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Written in the Sky: How Clouds Predict Weather

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My favorite movie growing up was The Wizard of Oz. I would watch it over and over again, awed by its magic. But what I found most fantastical about the movie wasn’t the Emerald City or the horse of a different color. What baffled me was: how the heck did Uncle Henry know the twister was coming?


It’s a foreign practice to those of us who have spent all our lives in a metropolitan area, but using clouds to predict the weather is common in rural areas and among farmers, whose livelihoods depend on what the skies are doing. Even though we use satellites to predict the weather now, people have been using the low-tech method of reading clouds to anticipate rain since at least the Babylonian era.


If you want to know whether you’ll need an umbrella without turning on the news, these weather-predicting tips just might help.


A Cloud Primer
Clouds are used to determine whether patterns because they change based on the water content in the air and the winds around them. Composed of large collections of water droplets and/or ice crystals that are small enough to float through the air, they are formed when warm air containing water vapor cools and the water vapor condenses into tiny droplets that come together. Clouds are white because sunlight shines through them. Because the light is scattered through clouds at the same frequency, it doesn’t usually break into component colors to form a rainbow.


If clouds get thick enough that they obscure sunlight, they appear gray and dark. Shadows from other clouds also contribute to this darkening. Clouds move with the wind, so you can tell which way the wind is blowing and how quickly by watching the clouds. Thunderstorm clouds typically move about 30–40 mph.


The Many Types of Clouds
There are three levels and four types of clouds. Cirrus clouds are the highest, above 18,000 feet. Below them are alto clouds, which hover between 6,500 and 18,000 feet. Lower down are stratus clouds, which stay below 6,500 feet. Cumulus clouds—the white, puffy clouds that look like bunches of cotton balls—are the fourth type of cloud and are classified by their vertical structure rather than their location.


These categories break down further, and the differences between them are used to determine weather patterns. If you want a hint at the weather, these are the clouds to recognize.


Cirrus
The high cirrus clouds are made entirely of ice crystals and appear as thin wisps that are spread out by high winds. They are usually white and predict fair weather if still, but if they are moving quickly across the sky you can expect a change in the weather within twenty-four hours. The direction of drift will also tell you from which direction the weather system is coming.




Cirrostratus
Stratus clouds are so named because they form a layer over the entire sky (from stratum, which is Latin for “horizontal layer”). Cirrostratus clouds are spread like a sheet across the entire sky and are thin enough that you can see the sun or moon through them. If you see this type of cloud in the sky, expect a storm within twelve to twenty-four hours.


Cirrocumulus
Like all cumulus clouds, these are small, white, and puffy; they look like cotton balls that are stuck together. Cirrocumulus clouds are usually seen in winter and indicate fair but cold weather. If they appear in tropical regions, however, they can indicate an advancing hurricane.


Altostratus
Altostratus clouds are blue-gray and consist of a combination of water droplets and ice crystals. They usually cover the entire sky, and signal an approaching storm of continuous rain or snow.


Altocumulus
These gray, puffy clouds consist only of water droplets. The temperature and humidity of the skies determine whether their clumps mean anything or not. If the weather is hot and sticky, expect a thunderstorm within twelve hours.


Stratus
These “layer” clouds cover the whole of the sky with a uniform gray. They hang low, like a fog that doesn’t touch the ground, and are usually accompanied by a light mist or drizzle, but nothing more.


Stratocumulus
Low, puffy, and gray, these clouds may appear clumped in rows with blue sky visible in between the rows. They do not predict any serious weather by themselves, but can eventually turn into nimbostratus clouds.


Nimbostratus
Nimbus is the Latin word for rain cloud, so you can guess what these guys bring. They appear as a dark gray layer and are accompanied by continuously falling light rain or snow.


Cumulus
Cumulus clouds are commonly known as fair weather clouds. As mentioned before, they are white and puffy, and float across the sky like fake snow. They have a flat base with rounded towers at the top that sometimes look like heads of cauliflower. Though they indicate nothing but pleasant weather themselves, they may grow upward into cumulonimbus, thunderstorm clouds.




Cumulonimbus
When cumulus clouds develop an anvil-like shape because their tops have been flattened by high winds, they indicate a coming thunderstorm with heavy rain or snow, hail, thunder and lightning, and possibly tornadoes. You can tell which direction the storm is moving by the direction the anvil is pointing. Sometimes, cumulonimbus clouds form low-hanging bulges called Mammatus clouds that indicate very severe weather is coming.


What Uncle Henry probably saw in the Kansas sky were Green clouds, cumulonimbus clouds that appear green when they reflect light off the ground vegetation. If you’re in the Great Plains Region of the United States, and you see green clouds, take cover because there’s a twister a-comin’!


Be Your Own Weatherperson
The skies have so much to tell us, if we only look. Expensive satellites and weatherpersons can’t tell us much more than what humans have read in the clouds for centuries. Just turn your head skyward and you won’t be caught off guard by a seemingly wayward rainstorm. It’s all in the clouds.

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  • Old Wives’ Tales about weather
  • Holy Twister! The Truth About Tornados
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