Thunderstorms and Tornadoes

WHEN SKIES DARKEN OR THUNDERSTORMS ARE FORECASTED, LOOK and LISTEN FOR......

Increasing wind

Flashes of lightning

Sound of thunder

Static on your AM radio

Thunderstorms are a common occurrence during changing seasons, primarily from winter to spring and again from fall to winter. All thunderstorms are dangerous. Strong winds, hail and tornadoes are also hazards associated with some thunderstorms.

The National Weather Service considers a thunderstorm severe, if it produces hail at least 3/4 inch in diameter, wind greater than 58 mph, or tornadoes.

(U.S. Department of Commerce)

LIFE CYCLE OF A THUNDERSTORM:

DEVELOPING STORM:

• Towering cumulus cloud indicates rising air
• Usually little if any rain
• Occasional lightning

MATURE STAGE OF A STORM:

• Most likely time for hail, heavy rain, frequent lightning, strong winds
• Possible tornadoes
• Storm occasionally has a black or green appearance
• Lasts an average of 10 to 20 minutes but may last much longer in some storms

DISSIPATING STAGE OF A STORM:

• Rainfall decreases in intensity
• Some thunderstorms produce a burst of strong winds
• Lightning remains a danger

Every thunderstorm needs moisture to form clouds and rain. Unstable air with relatively warm air that can rise rapidly is needed for the development of thunderstorms. Lift is also needed from fronts, sea breezes and mountains to help form thunderstorms.

LIGHTNING:
A cloud-to-ground lightning strike begins as an invisible channel of electricity charged air moving from the cloud toward the ground. When one channel nears an object on the ground, a powerful surge of electricity from the ground moves upward to the cloud and produces the visible lightning strike.

• Lightning results from the buildup and discharge of electrical energy between positively and negatively charged areas.
• The average flash could light a 100-watt light bulb for more than 3 months.
• Most lightning occurs within the cloud or between the cloud and ground.
• Your chances of being struck by lightning are estimated to be 1 in 600,000 but could be reduced by following safety rules:
• If outside: move indoors. Once inside avoid doors, windows, and metal objects. Avoid using electrical appliances. Use the telephone only in an emergency.
• If driving: Stay in your automobile. An enclosed automobile offers reasonably good protection from lightning.
• Outdoors: stay away from isolated trees. If your hair stands on end or your skin tingles, lightning may be about to strike. CROUCH down quickly and make a low target. DO NOT LIE DOWN FLAT.
• Most lightning deaths and injuries occur when people are caught outdoors.
• (boating, swimming, golfing, bike riding, standing under a tree, riding a lawnmower, talking on the telephone, loading a truck, playing soccer, fishing in a boat, mountain climbing)
• Most lightning casualties occur in the summer months and during the afternoon and early evening.
• The air near a lightning strike is heated to 50,000 degrees Fahrenheit -- hotter than the surface of the sun! The rapid heating and cooling of air near the lightning channel causes a shock wave that results in thunder.

LIGHTNING MYTHS AND FACTS:

MYTH:If it is not raining, then there is no danger from lightning
FACT:Lightning often strikes outside of heavy rain and may occur as far as 10 miles away from any rainfall.

MYTH:The rubber soles of shoes or rubber tires on a car will protect you from being struck by lightning.
FACT:Rubber-soled shoes and rubber tires provide NO protection from lightning. However, the steel frame of a hard-topped vehicle provides increased protection if you are not touching metal. Although you may be injured if lightning strikes your car, you are much safer inside a vehicle than outside.

MYTH:People struck by lightning carry an electrical charge and should not be touched.
FACT:Lightning-strike victims carry no electrical charge and should be attended to immediately.

MYTH:"Heat-lightning" occurs after very hot summer days and poses no threat.
FACT:What is referred to as "heat-lightning" is actually lightning from a thunderstorm too far away for thunder to be heard. However, the storm may be moving in your direction!

