Wind and Clouds Aloft – Background

Watching the sky can be peaceful or exciting, depending on the day or hour. Sometimes the sky is as still as a painting with snow-white clouds on a deep blue canvas. At other times ragged clouds race across the sky, propelled by high winds aloft. Sky-watching may take us on flights of fancy or lead us to ponder why the wind blows, or how clouds take such different shapes, or what wind and clouds can tell us about the weather.

Winds, or flowing air currents, are caused by warm air rising and cool air flowing in to take its place. Because the sun heats the Earth unequally, the atmosphere is heated unequally too, warming and rising in some parts, cooling and sinking in others. This creates areas of low pressure, where the air has expanded and spread out, and high pressure, where it has become compressed. As we can easily see when we pop a balloon, air always moves from high pressure to low. The same is true of winds. The direction in which the wind blows can tell us where high pressure and low pressure areas are located, which is important for weather forecasting. On a global scale, the spinning of the Earth causes predictable patterns of prevailing winds at different latitudes. Where we live, the prevailing winds are from the west, and so we can usually look to our west to see what the weather will bring.

Winds are named for the direction from which they are blowing. In the northern hemisphere, north winds usually bring colder weather, flowing to us from polar regions, and south winds bring warmer, moisture-laden air from the tropics. A change in the wind signals a change in the weather to come. Winds from the northwest, west, or southwest tend to bring good weather, but winds from the northeast, east, or southeast often mean bad weather is on the way, like the nor’easters that bring days of stormy weather to Atlantic coast states. Many factors affect the air currents at a local level. Snow and ice reflect heat away, water warms and cools more slowly than land, clouds block out the sun in some places and let it through in others. Mountains, valleys, and even buildings affect wind patterns.

High winds aloft cause clouds to scud across the sky while gentle breezes rustle the leaves on trees. In still air, smoke rises straight up from chimneys. Meteorologists use anemometers to measure wind speed, but we can make an estimate without instruments, using a wind-force scale devised in the nineteenth century by an admiral in the English navy. Ranging from zero for calm air to twelve for hurricane-force winds, the Beaufort Wind Force scale relies on visual clues like sails and waves to estimate wind speed. On land, it lets us judge wind speed from evidence like waving branches or rippling flags.

People have invented many devices that are moved by wind. Wind vanes tell us about wind direction, if we know how to read them. A wind vane swings on an axis and points into the wind, remaining stationary unless the wind changes direction. Toy pinwheels and anemometers, on the other hand, spin continuously in the wind, faster or slower depending on the wind’s speed. Wind turbines operate somewhat on the principle of a pinwheel. The blades are designed to catch and be pushed by the wind so that they turn when it blows. When the blades are attached to a shaft, the windmill can do work for us, converting wind energy into power. For many centuries, windmills lifted water from wells or operated the grinding stones in gristmills. Modern-day wind turbines turn wind energy into electricity, a renewable source of energy.

The Earth’s atmosphere extends a thousand miles out into space, but the troposphere, the part that’s within five or ten miles of the ground, is where winds blow, storms are born, and clouds form and drift. A cloud is a mass of tiny water droplets or ice crystals, floating in the sky. Clouds form when rising warm air meets cold air, and the moisture present in the air condenses onto tiny floating particles like dust or pollen. Clouds appear white or gray because water droplets scatter the light in all directions.

Clouds are classified both by their appearance and by their altitude – low, middle, and high clouds. Cloud names are constructed from five Latin words used alone or in combination. Cumulus means “heap.”  Cumulus clouds look like heaps of cotton in the sky, with flat bases and bulging tops, separated by spaces of blue sky. Often called fair weather clouds, they form on sunny days when the sunlight heats the Earth and warm air begins to rise in columns or “thermals.” As the warm air column meets colder air above, the moisture in it condenses and a puffy cloud grows vertically, expanding upward as warm air continues to rise.

Stratus clouds, from the Latin for “layer,” are low clouds that form a blanket-like layer, covering large areas with a solid cloud cover. Stratus clouds form when a mass of rising warm air meets an extensive layer of colder air, and the moisture in it condenses over a broad stretch of sky. Stratus clouds give us overcast days and sometimes rain or drizzle. Fog is simply a stratus cloud that forms on or near the ground. Stratocumulus, so named because they are both layered like stratus and heaped like cumulus, form a thick layer of fluffy gray rolls, low in the sky.

Middle clouds, found between one and four miles high, have the prefix alto, meaning “high,” before their names. Altostratus clouds form a gray layer over the sky, making the sun appear dim, as though behind frosted glass. Altocumulus are puffy mid-level clouds, lined up like sheep in rows. Either may precede a change in the weather.

Cirrus, the Latin word for “curl,” is used for high clouds with a wispy, hair-like appearance. Because they form above twenty thousand feet where the temperature is well below freezing, Cirrus clouds are made of ice crystals rather than water droplets. High winds at this altitude blow them into curving strands known as mare’s tails, which are actually streaks of falling ice crystals. Cirrus clouds are seen on sunny clear days, but because they often occur at the leading edge of a warm front, they may signal rain in a day or two.

Two other clouds fall in the cirrus family. Cirrostratus form a fine, veil-like layer over the sky. Because of the way the ice crystals bend the light, we may see sun dogs, small rainbows on either side of the sun, or a halo around the moon or sun when they are present. Cirrostratus often precede storms, as we are reminded in this saying: “Ring around the sun or moon, rain or snow is coming soon.” Cirrocumulus, high heap-like clouds, form a mottled pattern in the sky called a “mackerel sky,” which is striped like the skin of a mackerel fish. Following on the heels of cirrus clouds, they also portend bad weather, giving us adages like, “Mackerel sky, not twenty-four hours dry.” Contrails (short for “condensation trail”) from airplanes are human-made cirrus-type clouds, made of ice crystals or droplets from moisture in the jet exhaust.

Finally, nimbus means rain cloud to meteorologists. Nimbostratus clouds are the low, gray clouds that cover the sky when it is raining or snowing. Cumulonimbus clouds are thunderheads, towering cumulus clouds that grow to enormous heights, typically five miles high but sometimes higher, and often with anvil-shaped tops. With tremendous updrafts and high winds, these are the clouds that produce the most extreme weather – lightning, thunder, rain, hail, fierce winds, and sometimes tornados. Cumulonimbus is truly the giant of this cloud assemblage.

Wind and clouds, though they are only two pieces of a very large puzzle, can tell us a lot about the weather, if we learn to read them and watch their progression from day to day. Whether or not we can predict the weather, we can enjoy gazing at the sky, appreciating clouds and watching the weather unfold.

 

Suggested reading:

Ahrens, Donald, C. Meteorology Today: An Introduction to Weather, Climate and the Environment. 10th ed. Independence, KY: Cengage Learning, 2013.

Breen, Mark and Kathleen Friestad. The Kids’ Book of Weather Forecasting. Nashville: Williamson Books, 2008.

Day, John A. and Vincent J. Schaefer. Peterson First Guide to Clouds and Weather. Boston: Houghton Mifflin, 1991.

Lehr, Paul, R. Will Barnet, and Herbert S. Zim. Weather (Golden Guide). New York: St. Martin’s Press, 2001.

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