Clouds are visible collections of small particles of water or ice, or both, suspended in the atmosphere. They are one of the most obvious and influential features of Earth's climate system. They are also one of its most variable components. The natural diversity and variability of clouds has intrigued and challenged researchers for centuries.

How Do Clouds Form?
In order to form, clouds require the presence of water vapor and aerosols (tiny solid or liquid particles suspended in the atmosphere)-both are found abundantly in Earth's atmosphere. Water vapor, or water in its gaseous state, is transferred from the surface to the atmosphere primarily via either evaporation or "evapotranspiration," (the process by which water is evaporated from the tiny openings on the leaves of plants during respiration).

Figures 1a and 1b: Hygroscopic nuclei (1a) consist of particles such as sea salt and common table salt which have an affinity (attraction) for water. Water vapor condenses on these particles when the relative humidity it considerably less than 100 percent. On humid days, you may find it difficult to pour salt from the shaker because water vapor has condensed on the salt crystals, sticking them together. Hydrophobic nuclei (1b) consist of particles such as oils and Teflon. These nuclei resist condensation even when the relative humidity is greater than 100 percent.

As air currents rise, taking water vapor molecules along with them, they tend to cool. This vertical motion of air currents helps clouds form, by exposing its water vapor to both cooler temperatures and cloud condensation nuclei (CCN-aerosol particles up to one-millionth of a meter in size) also suspended in the atmosphere. There are many types of CCN, ranging from sea salt to windblown dust to industrial pollution. These nuclei are hygroscopic ("water-attracting") while others (oils, Teflon) are hydrophobic ("water-repelling") (Figures 1a and 1b). Since hygroscopic nuclei have an affinity for water, they act as "seeds" to accelerate the condensation process to convert water from its gaseous to its liquid phase.

In the presence of CCN, clouds form in either of two ways: (1) when a region of atmosphere cools to the temperature at which water vapor condenses into water droplets or is deposited into ice crystals; or (2) when sufficient amounts of water vapor are added to a given region of atmosphere, yielding the formation of water droplets (condensation) or ice crystals (deposition).

next: Clouds and Climate Change

by Steve Graham
May 13, 1999

Changing Global Cloudiness

How do Clouds Form?

Clouds and Climate Change

Surface-Based and Satellite Cloud Observations

Terra and Cloud Observations