Hail: A Unit Study

During a thunderstorm on September 3, 1970, hail formed, creating the heaviest hailstone of the 20th century. The hailstone fell on Coffeyville, Kansas. It weighed 1.67 pounds and had a circumference of 17.5 inches. That record has now been broken, although the Coffeyville hailstone still ranks among the world’s largest.

What Causes Hail?

Below an anvil-shaped cumulonimbus cloud, the water remains in a liquid state. But a cloud at high altitude (as high as 50,000 feet) is actually made of supercooled water droplets below freezing.

When one of these supercooled water droplets collides with another object, such as a speck of dust or ocean salt, it instantly freezes into a pellet of ice. A hailstone is formed as this process is repeated.

Hailstones travel up and down in the cumulonimbus cloud on convective air currents as the warm air rises. The current of upward-moving air, or updraft, in a thunderstorm can be quite strong, sometimes reaching speeds of over 110 miles per hour. Hailstones within the cloud are hefted to the top of the storm by these powerful updrafts. They descend again on downdrafts, caused by cool air sinking toward the ground.

The hailstones travel through the cloud until the updraft can no longer support them. Stronger updrafts give the hailstones more time to form. This means that larger hail is produced by storms with more convective energy.

Gravity eventually overcomes the force of the updraft and the hail falls to earth. Large hailstones may fall at speeds of over 100 miles per hour.

Predicting Hail Storms

Hail is a destructive force, devastating vehicles, buildings, and crops. Large hail is also a threat to humans and animals.

However, hail formation quite predictable. Meteorologists can detect falling hail early enough to give up to a 15-minute warning.

Hail Safety

To prevent injury and minimize property damage due to hail, experts recommend the following steps:

Consider impact-resistant roofing materials in hail-prone areas.

Place property that could be damaged by hail, such as vehicles, under shelter prior to the storm’s arrival if time allows. If you do not have a garage for your car, a tarp or similar vehicle cover may help reduce the damage.

Provide shelter to pets before the storm arrives.

If a severe thunderstorm is in your vicinity, go indoors and stay there until the storm has passed.

Stay away from windows, skylights, and glass doors when hail is falling. If you can, shut the drapes or blinds to further guard against injury from breaking glass.

If you are caught outdoors in a hailstorm, cover your head. Do not take shelter under a tree unless strictly necessary, as trees may be struck by lightning or lose large branches during a severe storm. Never take shelter in a culvert under any circumstances.

If you are in your car during a hailstorm, slow down or pull over in a safe place to avoid an accident due to poor visibility and slippery road surfaces, but stay in your vehicle. Lie face-down on the floor or wrap yourself in a blanket or coat to protect yourself from breaking glass.

Be sure to assess the condition of your roof after a severe hailstorm.

Hail Facts

To be considered hail, ice particles must be at least five millimeters in diameter. Smaller particles are classified as ice pellets or sleet.

Any storm that is capable of producing hail over one inch in diameter (quarter-sized) is classified as a severe thunderstorm.

“Plowable” hail accumulations, those with depths over 15 centimeters, are particularly dangerous. Besides the obvious damage risk from that much falling hail, drifting hail can close highways and produce significant flooding as it melts.
Colorado is considered the hail capital of the United States. The cool air at the high elevations in the lee of the Rocky Mountains keeps the hail from melting before hitting the ground.

Besides the Great Plains of the United States and Canada, other places around the world prone to hail include Kenya, Australia, China, India, Italy, and Russia.

Suggestions

Read the following from the Book of Knowledge:

Hail is quite different from snow. In the case of snow the ice forms in the process of condensation, but with hail it forms afterward. Hailstones are therefore simply frozen raindrops, or else flakes of snow surrounded by ice. Hail is caused by the drops being blown upward instead of falling downward and thus being carried into the colder moist upper air. True hail occurs only when the updraft of air is very powerful.

Hailstones are frequently carried upward several times after they have begun to fall; and as they receive a fresh coat of ice every time this happens, they may become very large. If a large hailstone is carefully cut in two the separate coats can be seen.

“Clouds, Rain and Snow”

Provide a narration (written or oral) explaining everything you know about hail.
Provide a compare and contrast chart showing the difference between hail and snow.
Point out the type of cloud that is likely to produce hail.

Further Investigation

World’s Heaviest Hailstone
Model of the former record holder.

Hail
Information for students at National Geographic.

Hail Basics
From NOAA.

Experimental Explanation of Supercooling
Why water does not freeze in the clouds.

Basic Ten
The basic ten clouds including cumulonimbus.

Activities

Amazing Hail Storm
Interesting video at ScienceKids.

Activity: Chart the Weather
Lots of resources for understanding and tracking hail and other types of weather.

Books

The New Weather Book by Michael Oard
The causes of weather, world climates, how to read a weather map, the atmosphere, warm and cold fronts, thunderstorms, tornadoes, hurricanes, and climates of the past and future. The Weather Book Study Guide