Severe Weather Awareness Week, day 2 (Severe Thunderstorms)

Severe thunderstorms are a way of life here in Texas, and yet far too often we underestimate the potential they have for damaging property and causing significant injuries or death. How are thunderstorms caused, and what’s the difference between a regular thunderstorm and a severe thunderstorm? The National Severe Storms Laboratory has an excellent website if you’d like to learn more. Today’s information comes courtesy of them.

For brevitys sake, we won’t differentiate between gust fronts, single cells, squall lines, supercell, or mesoscale here. We’re just talking about run of the mill severe thunderstorms. We’ll cover convection a bit in our Tornado article.

How does a thunderstorm form?
Three basic ingredients are required for a thunderstorm to form: moisture, rising unstable air (air that keeps rising when given a nudge), and a lifting mechanism to provide the “nudge.”

The sun heats the surface of the earth, which warms the air above it. If this warm surface air is forced to rise—hills or mountains, or areas where warm/cold or wet/dry air bump together can cause rising motion—it will continue to rise as long as it weighs less and stays warmer than the air around it.

As the air rises, it transfers heat from the surface of the earth to the upper levels of the atmosphere (the process of convection). The water vapor it contains begins to cool, releases the heat, condenses and forms a cloud. The cloud eventually grows upward into areas where the temperature is below freezing.

As a storm rises into freezing air, different types of ice particles can be created from freezing liquid drops. The ice particles can grow by condensing vapor (like frost) and by collecting smaller liquid drops that haven’t frozen yet (a state called “supercooled”). When two ice particles collide, they usually bounce off each other, but one particle can rip off a little bit of ice from the other one and grab some electric charge. Lots of these collisions build up big regions of electric charges to cause a bolt of lightning, which creates the sound waves we hear as thunder.

The National Weather Service issues a Severe Thunderstorm Warning when the following criteria is met;

Hail of 1” or higher in diameter
Winds exceeding 58MPH

Lightning, though deadly, is NOT a factor in determining if a thunderstorm is severe or not. Another misnomer is that all severe thunderstorms produce tornadoes. While not all severe thunderstorms produce tornadoes, they can produce serious straight line wind damage as severe as a tornado, which can actually cover a much wider area than a tornado usually does. Take shelter during a Severe Thunderstorm as you would during a tornado.

Downbursts are especially damaging. Downbursts are a type of straight line wind which is a small area of rapidly descending rain-cooled air and rain beneath a thunderstorm. A downburst can cause damage equivalent to a strong tornado! Since most thunderstorms produce some straight-line winds as a result of outflow generated by the thunderstorm downdraft, anyone living in thunderstorm-prone areas of the world is at risk for experiencing this hazard.  People living in mobile homes are especially at risk for injury and death. Even anchored mobile homes can be seriously damaged when winds gust over 80 mph.

There’s a saying “When Thunder Roars, Go Indoors” because if you hear thunder, you are likely within striking distance of the storm. NO PLACE outside is safe when thunderstorms are in the area!!

If you hear thunder, lightning is close enough to strike you. When you hear thunder, immediately move to safe shelter: a substantial building with electricity or plumbing or an enclosed, metal-topped vehicle with windows up.  Stay in safe shelter at least 30 minutes after you hear the last sound of thunder.

Indoor Lightning Safety

  • Stay off corded phones, computers and other electrical equipment that put you in direct contact with electricity.
  • Avoid plumbing, including sinks, baths and faucets.
  • Stay away from windows and doors, and stay off porches. Do not lie on concrete floors, and do not lean against concrete walls.

Last Resort Outdoor Risk Reduction Tips

If you are caught outside with no safe shelter anywhere nearby the following actions may reduce your risk:

  • Immediately get off elevated areas such as hills, mountain ridges or peaks
  • Never lie flat on the ground
  • Never shelter under an isolated tree
  • Never use a cliff or rocky overhang for shelter
  • Immediately get out and away from ponds, lakes and other bodies of water
  • Stay away from objects that conduct electricity (barbed wire fences, power lines, windmills, etc.)

Hail is another damaging factor in thunderstorms and is caused by updrafts in thunderstorms carrying raindrops upwards into the extremely cold portions of the atmosphere, tens of thousands of feet up. Here’s some more good information from the NSSL;

How does hail form?
Hailstones grow by colliding with supercooled water drops. Supercooled water will freeze on contact with ice crystals, frozen raindrops, dust or some other nuclei. Thunderstorms that have a strong updraft keep lifting the hailstones up to the top of the cloud where they encounter more supercooled water and continue to grow. The hail falls when the thunderstorm’s updraft can no longer support the weight of the ice or the updraft weakens. The stronger the updraft the larger the hailstone can grow.

Hailstones can have layers like an onion if they travel up and down in an updraft, or they can have few or no layers if they are “balanced” in an updraft. One can tell how many times a hailstone traveled to the top of the storm by counting the layers. Hailstones can begin to melt and then re-freeze together – forming large and very irregularly shaped hail.

How does hail fall to the ground?
Hail falls when it becomes heavy enough to overcome the strength of the updraft and is pulled by gravity towards the earth. How it falls is dependent on what is going on inside the thunderstorm. Hailstones bump into other raindrops and other hailstones inside the thunderstorm, and this bumping slows down their fall. Drag and friction also slow their fall, so it is a complicated question! If the winds are strong enough, they can even blow hail so that it falls at an angle. This would explain why the screens on one side of a house can be shredded by hail and the rest are unharmed!

How fast does hail fall?
We really only have estimates about the speed hail falls. One estimate is that a 1cm hailstone falls at 9 m/s, and an 8cm stone, weighing .7kg falls at 48 m/s (171 km/h). However, the hailstone is not likely to reach terminal velocity due to friction, collisions with other hailstones or raindrops, wind, the viscosity of the wind, and melting. Also, the formula to calculate terminal velocity is based on the assumption that you are dealing with a perfect sphere. Hail is generally not a perfect sphere!

If a severe storm is producing large hail stones, seek a sturdy shelter and stay away from windows that can
easily be smashed. If you are in your vehicle before the hail storm starts, get out of it and go to a sturdy shelter. Glass windows in vehicles can easily be smashed by the hail stones. If you can’t get out of your vehicle, then come to a stop and cover your head with your arms and hands.

Finally, if you’ve lived in our area long enough, you know it only takes a thimble of rain to flood 4th Street. We’re going to touch on flooding later this week.

Wednesday (Tornadoes)
Thursday (Flooding)
Friday (Warm Weather Safety tips, resources for further learning)

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