FAQ

Severe Thunderstorms and Hailstorms

A thunderstorm is formed by atmospheric imbalance and turbulence caused by: (1) the rapid rising of unstable warm air into the atmosphere, (2) a sufficient amount of moisture to form clouds and produce rain, and (3) the collision of separate weather fronts (warm and cold) creating an upward lift of air currents. These conditions may result in severe thunderstorms, heavy rains (which may cause flash flooding), and strong winds reaching or exceeding 58 mph, tornadoes, or hail of at least 0.75 inches in diameter. 

Thunderstorms are common in Iowa and can occur singly, in clusters, or in lines. Most thunderstorms produce only thunder, lightning, and rain. Severe storms however, can produce tornadoes, high straight-line winds above 58 mph, microburst, lightning, hailstorms, and flooding. High straight-line winds, which can often exceed 60 mph, are common occurrences and are often mistaken for tornadoes.  Lightning occurs with all thunderstorms even if the buildup of electricity isn’t strong enough to send a bolt to the ground.

Between 1950 and 2010 there have been 455 severe thunderstorm, high wind, lightning, and hailstorm events that have affected Linn County.  These severe weather events were responsible for 2 deaths, 18 injuries, and $26 million in damages.

Common Hailstone Size Comparisons

 

Common Coin Sizes  

Dime

0.70 in

Penny

0.75 in

Nickel

0.83 in

Quarter

0.955 in

Dollar

1.04

Half Dollar

1.02 in

 

Other Objects  

Object

  

  Diameter  

Inches

Meters

Pea

0.24

5-9

Small Marble  

0.51

10-15

Walnut/Ping Pong Ball

1.5

16-20

Golfball

1.7

21-30

Lime/Hen Egg

2.0

31-40

Tennis Ball

2.5

41-50

Baseball

2.8

51-60

Apple/Tea Cup

3.0

61-75

Grapefruit

4.3

76-90

Softball

 4.5

91-100

Computer CD

5.0

>100

 

Typical Hail

Diameter (mm)  

Typical Damage Impacts

5

No damage

5-15

Slight general damage to plants, crops

10-20

Significant damage to fruit, crops, vegetation

20-30

Severe damage to fruit and crops, damage to glass and plastic structures, paint and wood scored

25-40

Widespread glass damage, vehicle bodywork damage

30-50

Wholesale destruction of glass, damage to tiled roofs, significant risk of injuries

40-60

Bodywork of grounded aircraft dented, brick walls pitted

50-75

Severe roof damage, risk of serious injuries

60-90

Severe damage to aircraft bodywork

75-100

Extensive structural damage. Risk of severe or even fatal injuries to persons caught in the open

>100

Extensive structural damage. Risk of severe or even fatal injuries to persons caught in the open

© 2009 Linn County EMA

  

Severe Thunderstorm Watch

Conditions are favorable for the development of severe thunderstorms in and close to the watch area within the next 12 hours.

Severe Thunderstorm Warning

This is issued when either a severe thunderstorm is indicated by radar or a spotter reports a thunderstorm producing hail 3/4 inch or larger in diameter and/or winds equal or exceed 58 miles an hour; therefore, people in the affected area should seek safe shelter immediately.

Severe Thunderstorm Safety Tips

Get inside a home, building, or hard top automobile (not a convertible). Although you may be injured if lightning strikes your car, you are much safer inside a vehicle than outside.

Secure outdoor objects that could blow away or cause damage.

Avoid showering or bathing. Plumbing and bathroom fixtures can conduct electricity.

Use a corded telephone only for emergencies. Cordless and cellular telephones are safe to use.

Unplug appliances and other electrical items such as computers and turn off air conditioners. Power surges from lightning can cause serious damage.

Use your battery-operated NOAA Weather Radio for updates from local officials.

Avoid the following:

Natural lightning rods such as a tall, isolated tree in an open area

Hilltops, open fields, the beach, or a boat on the water

Isolated sheds or other small structures in open areas

Anything metal—tractors, farm equipment, motorcycles, golf carts, golf clubs, and bicycles

Source: NOAA

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Linn County Emergency Management Agency, 6301 Kirkwood Blvd. S.W., Cedar Rapids, Iowa 52404