measure between 5 millimetres (0.20 in) and 150 millimetres (5.9 in) in diameter, with the larger stones coming from severe thunderstorms. The METAR reporting code for hail 5 millimetres (0.20 in) or greater in diameter is GR, while smaller hailstones and graupel are coded GS. Hail is possible with most thunderstorms as it is produced by cumulonimbi (thunderclouds),[1] usually at the leading edge of a severe storm system. Hail is possible within 2 nautical miles (3.7 km) of its parent thunderstorm. Hail formation requires environments of strong, upward motion of air with the parent thunderstorm (similar to tornadoes) and lowered heights of the freezing level. Hail is most frequently formed in the interior of continents within the mid-latitudes of Earth, with hail generally confined to higher elevations within the tropics. .
Unlike ice pellets, hail stones 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 millimetre (0.039 in) thick, which are deposited upon the hail stone as it cycles through the cloud multiple times, suspended aloft by air with strong upward motion until its weight overcomes the updraft and falls to the ground. There are methods available to detect hail-producing thunderstorms using weather satellites and radar imagery. Hail stones generally fall at faster rates as they grow in size, though complicating factors such as melting, friction with air, wind, and interaction with rain and other hail stones can slow down 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 man-made structures, and most commonly, farmers' crops. In the United States, the National Weather Service issues severe thunderstorm warnings for hail 1" or greater in diameter. This threshold, effective January 2010, marked an increase over the previous threshold of 3/4" hail. The change was made for two main reasons: a) public complacency and, b) recent research suggesting that damage does not occur until a hailstone reaches 1" in diameter.


Everyone knows that it doesn't take a large hailstone to cause much damage. But out of curiosity, how big can hailstones get?
While most hailstones are the size of peas (about 0.25 in [0.63 cm] in diameter), they sometimes grow larger than softballs. Large hailstones have been responsible for destroying crops, breaking windows, and denting cars, and have caused the deaths of many people and animals.
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.). Hail falls when it becomes heavy enough to overcome the strength of the updraft and is pulled by gravity towards the earth. How it falls is dependent on what is going on inside the thunderstorm. Hailstones bump into other raindrops and other hailstones inside the thunderstorm, and this bumping slows down their fall.
Drag and friction also slow their fall, so it is a complicated question! If the winds are strong enough, they can even blow hail so that it falls at an angle. This would explain why the screens on one side of a house can be shredded by hail and the rest are unharmed! Hail is a form of precipitation that occurs when updrafts in thunderstorms carry raindrops upward into extremely cold areas of the atmosphere where they freeze into ice.
The National Climate Extremes Committee, which is responsible for validating national records, formally accepted the measurements for the largest hailstone ever to fall in the U.S.: seven inches in diameter (17.8 centimeters) and a circumference of 18.75 inches (47.6 centimeters). The old record for the largest hailstone had a diameter of 5.7 (14.5 centimeters) inches, a circumference of 17.5 inches (44.5 centimeters), and was found in Coffeyville, Kansas, on September 3, 1970. The previous longstanding record was believed to be a hailstone which fell at Potter, Nebraska on July 6, 1928. It measured around 7 inches (17.8 centimeters) in diameter and weighed about 1.5 pounds (680 grams).
The largest hailstones ever reported, weighing up to 7.5 pounds (3.4 grams), fell in the state of Hyderabad, India, in 1939. However, scientists believe that these huge hailstones may have been several stones that partially melted and stuck together. On April 14, 1986, hailstones weighing 2.5 pounds (1 kilogram) each were reported to have fallen in the Gopalgang district of Bangladesh.
Clearly hailstones can become quite large given the right meteorological conditions. All of which begs the question, with the current environmental changes occurring...can we expect hailstones to get heavier and larger as the years roll on?

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