What clouds are all about is water. They are visible collections of tiny water droplets and/or ice crystals (frozen water). From cloud, rain and snow fall.
Water is critical for life here as we know it. We drink it, shower or bathe in it, swim in it, boat on it, use it on our plants. In winter, when snow piles high, we build snowmen and sled or ski on it. We depend on clouds to replenish the water we use.
Even when there is no cloud, there is water in the air in invisible (vapor) form. Warm air is capable of holding more water vapor than cold air.
When air rises as in storms, ahead of fronts or when forced over mountains, it cools and becomes less able to hold water in vapor form. The vapor condenses into droplets, forming clouds. As it continues to rise, the drops grow in number and size. When the drops get sufficiently large and heavy, they begin to fall through the rising air. They may collide with rising droplets and increase in size. When they reach the ground, we call it rain.
In winter, when the air is cold, ice crystals form in the cloud. They tend to grow faster than the water droplets in the cloud. When the air is very cold, they may fall as tiny snowflakes. When the air is not as cold, the snowflakes are wetter and tend to stick together as they fall, forming the bigger snowflakes that flutter as they fall.
All children know, in very cold weather, the snow is often more difficult to make a snowball or snowman than when the air is milder and the snow wetter.
THE DIFFERENT CLOUDS
Clouds take many different shapes and forms. The cloud names we use today come from an English pharmacist and meteorologist, Luke Howard in 1803.
HIGH CLOUDS (18000 feet and above)
CIRRUS – white, ice crystal clouds, wispy in appearance. They may take on the appearance of a mare’s tail. This is because as ice crystals fall from the cloud they often encounter changing winds and so "tail" off.
CIRROSTRATUS – a thin veil of ice crystals usually covering most or all of the sky. A "halo" may appear about the sun or moon due to refraction (or bending) or light rays in the ice crystals. Often a sign of approaching storms.
CIRROCUMULUS – a milky-white cloud with a dappled or wave like appearance. During the day, refraction of the sun’s rays may cause one or more "mock-suns" to appear on either side of the sun.
MIDDLE CLOUDS (7000-18000 feet)
ALTOSTRATUS – gray or blue-gray clouds composed of water droplets or a mix of water droplets and ice crystals. Sun may be dimly visible as through ground glass. Often a sign of approaching rain or snow.
ALTOCUMULUS - a patchy cloud with a wave-like or dappled appearance. Distinguished from Cirrocumulus by the color which is gray or blue gray (Cirrocumulus is milky-white). Sometimes referred to as a "Mackerel sky".
ALTOSTRATUS LENTICULARIS – a special type of altostratus clouds usually found downwind from mountains. It is a lens shaped cloud. May look like a flying saucer and may have in the past been responsible for UFO reports. The cloud forms from a wave in the wind flow induced by the mountains. Often these waves are stationary and the clouds form at one end and dissipate on the downwind side, and so the clouds do not move although the winds are typically blowing strongly through them. Sometimes when moisture varies in the vertical, several of these clouds may be stacked vertically and appear as a stack of pancakes.
LOW CLOUDS (below 7000 feet)
STRATUS – low, gray, flat water droplet cloud (called fog if it touches the ground) with no clear structure. Drizzle may occur from these clouds.
STRATOCUMULUS – a low layered cloud with some observed structure and varying color. Often stratocumulus is the result of both mixing and low-level instability. A sign of blustery and relatively cool conditions with possible showers or rain or snow.
CUMULUS – small heaped clouds with flat bottoms and rounded tops. Most commonly seen in an otherwise blue sky. Usually a sign of fair weather and thus often referred to as Fair Weather Cumulus.
NIMBOSTRATUS - thick, layered cloud, usually gray or dark-gray. Likely to produce rain or snow that covers a large area and lasts a relatively long time (hours).
CUMULONIMBUS - very tall cumulus cloud, with a large cauliflower shape often with an anvil top which produces showers and thunderstorms. Large cumulonimbus clouds may extend to 60,000 feet or higher and may produce very heavy rains, hail, strong winds and even tornadoes.
THE SEQUENCE OF CLOUDS DURING A TYPICAL STORM PASSAGE
If you study clouds carefully and observe how they change with the weather, you will see some very consistent patterns.
