Weather 101

Aristotle was one of the first people to systematically study the weather and try to understand what caused different kinds of weather. Although his understanding of natural science was limited—he thought there were only four elements: earth, wind, air, and fire (in fact, there are 118)—he did correctly conclude that the Sun could put air masses into motion (resulting in wind), and that clouds were condensed water vapor.

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In 1812, when the Emperor Napoleon of France invaded Russia, weather came to the aid of his enemies and played a significant role in his defeat. The Russians drew back before him, destroying food stocks in the process. He reached Moscow in September and remained in the Russian capital for five weeks before beginning the return to France. Winter set in and the French soldiers, without proper clothing, walked in snow up to their knees. Half the French army froze to death or died of weather-related illnesses.

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Cumulus clouds are among the most common kinds of clouds; they’re puffy with flat bottoms. They often give rise to cumulonimbus clouds, which are much larger and taller and are more likely to bring rain or snow.

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If the Sun is shining while there are a lot of water droplets in the sky, the droplets refract, reflect, and disperse the white light of the Sun into its various component colors. The result is a rainbow. In some rare instances, fog rather than rain can produce a rainbow; the water particles in the fog are so small that the rainbow appears white.

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Supercells start as ordinary thunderstorms, but because of winds blowing at different speeds at different levels of the atmosphere, the storm tilts, pushing out the colder air and allowing warm air to rush in. The result is winds moving up to 150 miles per hour. In this picture a supercell is beginning to form.

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If snowfall is accompanied by winds of more than 35 mph and visibility is down to a quarter mile or less, the snowstorm has officially become a blizzard. The Blizzard of 1978 that hit New England closed much of Boston for ten days.

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The Tacoma Narrows Bridge, spanning the Puget Sound south of Seattle, was completed in July 1940. Four months later, it collapsed as a result of an aeroelastic flutter, a sudden instability caused by 40-mile-an-hour wind gusts. Although no people were killed, the disaster led to radical changes in the way bridges are designed to avoid such accidents in the future.

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Lightning, an electrical discharge during a thunderstorm, is caused by the upper part of the cloud becoming positively charged while the middle and lower parts of the cloud are negative. It’s the reaction between these parts of the cloud that results in the lightning. Most lightning bolts are isolated, but during especially severe storms, multiple bolts can occur almost simultaneously.

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Floods can occur when the ground is so soaked that it is unable to absorb water or when the ground is so dry that water can’t penetrate it. Flooding also happens when rivers or lakes overrun their boundaries. Each year, floods cause millions of dollars in damage and many deaths across the globe.

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Droughts can damage the ecosystem and cause wildfires that burn millions of acres.

The peak of the Atlantic hurricane season is in September and October. During that time, large masses of humid air over warm waters form tropical storms. If one of these strengthens and starts to rotate, it can become a hurricane. Such storms in the Atlantic and northeast Pacific are called hurricanes; similar storms that form in the southern Pacific are called typhoons.

An undersea earthquake is one of many events that can create a tsunami—a wave that can overwhelm everything in its path. A tsunami’s size depends on the depth of the water where the event occurs, the violence of the event that caused it, and the distance the wave has to travel to reach land. Large tsunamis that strike populated areas can be deadly. In 2004, an earthquake in the Indian Ocean created a tsunami that killed more than 280,000 people.

The development of radar during World War II was a watershed for weather forecasting since it showed the direction and velocity of moving air masses. In this radar image of Hurricane Katrina as it struck the Gulf Coast in 2005, the center of the storm is passing over the greater New Orleans area.

Hurricane Katrina struck the Gulf Coast in August 2005. By the time it was over, 80 percent of New Orleans was flooded; levees and floodwalls had been breached; and more than 700 people in New Orleans were killed. Many of the houses in poorer districts were entirely destroyed by wind and water.