How do We Stay Safe from Tornadoes?

Posted on May 8, 2011 by Weather Guys Editor

The U.S. has a wonderful tornado warning systems. The national system of severe weather watches and warnings has saved untold numbers of lives. In addition to the sirens, the National Oceanic and Atmospheric Administration’s (NOAA) Weather Radio network puts nearly everyone within range of government weather broadcasts. These radios can sound an alarm whenever severe weather warnings are issued for your area. Even with this excellent warning system, there are fatalities as we saw in these recent storms. Many deaths result directly from the force of the tornadoes.

In the event of a tornado, go into a tornado shelter, or the basement, or into a small interior room on the lowest floor of a building, such as a bathroom or closet. Protect yourself from flying debris and stay away from windows. If you are away from a sturdy home, you must seek adequate shelter. Avoid auditoriums, gymnasiums, and eating areas; their large, high roofs can blow off and the walls can collapse. If you are in a mobile home or car, leave it and go to a strong building. Many people are killed when cars and mobile homes are overturned in high winds. If there are no shelters nearby, get into the nearest ditch or depression and protect yourself from flying debris.

Videos have wrongly popularized the notion that it is safe to hide under a highway overpass as the tornado passes overhead. Don’t do it. It is far safer to take shelter in a sturdy building instead. A highway overpass creates a “wind tunnel” effect underneath it and can increase the amount of damage from a tornado.

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What Causes Tornadoes?

Posted on May 1, 2011 by Weather Guys Editor

We don’t know if a particular storm will produce a tornado so the truth is we really don’t know what causes a tornado. We do know the necessary conditions needed for tornado formation.

A tornado is a powerful column of winds that rotate around a center of low pressure. The winds inside a tornado spiral inward and upward, often exceeding speeds of 300 mph. For a thunderstorm to produce a tornado requires warm humid air near the surface with cold dry air above. These conditions make the atmosphere very unstable, in the sense that once air near the ground is forced upward, it moves upward quickly and forms a storm. Severe thunderstorm conditions also include a layer of hot dry air between the warm humid air near the ground and the cool dry air aloft. This hot layer acts as a lid that allows the sun to further heat the warm humid air — making the atmosphere even more unstable.

To form a tornado, the host thunderstorm must also rotate. From below, a rotating cloud base looks like someone is stirring the storm from above. This happens in a storm when wind at the ground is moving in a different direction and speed than the air above. The change in wind speed and direction with height is known as wind shear. This wind shear develops the rotation in the thunderstorm needed for tornado formation.

At this time of year, warm moist winds from the Gulf of Mexico move northward while above the jet stream warm dry winds from the Great Plains move eastward, providing the necessary conditions for severe thunderstorms.

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When is the Severe Weather Season?

Posted on April 23, 2011 by Weather Guys Editor

As the threat of winter snows recedes across the country, it is replaced by the threat of severe weather (i.e. thunderstorms with hail, damaging winds and tornadoes).  The severe weather season, though broadly spanning March – August across the United States, is actually quite regional.  It begins in March in the southern states, moves to the southern Plains during April and May, and then further north toward the Great Lakes states during the summer.  One of the basic underlying reasons for this northward migration of the severe weather threat during the spring and summer is the fact that the jet stream follows a similar seasonal cycle.

The jet stream is a ribbon of high wind speeds located near the top of the troposphere (~ 6 miles above the surface of the Earth).  The jet stream position is strongly tied to the southern edge of the dome of cold air that is centered on the North Pole.  During the depths of winter, that cold dome expands considerably, extending nearly to the Gulf of Mexico.  As the winter ends and spring approaches, the hemisphere begins to warm up and the cold dome shrinks dramatically.  Its southern edge moves to central Canada by early summer.  The jet stream is associated with vigorous upward and downward vertical motions.  The upward vertical motions are instrumental in producing thunderstorms.  Thus, when the jet stream migrates northward as the weather warms in spring/summer, so does the greatest concentration of severe weather outbreaks.

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How Does Hail Form?

Posted on April 10, 2011 by Weather Guys Editor

Hail is precipitation in the form of large balls or lumps of ice. There was a storm that produced hail early last week. Hailstones begin as small ice particles that grow primarily by accretion; to grow large, they require abundant water droplets. As the hailstone moves up and down through a storm, it collides with water droplets, growing larger with each collision.  Hailstones can be as large as oranges and grapefruits.

When a hailstone is cut in half, you can see rings of ice. Some rings are milky white; others are clear. This ringed structure suggests that a hailstone can grow by two different processes, wet growth and dry growth.

In wet growth, the hailstone is in a region of the storm where the air temperature is below freezing, but not super cold. When the hailstone collides with a drop of water, the water does not freeze on the ice immediately. Instead, the liquid water spreads over the hailstones and slowly freezes. Because the water freezes slowly, air bubbles can escape, resulting in a layer of clear ice.

Dry growth of hailstones occurs when the air temperature is well below freezing. In these conditions a water droplet freezes immediately as it collides with the hailstone. This quick freezing leads to air bubbles “frozen” in place, leaving cloudy ice. Counting the layers of clear and milky white ice gives an indication of how many times the hailstone traveled to the top of the storm.

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What is Freezing Rain?

Posted on March 27, 2011 by Weather Guys Editor

When particles falling from clouds reach the surface as precipitation, they do so primarily as rain, snow, freezing rain, or sleet. Why are there different types of precipitation? It has to do with the ability of water to change phase. Where we live, precipitation usually begins falling out of a cloud as ice particles. What determines the precipitation type is the temperature between the cloud bottom and the ground.

If the temperature remains above freezing from the cloud bottom to the ground, precipitation particles melt into liquid droplets and are called rain. If the temperature underneath a cloud stays below freezing all the way to the ground, the ice crystals never melt and snow falls. Ice storms occur when precipitation particles melt and then fall through a layer of cold air near the ground. The two precipitation types most common during ice storms are freezing rain and sleet. Freezing rain forms when a thin layer of cold air near the surface causes melted precipitation to become super cold. It then freezes on contact with exposed objects on the ground whose temperature is below freezing. Sleet consists of translucent balls of ice that are frozen raindrops. It occurs when the layer of subfreezing air at the surface is deep enough for the raindrop to freeze.

When sleet hits the surface, it bounces and does not coat objects with a sheet of ice, as freezing rain does. Freezing rain covers everything in a sheet of ice, creating shimmering landscapes. However, even a little freezing rain causes treacherous road conditions and tree and power lines to snap.

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