Page "Cloud" Paragraph 8

There are three main agents of vertical lift.

One comprises two closely related processes which work together.

Frontal lift and cyclonic lift occur when stable or slightly unstable air, which has been subjected to little or no surface heating, is forced aloft at weather fronts and around centers of low pressure.

Newly formed cloud droplets that are lifted beyond the condensation level tend to increase in number and coalesce until they grow to a size of up to. 04 mm (. 002 in ).

They remain aloft as long as the drag force of the air dominates over the gravitational force for small particles.

If the cloud droplets continue to grow past this size, they become too heavy to be held aloft as the gravitational force overcomes the atmospheric drag, and they fall from the cloud as rain.

When this process takes place just above the freezing level, the vapor tends to condense into supercooled water droplets, which with additional lifting and growth in size, can eventually turn into freezing rain.

At temperatures well below freezing, the vapor desublimates into ice crystals that average about 0. 25 mm in length.

Continuing lift and desublimation will tend to increase the number of ice crystals which may combine until they are too heavy to be supported by the vertical air currents and fall out as snow.