![]() |
||
![]() |
![]() |
|
![]() |
![]() |
![]() |
How Do Clouds Form?
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). |
![]()
|
Subscribe to the Earth Observatory About the Earth Observatory Contact Us Privacy Policy and Important Notices Responsible NASA Official: Lorraine A. Remer Webmaster: Goran Halusa We're a part of the Science Mission Directorate |
![]() |