Forecasting Weather

What Clouds Mean
Gorp.com
Stratus Clouds

You do not have to have an oncoming weather system to have clouds form in your area. Local conditions can cause thunderheads and other cloud types to occur. For example, in Florida, residents can expect brief but often intense afternoon thunderstorms almost daily between April and September. This is due to humid, unstable Caribbean, Gulf of Mexico, and moist subtropical Atlantic air moving into the heat rising off the land. The atmosphere can't hold all this energy, so it dumps it off in the form of a daily thundershower. Still, clouds are a good indicator of weather to come.

There are four categories of clouds: those that form in the upper reaches of the atmosphere between about 16,000 and 43,000 feet (high clouds); those that form between approximately 6,500 and 23,000 feet (middle clouds) (and yes, the two can overlap); and low clouds, which form below 6,500 feet or so. The fourth category of clouds, those that have vertical development such as cumulus and cumulonimbus, is a renegade group that operates under special rules.

You can't tell the exact height of a certain cloud by looking at it, but you can determine its general height by knowing what type it is, i.e., the altitude ranges just mentioned. So, know the clouds and know their altitude range. The best way to do this is to get with someone who knows them and have along guide with illustrations and descriptions. It takes a little practice but can be learned.

High Clouds

These include cirrus, cirrostratus, and cirrocumulus clouds, all of which are made up almost entirely of ice crystals due to the extreme altitudes where they are found. This is the height at which most trans-continental airliners fly.

Cirrus

Fair weather is likely ahead when you see these wispy, thin clouds that look likes mares' tails and are blown about by winds high overhead. They almost always come out of the west and drift across a bright blue sky to the east. When they begin to thicken substantially due to a pressure drop, the likelihood of precipitation is increased. Since these are high clouds and wind directions may be different at various altitudes, they do not always point in the direction of the wind where you are standing.

Cirrostratus

Unlike cirrus clouds, which portend fair weather, cirrostratus clouds can mean rain or snow within twelve to twenty-four hours. The thicker the clouds, the greater the chance of precipitation. And if some middle range clouds follow their cirrostratus friends, you can just about count on getting rained or snowed upon. Like sheets, these clouds may cover the entire sky and form haloes around the still visible sun or moon due to the ice crystals reflecting light.

Cirrocumulus

You have seen these white rows of clouds sailing across the sky in rows or scattered about singly. When they are overhead, you can go ahead and plan an outing for tomorrow because they mean that fair weather is coming.

Middle Clouds

Middle elevation clouds include altostratus and altocumulus, and are usually made up of droplets of water. However, if temperatures are cold enough they can be formed of ice crystals.

Altostratus

When you see these grayish (sometimes with a blue tinge) clouds covering the entire sky, with or without the sun or moon visible through them, you can expect storms to blow through soon; such storms are usually widespread.

Altocumulus

A thunderstorm in the afternoon may be preceded by these grayish, puffy clouds that form in masses in the morning, especially if that morning was humid. They may appear to come rolling along the sky in waves, with some parts of the clouds lighter than other parts.

Low Clouds

Low-range clouds are most often made of water droplets, but snow can be produced during periods of cold weather. There are three types: stratus, stratocumulus, and nimbostratus.

Stratus

Stratus clouds don't often portend hard rain, but are more likely to produce light rain or drizzle. They are a uniform gray in color, and frequently cover the entire sky. When fog lifts, these clouds may form, which explains why they look like fog.


Stratocumulus

Seldom producing rain, these low, grayish clouds appear lumpy and may not cover the entire sky. They can appear in patches, masses, or rows. As cumulus clouds disperse at sunset, stratocumulus may form in their place.

Nimbostratus

These are our traditional rain clouds, with their dark gray color and a look that just says"rain," which is usually continuous but not often extremely heavy. They cover the whole sky.

Clouds With Vertical Development

There are two major types in this category: cumulus and cumulonimbus. While one often means fair weather, the other means to stand by for a thunderstorm. Cumulus are low clouds that hang out below 4,000 feet, while cumulonimbus can run up to well over 60,000 feet in extreme cases, especially over the desert.

Cumulus

These well know cottonball clouds, with their flat base and puffy appearance, are clearly defined and therefore easy to identify. If they don't have too much vertical growth, fair weather is ahead, but the greater the vertical development, the greater the chance of that fair weather becoming showery with possible thunderstorms.

Cumulonimbus

Here is the classic thunderhead, with its flat base, towering center, and anvil-shaped top that is often diffused by wind. They can be destructive, given their penchant for heavy precipitation, thunder, lightning, and hail.

So we see here that although one type of cloud within an altitude range might mean one thing, another cloud within that range might mean just the opposite.


Published: 29 Apr 2002 | Last Updated: 15 Sep 2010
Details mentioned in this article were accurate at the time of publication

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