Before radio, television or newspaper forecasts were readily available, people whose livelihoods depended on the weather (like sailors or farmers) often used the clouds as a indicator of what might happen.
Low pressure weather systems especially have a prescribed sequence of clouds. After a spell of clear weather, usually the first signs of a change coming are high wispy cirrus clouds. If you check you barometer, it is probably high and about ready to fall.
These high clouds often thicken and may fill the sky. When they do they are called cirrostratus. Cirrostratus is a white milky ice-crystal cloud. A halo may appear around the sun or moon with cirrostratus clouds. Usually precipitation is 12 to 24 hours away when the sky becomes completely covered by cirrostratus.
The clouds often then continue to lower and thicken. Altostratus or middle layer clouds are often next. They are gray or bluish gray in color. The sun often is still dimly visible through altostratus clouds but gradually fades. Precipitation is often just hours away. Winds freshen from the east or southeast.
When rain or snow begins, the clouds are nimbostratus (nimbus means rain). Precipitation is usually steady and light to moderate. Sometimes, especially in spring, heavy downpours and thunder may signify some imbedded cumulonimbus in the nimbostratus clouds.
Clouds continue to lower during the precipitation. When the air becomes saturated near the ground, fog may form. This is usually a sign you are near a warm front. If the precipitation ends, winds shift to the south or southwest and the skies brighten, the warm front is likely to have passed. The air in the warm sector of storms is usually hazy, unseasonably warm and humid.
Darkening skies and freshening winds are usually the signs that the next change associated with a storm passage, the cold front is approaching. Cumulonimbus clouds often precede or accompany the cold front and bring showers and thunderstorms.
If you tap your barometer, you should see pressures now near its lowest point. A gusty wind shift to the northwest usually signifies the passage of the cold front. Typically, the pressure begins to rise rapidly while temperatures and humidities drop and skies partially clear.
Cumulus clouds often follow generally signifying fair weather. If the air is cold and winds are strong and gusty, the cumulus clouds may fill the sky during the day. These are called stratocumulus. Sprinkles or flurries can fall from these clouds if the air is sufficiently unstable and moist.
Eventually winds diminish, the pressure rise slow and blue skies dominate, signifying continental polar or, in the west, maritime polar air high pressure is approaching.
After a day or so of fine weather, cirrus clouds may appear signifying the approach of the next weather system.
CLOUDS ON SATELLITE IMAGERY
Satellites provide a continuous monitoring of the earth’s clouds and weather systems. Geostationary (GOES) satellite provide the most timely and extensive imagery. Polar orbiter satellites provide strip coverage as they rotate around the earth. Coverage over any given point is provided just several times per day from these satellites.
One set of sensors onboard the weather satellites measures the amount of radiation emitted which is proportional to the temperature. Warm ground emits more radiation than high cold clouds. We can with the right choice of a color table see clouds over the earth’s surface. The typical color table uses dark colors for warm and white for cold temperatures. When you see very bright clouds on an infrared image, they are likely high clouds. They may be high cirrus clouds associated with fine weather beneath or high ice crystal blowoff from huge thunderstorms which are producing violent weather at the surface. Dark areas may be clear (we are seeing the ground) or may be areas of low (thus relatively warm) clouds or fog.These clouds will only be visible if their temperatures differ from the surrounding ground.
Another radiation band sensed is the visible band. This is close to what you would see with the naked eye if you were a space traveler. When we see white here, it is usually reflected solar radiation from the reflecting surface back to the satellite. The more water droplets, the thicker the cloud, the more light is reflected and the brighter the appearance.
Comparison of visible and IR images in the two figures on the left shows how the IR emphasizes the higher colder cloud tops associated with cumulonimbus clouds off the coast and the low clouds over New England which have temperatures very similar to the land are barely visible (stealth clouds). The visible image on the right on the other hand shows the lower (wet and thus reflective) clouds over New England equally well as the thunderstorms.
Extensive deep snowcover, ice, or sandy soil without vegetation (e.g. deserts) also may appear bright.
*Visible imagery is available only during the daylight hours.