Weather: Forecasting by Clouds

Forecasting by Clouds

Weather Words

"It sits looking
over the harbor and city
on silent haunches
and then moves on."

—"Fog," Carl Sandburg


If you want an inexpensive radar for thunderous occasions, try an AM-band radio. Even before those towers develop into rip-roaring thunderstorms, static will be given off that can be picked up on the AM-band, especially in the low frequencies, between stations. It really works. After a while, you'll be able to relate the intensity of the static to the proximity of the storm. But if the dew point is in the 60s or above, and those cumulus clouds start building towers by late morning, I would pay attention to the sky throughout the day.

I'm constantly looking at the sky. Sure, the computer offers a lot of information and insight, but the first step in any forecast is observation. How does the sky appear? Is it deep blue or is it hazy? That immediately tells me about the humidity, because with increasing moisture, the sky will lose its ability to scatter blue light. As the humidity increases, the sky has that hazy appearance.

Are cirrus clouds drifting by? That by itself is okay, but if the cirrus clouds appear to be thickening into cirrostratus on the western horizon, I know changes are about to occur. And if the thicker cirrostratus covers the sky, and if that gives way to cirrocumulus, I would bet my last thermometer that something is up. The changes might indicate a warming trend or that some rain is a short 24 hours away. If the clouds continue to lower to those altostratus and altocumulus types, without knowing anything else, I would take out the umbrella.

Of course, sometimes the sky doesn't give solid clues, and we must wait to see more before we can confidently predict the weather's next moves. For example, there will be days when innocent-looking cumulus clouds appear during the middle or late morning. The clouds seem to be capable of doing absolutely no wrong. But then, without much notice, the cumulus clouds develop into towers. That is when we should begin to pay attention. Thunderstorms can rapidly form. These convective storms do not announce their arrival 24 or 36 hours in advance—they're not bringing your normal rainy day.

The following table gives you the lowdown on how you can forecast the weather based on cloud appearance.

Cloud Type Description Forecast
Cirrus Scattered, high thin clouds Mostly fair, but watch for cirrostratus or altostratus
Cirrostratus altostratus Halo around sun or moon, sometimes a rainbow effect Precipitation within 24 hours if clouds are increasing; if clouds are decreasing, moon becomes more discernible, skies clearing
Cumulus Scattered puffy clouds Fair
Cumulus congestus Cumulus increasing and covering sky, also Showers within two to four hours forming in towers
Cumulonimbus Tall, mountainous clouds with a flat anvil top Rain, thunderstorm imminent
Cirrocumulus and altocumulus High, puffy, cotton-ball-shaped clouds Precipitation within 12 to 24 hours if increasing and accompanied by cirrostratus and altostratus clouds
Jet contrails Trails of condensed vapor from high-flying jets Fair weather, but watch for increasing cirrus or cirrostratus
Mammatus Puffy rolls of dark clouds, frequently at the bottom of a cumulonimbus cloud Violent thunderstorms, even tornadoes, are imminent; watch for rotation
Stratocumulus Long, dark, rolling cloud; no precipitation with it Remaining overcast, or slow clearing
Stratus Low cloud upon ground, or fog No precipitation by itself; clearing likely if fog developed overnight; otherwise, remaining cloudy

Air-pressure trends and wind are also important. Later we will explore those variables. But if you know your cloud types, it's like knowing your chords if you want to play jazz. It's a good beginning.

book cover

Excerpted from The Complete Idiot's Guide to Weather © 2002 by Mel Goldstein, Ph.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.