Clouds exist on any planet in the solar system with an atmosphere, such as Venus, Mars, Jupiter and Saturn. Though clouds are observed high in the sky, the process of cloud formation starts right at ground level. Understanding the cloud formation process is not only fascinating, but it’s important when forecasting weather.

Instrument-laden aircraft and Doppler radar have probed clouds and presented scientists with a clear depiction of how clouds form. Clouds are giant masses of frozen crystals, water droplets or both. With a microscopic diameter of about 0.01mm, these frozen crystals and water droplets are created by water vapor, which is an important element in the process of forming clouds. Water vapor is suspended in the air, attracted by hygroscopic particles, such as salt. Even on a clear and sunny day, the air is filled with these particles (there are up to 150,000 in the volume of an index finger), which are called condensation nuclei. The condensation nuclei help water vapor condensate to form droplets, which will soon cause precipitation.

The water droplets vary in shape and size, making each cloud unique in shape and density. The differing cloud densities affect the range of white to dark coloration. Dense clouds project a reflectance rate of 70-95% throughout the visible range of wavelengths, thus creating a white appearance from above. The base of clouds may appear dark and gray because light is scattered more efficiently by water droplets, causing a decrease in the solar radiation with depth in the cloud.

Cloud formation highly depends on the differences in air density, which changes due to temperature. Cold air has a higher density than warm air, and therefore cold air sinks, whereas warm air rises. This is called convection and is part of the cloud-forming process. The sun plays a major role in convection; when the sun shines, ground-level air that contains water vapor heats up and begins to rise. As air rises, it begins to cool, nearing its saturation point. The saturation point is when air contains as much water vapor as it can hold. It is either reached by maximum moisture accumulation the volume of air can hold or by the reduction in temperature of air already filled with moisture. Once the air reaches the saturation point, moisture turns into visible water droplets, forming a cloud.

Clouds may form because of topography. For example, mountains may force warm air to rise and then cool below the saturation point as the elevation increases. These types of clouds are called orographic clouds. Once this cool air becomes saturated with water vapor, condensation occurs and is visible in the form of a cloud. The air mass then travels to the other side of the mountain to descend in a drier and warmer climate.

A typical cloud may have a water mass of up to several million tons. Clouds are of high volume, yet the net density is low, so air currents below and within it are capable of keeping the cloud suspended. The water droplets’ microscopic radius and terminal velocity of about 1-3 cm per second allows plenty of time for water droplets to re-evaporate as they fall beneath the cloud into warmer air. Therefore, water droplets are constantly forming and re-evaporating within a cloud.

Clouds are categorized based on their appearance and their relative location above the earth's surface using the following international classification system.

High clouds, above 18,000-20,000 feet, are composed of ice crystals. These include:

• Cirrus (Ci), Detached clouds that look like delicate white wisps, or white patches or narrow bands. They are often the first sign of a warm front.
• Cirrocumulus (Cc), thin, small, globular white puffs that appear as grains or ripples resembling fish scales. Usually seen in winter, they indicate fair, cold weather ahead.
• Cirrostratus (Cs), whitish, sheetlike clouds often covering the entire sky and transparent enough to see the sun and moon above them; may produce halos. Visible 12-24 hours before a rainstorm or snowstorm.

Middle clouds, in the 7,000-20,000-foot range, contain water droplets. They include:

• Altocumulus (Ac), patches or puffs of white, gray or both, usually formed in groups, or rolls; may or may not be merged. They often form a fish scale pattern, called "mackerel sky." Not precipitation clouds, but they may signal an oncoming thunderstorm.
• Altostratus (As), gray or blue-gray clouds composed of both water droplets and ice crystals that completely or partially cover the sky and contain thin patches through which the sun or moon is visible in the form of a round disc. Precursor of storms with continuous rain or snow.

Low clouds are found from the earth's surface up to about 7,000 feet. Included are:

• Nimbostratus (Ns), a dark gray, wet-looking cloud layer thick enough to blot out the sun and generally accompanied by more or less continuous precipitation in the form of rain or snow. Additional low, ragged clouds may appear below the main layer.
• Stratocumulus (Sc), a patch, sheet or layer of white with dark areas where the clouds are thicker and patches of blue sky are visible in between. Rarely a harbinger of rain, although they may become nimbostratus clouds and eventually bring precipitation.
• Stratus (St), a uniform gray layer of clouds that covers the entire sky, generally indicate stable air, although they can produce precipitation in the form of light mist or drizzle.

Vertical clouds are a unique category, formed when portions of cumulus and cumulonimbus clouds become middle clouds:

• Cumulus (Cu), white, cottony puffs with flat bases and rounded towers that are signs of fair weather. Cumulus clouds build vertically and may develop into giant cumulonimbus, or thunderstorm clouds.
• Cumulonimbus (Cb), heavy, dense towers, usually with smooth upper portions that are flattened and spread into an anvil shape or plume. Base is usually very dark and merged with low, ragged clouds. Associated with heavy rain, snow, hail, lightning and sometimes heavy winds and tornadoes. Anvil generally points in the direction the storm is moving. May produce virga, streaks of droplets or ice particles that evaporate before reaching the earth's surface.

• Mammatus, a cellular pattern of breast-shaped pouches hanging below the base of a cloud that tends to form during warmer months in the mid- and southwest United States. These smooth, ragged or lumpy lobes often indicate the approach of a particularly violent storm.
• Lenticular clouds are lens-shaped, stationary clouds that form at high altitudes and normally align at right angles to the direction of the wind. Often appear as flat discs formed near mountain ranges.

- Eri Hino