Whirling Winds (Part One)
Wind is moving air. The air may move slowly, or it may move fast. Gentle breezes and storm winds both get their start in the same way. All winds are caused by differences in air pressure.
As a general rule, warm air rises and cool air sinks. When air is heated, it expands and becomes lighter as the molecules of air move farther apart. This makes the air pressure lower. When air is cooled, the opposite happens. Cool air molecules move closer together, making it more dense and thus heavier, so the air pressure is higher.
As warm air rises, cooler air rushes in to fill the space left behind. Thus, air tends to move from cooler places to warmer places, and winds blow from high pressure areas to low pressure areas. Surface air is always on the move between these high and low pressure areas. This movement of air called wind can occur on a small local scale or on a large scale all over the Earth.
As the Earth is heated by the sun, the warmed air rises, thus reducing the air pressure on the Earth’s surface. More warm air rises in some places than in others due to differences in the landscape. Since this heating of the Earth’s surface is uneven, the air pressure is constantly changing. The Earth’s spin and the amount of water in the air also contribute to complex and variable wind patterns.
Regional winds are caused by local conditions, and not by large scale air movements. A sea breeze is a local wind caused by differences in heat and air pressure between the land and ocean. During the daytime, the land absorbs heat faster than the water. As the air above the land warms up, the air pressure over the land becomes lower, and a cool breeze blows in from the ocean. At night, the land gives off heat more quickly than the water. The air pressure over the ocean is now lower, and a breeze blows from the land to the water.
Local winds are commonly caused by air movements around mountains. The tops and sides of mountains become heated and cooled at different times. Air from cooler areas blows into the warmer areas. During the day, cool air usually blows from the valleys up the mountain sides. At night, cool air blows from the mountains down into the valleys. (I remember our house painter complaining that it’s always windy in New River!) Santa Anas are strong winds caused by cold air masses being pushed down a mountainside. Chinooks are hot, dry winds caused when large masses of air blow down a mountainside and become heated by the pressure of their own weight.
At certain latitudes of the Earth’s surface, winds blow steadily from the same direction. These winds are called prevailing winds. They are caused by the effect of the Earth’s rotation on large-scale air movements from cooler to warmer areas. Trade winds are the prevailing easterly winds from north of the equator to 30 degrees latitude. Westerlies are the prevailing winds between 30 to 60 degrees latitude. Jet streams are belts of high-speed winds in the upper atmosphere.
A wind gets its name from the direction it comes from. A wind from the north is a north wind. A wind from the southeast is a southeast wind. In the United States, a south wind often brings warm weather. A north wind may bring a cold wave. East winds often mean cloudy skies are likely, and west winds mean fair weather. A wind vane indicates the direction in which the wind is blowing.
In 1805, a British Admiral named Sir Francis Beaufort made a scale for estimating wind speed:
Calm = less than 1 mph; smoke rises straight up.
Light air = 1-3 mph; smoke drifts gently.
Light breeze = 4-7 mph; wind felt on face and leaves rustle a little.
Gentle breeze = 8-12 mph; flags blow straight out, leaves move, but no branches sway.
Moderate breeze = 13-18 mph; small branches sway, paper and dust is blown about.
Fresh breeze = 19-24 mph; small trees sway and there are whitecaps on bodies of water.
Strong breeze = 25-31 mph; large branches move and umbrellas are hard to hold.
Moderate gale = 32-38 mph; whole trees move and walking is difficult.
Fresh gale = 39-46 mph; tree branches break.
Strong gale = 47-54 mph; loose shingles may blow off.
Whole gale = 55-63 mph; trees are uprooted.
Storm = 64-75 mph; widespread damage.
Hurricane = above 75 mph; violent destruction.
Make an Anemometer
Wind speed is measured by an anemometer. To make one, attach (with tacks) four small cups to the ends of two short flat sticks (such as paint stirrers or old wooden rulers). Cross the sticks and fasten them at right angles by driving a long nail through the center. Use a third stick or wooden dowel as a support. In one end of the support, drill a hole slightly larger than the nail used in the crossbars. Wax the hole (using an old candle or crayon) and set the nail in it. When the wind blows, the anemometer should rotate. By counting the number of rotations per minute, you can figure out the wind speed. By holding the anemometer outside the window of a moving car, you can check the speed of rotation against the “wind speed” shown on the speedometer.
Thunderclouds form when warm air rising upward cools. As it cools, water vapor condenses into water droplets, which grows into a cloud. The cool upper air in the cloud then causes downdrafts. These updrafts and downdrafts in thunderstorms create rotating columns of air called mesocyclones. If a mesocyclone starts spinning fast enough, it may produce a tornado.
Ninety percent of the world's tornadoes occur in the United States. Most of these occur in “Tornado Alley,” which extends from Texas up to Nebraska and Iowa. They usually occur in late spring and summer. Tornadoes travel at speeds of 35 mph to as fast as 70 mph across land.
There are six ratings for tornadoes, based on their wind speed and the damage they do:
F0 tornadoes do light damage to chimneys, TV antennas, and roof shingles. Small tree branches can be broken. Their winds are 40-72 miles per hour.
