Thunderstorms can stop truckers in their tracks, producing damaging hail, powerful winds, blinding rainfall and tornadoes. About 100,000 thunderstorms hit the U.S. each year, and around 10% of them are severe. Here’s how they form.
Birth of a thunderstorm
Simply put, a thunderstorm is a rain shower accompanied by thunder. Since thunder comes from lightning, all thunderstorms have lightning. Three things are necessary for a thunderstorm to develop: moisture, rising unstable air (air that keeps rising when given a nudge) and a lifting mechanism to provide the “nudge.”
Life cycle stages of a thunderstorm. (Image: NOAA)
The sun heats the surface of the Earth, which warms the air above it. If this warm surface air is forced to rise, it will continue to rise as long as it weighs less and stays warmer than the air around it. Hills and mountains, as well as areas where warm and cold or wet and dry air masses bump together, can cause this rising motion.
Up, up and away
As the air rises, it transfers heat from the surface to the upper atmosphere through convection. The water vapor it contains cools, releasing the heat. This leads to condensation and cloud formation. The cloud eventually grows upward into much colder temperatures, which are often below freezing.
As the cloud continues its vertical buildup into the very cold air, ice particles grow and collide with each other. This results in big regions of electric charge, creating a bolt of lightning. The lightning heats and expands a narrow channel of air, creating sound waves that we hear as thunder.
Types of thunderstorms
• Multicell: A common, garden-variety thunderstorm in which new updrafts (rising currents of air) form along the leading edge of rain-cooled air. Individual cells usually last 30 to 60 minutes and may produce hail, strong winds, brief tornadoes and/or flooding.
Features of a supercell thunderstorm. (Image: NOAA)
• Supercell: A long-lived (greater than one hour), highly organized storm feeding off an updraft that is tilted and rotating. This rotating updraft, which can be up to 50,000 feet tall, can be present as much as 20 to 60 minutes before a tornado forms. Scientists call this rotation a mesocyclone when it’s detected by Doppler radar. The tornado is a very small extension of this larger rotation. Most large and violent tornadoes come from supercells.
• Squall line: A group of storms arranged in a line, often accompanied by “squalls” of high wind and heavy rain. Squall lines tend to pass quickly and are less prone to produce tornadoes than are supercells. They can be hundreds of miles long but are typically only 10 or 20 miles wide.
Learn about other types of thunderstorms here.
Severe limits
The main difference between a common thunderstorm and a severe thunderstorm is wind shear — a strong increase in wind speed and/or a change in wind direction from the ground up. This creates a twisting motion in the updraft of the storm.
Gradually, the updraft overpowers the downdraft and sustains itself for a long period of time. This can lead to large hail formation in the cloud top. It can also tilt the storm. The rotating air is ingested into the updraft, spawning a tornado. As the downdraft takes over, air comes crashing down in the form of severe winds, and the large hail hits the ground.
The National Weather Service defines a thunderstorm as severe if it produces any of the following, as indicated by radar or eyewitness reports:
• Hail 1 inch or greater in diameter.
• Winds of at least 58 mph.
• A tornado.
Tornadoes can move fast, with forward speeds up to 60 mph. They can also change direction. When behind the wheel, truckers should never try to outrun a tornado. Look for more tornado safety tips here and thunderstorms safety tips here.
Click here for more FreightWaves articles by Nick Austin.
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