Weather: Forecasting by Clouds

Forecasting by Clouds

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.