F1 tornadoes have winds from 73-112 miles per hour. They can uproot trees, overturn cars, and push trailers around. F1 tornadoes are the most common, followed by F2 tornadoes.
F2 tornadoes have winds from 113-157 miles per hour. They can blow roofs off homes, demolish sheds and small outbuildings, and cause wooden buildings to collapse.
F3 tornadoes have winds from 158-206 miles per hour. They can flatten all the trees in a forest and collapse metal buildings. They blow off roofs and tumble exterior walls made of concrete.
F4 tornadoes have wind speeds of 207-260 miles per hour. They will leave few, if any walls standing. They can pluck trees from their roots and break their trunks in half. They can pick up large building materials and hurl them with such force that they penetrate concrete.
F5 tornadoes have winds over 261 miles per hour and can be as wide as mile across. They make the land look like a bulldozer ran over it. Only one out of every hundred tornadoes are rated F5. The fastest tornado wind speed was clocked at 318 miles per hour.
Tornadoes that occur over water are called waterspouts. Waterspouts are not as big or fast as terrestrial tornadoes. Waterspouts can pick up fish and frogs and drop them on land!
Hurricanes have been called Nature’s most destructive force. Hurricanes are hundreds of miles across, with winds that whirl around in a great circle at speeds of 75-150 miles per hour.
Hurricanes form over oceans near the equator, where the air is moist. A thunderstorm may cause great quantities of moisture to condense. The condensing moisture gives up large amounts of heat, which warms the air. The warmed air rises rapidly, and as the Earth rotates, the column of warm air begins to spin, forming a hurricane. The whirling mass of air can last for several days.
In the Northern Hemisphere, a hurricane spins counterclockwise; in the Southern Hemisphere it spins clockwise. At the center of the hurricane is a slowly spinning column of air a few miles in diameter. This is the eye of the hurricane, where there is little wind and the sun shines through.
When a hurricane moves off the ocean and travels inland, friction from the rough land surface finally slows the storm down. While hurricane winds can do great damage to ships, buildings, and trees, the worst damage comes from the high waves and heavy rains which cause flooding.
Air Mass – a large volume of air with uniform temperature and humidity.
Air Pressure – a measure of the total weight of air above a certain area; barometric pressure.
Atmosphere – the envelope of air around the earth.
Cold Front – the boundary where a large mass of cold air is pushing beneath warmer air.
Cyclone – another term for a violent, whirling windstorm such as a tornado or hurricane.
Downdraft – a strong current of air that heads from within a storm cloud toward the ground.
Dust Devil – a spinning column of warm wind often seen in the desert on hot days.
Front – the boundary line between two large air masses that push each other back and forth.
Funnel Cloud – a rotating column of air dropping below a cloud but not reaching the ground.
Gust Front – a gust of cool wind moving out from the leading edge of a storm.
Mesocyclone – rotating air associated with a thunderstorm, from which tornadoes can form.
Meteorology – the scientific study of the atmosphere and weather.
Rotate – to move around in a circular motion.
Temperature Inversion – cool air trapped beneath warmer air.
Tornado – a violently rotating column of air that extends from a thunderhead cloud to the ground.
Twister – another name for a tornado.
Typhoon – a tropical hurricane specifically originating in the western Pacific Ocean.
Vortex – a column of swirling wind or water, such as the twisting winds of a tornado.
Warm Front – the boundary where moving warm air is overriding cooler air.
Waterspout – a weak tornado that forms from a cumulus cloud over a body of water.
Weather – the condition of the atmosphere at a particular place and time.
Whirlwind – a general term for a rotating column of air, such as a tornado or dust devil.
Wind – the movement of air.
A study of the weather includes how air moves and anything it might be carrying. The following books and websites explain more about winds and weather:
Do Tornadoes Really Twist? Questions and Answers About Tornadoes and Hurricanes, by Melvin and Gilda Berger, Scholastic, 2000.
Hurricanes: Earth’s Mightiest Storms, by Patricia Lauber, Scholastic, 1996.
National Audubon Society First Field Guide: Weather, by Jonathan D. W. Kahl, Scholastic, 1998.
Storms and Hurricanes, by Kathy Gemmell, Usborne, 1995.
The Weather Atlas, by Keith Lye, Running Press, 2001.
The Weather Book, by Jack Williams, Random House, 1992.
The Weather Channel: Tornadoes! By Sally Rose, Scholastic, 1999.
Tornado, by Catherine Chambers, Heinemann, 2002.
Tornadoes, by Seymour Simon, Scholastic, 1999.
Weather, by Seymour Simon, Scholastic, 1993.
Weather (Eyewitness Explorers), by John Farndon, Dorling Kindersley, 1992.
Weather and the Bible: 100 Questions and Answers, by Donald B. DeYoung, Baker Books, 1992.
Wild Weather: Hurricanes! By Lorraine Jean Hopping, Scholastic, 1995.
www.nws.noaa.gov (Weather Education from the National Weather Service.)
www.noaa.gov/wx.html (Everything you want to know about weather from the National Oceanic and Atmospheric Administration.)
www.wildwildweather.com (Dan’s Wild Wild Weather Page, an interactive weather site for kids.)
www.wxdude.com (Weather Basics and Meteorology Topics from A to Z.)
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