What is the difference between a frost and a hard freeze?

The National Weather Service issues a hard freeze warning when it predicts that regional temperatures will fall below 27°F (-2.8°C) degrees for four hours or more.

Such a freeze is of particular concern to gardeners and farmers because temperatures this low will destroy crops. The good news is that a hard freeze usually spells the end for mosquitoes and other pesky insects for the rest of the year.

Frosts, on the other hand, do not require such low temperatures. In fact, air temperatures can be several degrees above freezing, while surface temperatures are at or below freezing, causing frost to form on car windows and other surfaces.

What are glaze and rime?

Glaze, as one might infer, is the result of freezing rain or drizzle falling onto surfaces and forming a sheet of ice. Rime, on the other hand, is formed by freezing fog or mist during windy conditions. There are two types of rime: hard rime and soft rime. Soft rime has a milky appearance and forms sugar-like crystals shaped into scales, feathers, or needles on the windward side of thin objects such wires, poles, and tree branches. Hard rime is less milky, comb-like in appearance; it is also more dense and less fragile than soft rime. Hard rime can form in windy weather when the temperature is between 18 to 28°F (-2 to -8°C) and humidity is 90 percent or more, while soft rime can occur in similar conditions, but when the temperature is below 18°F (-2°C).

What is an ice storm?

Ice storms account for some of the most dangerous winter conditions one can experience. Ice storms occur when freezing rain accumulates on the ground, building up layers of glaze or rime that coat everything, from roads to buildings to telephone lines and vegetation. Naturally, this makes driving extremely hazardous, and many people have lost their lives while traveling in ice storms. Airports will often cancel flights as ice forming on wings can prevent airplane wing flaps from moving, even when workers repeatedly de-ice them. Ice storms can down power lines and even cause older trees to collapse under the sheer weight of frozen water. For example, a tree 50 feet tall with a branch circumference averaging 20 feet can be weighed down by 10,000 pounds (about 4,500 kilograms) of ice as a result of an ice storm.

How is hail formed?

Hail is precipitation consisting of balls of ice. Hailstones usually are made of concentric, or onion-like, layers of ice alternating with partially melted and refrozen snow, structured around a tiny central core. It is formed in cumu

Rime ice coats tree branches near Asheville, North Carolina. (photo by Grant W. Goodge, courtesy NOAA)

Rime ice coats tree branches near Asheville, North Carolina. (photo by Grant W. Goodge, courtesy NOAA)

Texas is well known for its large hailstones, like this one that fell during a June 8,1995, storm. (NOAA Photo Library, NOAA Central Library; OAR/ERL/National Severe Storms Laboratory)

Texas is well known for its large hailstones, like this one that fell during a June 8,1995, storm. (NOAA Photo Library, NOAA Central Library; OAR/ERL/National Severe Storms Laboratory)

lonimbus or thunderclouds when freezing water and ice cling to small particles in the air, such as dust. The winds in the clouds blow the particles through zones of different temperatures, causing them to accumulate additional layers of ice and melting snow and to increase in size.

Are hailstones always round?

Usually, hailstones are round or lumpy-round little ice balls. Sometimes, however, they can be oblong or have protruding spikes.

How large can hailstones become?

The average hailstone is about one-quarter inch (0.64 centimeter) in diameter. However, hailstones weighing up to 7.5 pounds (3.4 kilograms) are reported to have fallen in Hyderabad state in India in 1939, although scientists think these huge hailstones may be several stones that partly melted and stuck together. On April 14, 1986, hailstones weighing 2.5 pounds (one kilogram) were reported to have fallen in the Gopalgang district of Bangladesh; there was also a report of a hailstone weighing 4.5 pounds (2.04 kilograms) in Germany.

The current claim to fame for largest hailstone found in the United States was one that was measured to have a circumference of 18.75 inches (47.625 centimeters) that fell in Aurora, Nebraska, in June 2003. Before that, the record was held by a 17.5-inch (44.45-centimeter) stone found in Coffeyville, Kansas, in September 1970.

When it comes to weight, however, the Coffeyville stone was 1.67 pounds (0.76 kilograms) while the Aurora hailstone was a fluffier 1.3 pounds (0.59 kilograms).

 
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