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The water droplets in a cloud have a normal radius of about 0.002 mm (0.00008 in). The droplets may collide to form larger droplets, which remain aloft as long as the velocity of the rising air within the cloud is equal to or greater than the terminal velocity of the droplets. [9]
Cloud droplets normally form on aerosol particles that serve as CCN. Increasing the number density of CCN can lead to formation of more cloud droplets with a smaller size. The increase in number density increases the optical depth of the cloud, which results in an increase in the cloud albedo making clouds appear whiter. Satellite imagery often ...
When no CCNs are present, water vapour can be supercooled at about −13 °C (9 °F) for 5–6 hours before droplets spontaneously form. This is the basis of the cloud chamber for detecting subatomic particles. [3] The concept of CCN is used in cloud seeding, which tries to encourage rainfall by seeding the air with condensation nuclei.
The amount of water vapour that air can carry decreases with lower temperatures. The excess vapor begins to nucleate and to form small water droplets which form a cloud. Nucleation of the droplets of liquid water is heterogeneous, occurring on particles referred to as cloud condensation nuclei.
As a tropospheric cloud matures, the dense water droplets may combine to produce larger droplets. If the droplets become too large and heavy to be kept aloft by the air circulation, they will fall from the cloud as rain. By this process of accumulation, the space between droplets becomes increasingly larger, permitting light to penetrate ...
It is mechanically impossible to exceed D = 10 mm as the drop breaks at large diameters. From the general distribution, the diameter spectrum changes, μ = 0 inside the cloud, where the evaporation of small drops is negligible due to saturation conditions and μ = 2 out of the cloud, where the small drops evaporate because they are in drier air.
The droplet concentration of a cloud is the number of water droplets in a volume of cloud, typically a cubic centimeter (Wallace, 2006). The formula for the droplet concentration is as follows. = / In this equation, N is the total number of water droplets in the volume, and V is the total volume of the cloud being measured.
The most common way to form an ice crystal starts with an ice nucleus in the cloud. Ice crystals can form from heterogeneous deposition, contact, immersion, or freezing after condensation. In heterogeneous deposition, an ice nucleus is simply coated with water. For contact, ice nuclei will collide with water droplets that freeze upon impact.