Hail and More

Today

• Hail

• How to stop hail?

Hail

• Precipitation in the form of balls or irregular lumps of ice, only produced by convective clouds, usually cumulonimbus clouds.

• Growth by accretion.

• Embryo is the origin of the hailstone.

• Various paths in a thunderstorm.

Hail

• Falls from thunderstorms characterized by:– Strong updrafts– Great vertical development– Abundant supply of supercooled water

droplets

Hail• Hail fall is a rather rare event but it can be

catastrophic– Annual hail damage (US) ~ $700 Million– 5 May 1995 in Ft. Worth, TX ~ $2 Billion

• Hail can damage aircraft, homes, automobiles, crops, and people.

Hail Damage

© 1990 Trimedia Publishing Co. -- From: Eagleman , Severe and Unusual Weather

Hail Formation• Hail storms are not usually observed in the

winter– Need convective clouds which normally

develop in the warmer months– Hail requires strong updrafts to allow large

hailstones to grow. Such strong updrafts are usually found in the warmer months.

Hailstone Formation

• Recipe:– Cumulonimbus Cloud– High liquid water content– Strong and sustained updraft– Some type of hailstone embryo

• The embryo is the “seed” for the hailstone

• Hail grows by the collision-coalescence process

Hailstone Growth

• The embryo falls at a different speed than the supercooled cloud droplets.

• Some droplets collide with and stick to the embryo.

• Depending on the temperature we see two types of growth– Wet Growth– Dry Growth

• The layered structure in hailstones reflect the two different types of growth.

Fancy being hit by this?

Wet Growth

• When temperatures are “warm”, the supercooled water will not freeze on contact.

• The liquid will spread across the tumbling hailstone and slowly freeze.

• Since the freezing process is slow, any air bubbles in the ice can escape.

• The result is a layer of clear ice.

Dry Growth

• When temperatures are “cold”, the supercooled water will freeze on contact.

• Since the freezing process is fast, any air bubbles in the ice cannot escape.

• The result is a layer of bubbly or cloudy ice.• All acquired ice is frozen -- there is no

liquid involved in this process.

Hail

• Hail Growth– Graupel or frozen raindrops grow by the

collision-coalescence method.– Accretion– Held up in the cloud by the strong updraft– When hailstones are too big, they fall to the

ground, often continuing to grow as they fall.

Terminal Velocities• What updrafts are required to hold a

hailstone up?

3 cm 25 m s-1 56 mi hr-1

8 cm 55 m s-1 125 mi hr-1

10 cm 83 m s-1 185 mi hr-1

HailDiameter

Updraft SpeedTerminal Velocity

Terminal Velocities

Record Hailstone• 3 September 1970

• Coffeyville, KS

• Weight: 700g (1.5 pounds)

• Diameter: 14cm (5.5 inches)

• Obviously we don’t see updrafts that can hold this monster up!

• The hailstones must also have significant growth on the way down.

The Coffeyville Hailstone

© 1990 Trimedia Publishing Co. -- From: Eagleman , Severe and Unusual Weather

Broken Record

7 inch (17.5cm) hailstone: 1.33 lb

Aurora, NE June 22, 2003

Probably bigger: 10” divot!!!

Summary

• Lots of precipitation

• Rain

• Snow

• Hail