DOWNBURSTS:

• A small area of rapidly descending air beneath the thunderstorm
• Can cause damaging winds in excess of 100 mph
• The strong winds usually approach from one direction and may be known as "straight-line" winds.
• In extreme cases, straight-line winds can reach speeds equal to a strong tornado, causing significant damage to some buildings
• Strong winds may or may not be accompanied by rain

LARGE HAIL:

• The strong rising currents of air within a storm, called updrafts, carry water droplets to a height where freezing occurs.
• Ice particles grow in size, finally becoming too heavy to be supported by the updraft and fall to the ground.
• Large hailstones fall at speeds faster than 100 mph.

(U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Weather Service)

TORNADOES:
A funnel cloud is defined as a violently rotating column of air that is not in contact with the ground.

A tornado is defined as a violently rotating column of air in contact with the ground.

WHAT CAUSES TORNADOES?
Thunderstorms develop in warm, moist air in advance of eastward-moving cold fronts. These thunderstorms often produce large hail, strong winds, and tornadoes.

Tornadoes in the winter and early spring are often associated with strong, frontal systems that form in the Central States and move east. Occasionally, large outbreaks of tornadoes occur with this type of weather pattern. Several states may be affected by numerous severe thunderstorms and tornadoes.

During the spring in the Central Plains, thunderstorms frequently develop along a "dryline," which separates very warm, moist air to the east from hot, dry air to the west. Tornado producing thunderstorms may form, as the dryline moves east during the afternoon hours.

Along the front range of the Rocky Mountains, in the Texas panhandle, and in the southern High Plains, thunderstorms frequently form as air near the ground flows "upslope" toward higher terrain. If other favorable conditions exist, these thunderstorms can produce tornadoes.

Tornadoes occasionally accompany tropical storms and hurricanes that move over land. Tornadoes are most common to the right and ahead of the path of the storm center as it comes on-shore.

HOW DO TORNADOES FORM?
Before thunderstorms develop, a change in wind direction and an increase in wind speed with increasing height creates an invisible, horizontal spinning effect in the lower atmosphere.

Rising air within the thunderstorm updraft tilts the rotating air from horizontal to vertical.

An area of rotation, 2-6 miles wide, now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation.

FREQUENCY OF TORNADOES:

In the southern states, peak tornado occurrences is in March through May, while peak months in the northern states are during the summer.

Note, in some states, a secondary tornado maximum occurs in the fall, such is the case with Missouri.

Tornadoes are most likely to occur between 3 and 9 p.m. but have been known to occur at all hours of the day and night.

The average tornado moves from southwest to northeast, but tornadoes have been known to move in any direction. The average forward speed is 30 mph but may vary from nearly stationary to 70 mph.

The total number of tornadoes is probably higher than indicated in the western states. Sparse population reduces the number reported.

Some tornadoes appear as a visible funnel extending only partially to the ground. Look for signs of debris below the visible funnel.

Some tornadoes are clearly visible while others are obscured by rain or nearby low-hanging clouds.

DID YOU KNOW?
On average, the United States gets 100,000 thunderstorms each year. Approximately 1,000 tornadoes develop from these storms.

Tornadoes are the most violent winds on earth. These twisters can produce wind speeds as high as 300 miles per hour, travel longer than 100 miles and reach up to 20,000 feet above ground.

Each year about 50-70 people are killed because of tornadoes. The worst series of tornadoes occurred on March 18, 1925, when eight tornadoes in Missouri, Illinois, Indiana, Kentucky, Tennessee and Alabama caused 689 deaths.

ESTIMATING WIND SPEEDS:
Estimated MPH (Miles Per Hour) Observation

10: Leaves and Twigs Move
15: Small Branches Move
20: Small Trees Sway
25: Large Branches Sway
35: Twigs Break Off Trees
40: Whole Trees in Motion
50: Branches Break Off Trees
60: Branches Break Off Trees
70: Whole Trees Go Down
80: Hurricane Force Extreme Damages

The above information is currently being used by trained SKYWARN Weather Spotters throughout the region to estimate wind speeds, during severe thunderstorms. See Amateur Radio Emergency Service or National Weather Service for more information and links. Annually, the National Weather Service provides Basic and Advanced Weather Spotter Training Seminars. Contact St. Charles County Division of Emergency Management, National Weather Service or your local Amateur Radio Emergency Service or local Amateur Radio Clubs for more information.