A could is a visible aggregate of minute water droplets or ice particles in the atmosphere above the Earth's surface. Clouds are classified according to their height above and appearance (texture) from the ground.
Clouds form when air is cooled to its dewpoint—or the temperature at which, if the air is cooled, it reaches saturation with water. Air can reach saturation in a number of ways. The most common way is through lifting.
As a bubble or parcel of air rises it moves into an area of lower pressure (pressure decreases with height). As this occurs the parcel expands. This requires energy, or work, which takes heat away from the parcel. So as air rises it cools. This is called an adiabatic process.
The rate at which the parcel cools with increasing elevation is called the "lapse rate". The lapse rate of unsaturated air (air with relative humidity <100%) is 5.4 °F per 1000 feet (9.8 °C per kilometer). This is called the dry lapse rate. This means for each 1000 feet increase in elevation, the air temperature will decrease 5.4°F.
Since cold air can hold less water vapor than warm air, some of the vapor will condense onto tiny clay and salt particles called condensation nuclei. The reverse is also true. As a parcel of air sinks it encounters increasing pressure so it is squeezed inward.
This adds heat to the parcel so it warms as it sinks. Warm air can hold more water vapor than cold air, so clouds tend to evaporate as air sinks.
Types of Clouds
There are four basic cloud categories observed in our atmosphere:
|High-level clouds which form above 20,000 feet (6,000 meters) and are usually composed of ice crystals. High-level clouds are typically thin and white in appearance, but can create an array of colors when the sun is low on the horizon. Cirrus generally occur in fair weather and point in the direction of air movement at their elevation.|
|Nimbus comes from the Latin word meaning "rain". These clouds typically form between 7,000 and 15,000 feet (2,100 to 4,600 meters) and bring steady precipitation. As the clouds thicken and precipitation begins to fall, the bases of the clouds tend to lower toward the groun|
|Clouds look like white fluffy cotton balls or heaps and show the vertical motion or thermal uplift of air taking place in the atmosphere. The level at which condensation and cloud formation begins is indicated by a flat cloud base, and its height will depend upon the humidity of the rising air. The more humid the air, the lower the cloud base. The tops of these clouds can reach over 60,000 feet (18,000 meters).|
|"Stratus" is Latin for layer or blanket. The clouds consist of a feature-less low layer that can cover the entire sky like a blanket, bringing generally gray and dull weather. The cloud bases are usually only a few hundred feet above the ground. Over hills and mountains they can reach ground level when they may be called fog. Also, as fog "lifts" off the ground due to daytime heating, the fog forms a layer of low stratus clouds.|
- National Weather Service — Clouds.
- Also, see: Cloud Classification and Characteristics.