Clouds

Clouds form when the invisible water vapor in the air condenses into visible water droplets or ice crystals. This can happen in two ways.

1. The air is cooled below its saturation point. This happens when the air comes in contact with a cold surface or a surface that is cooling by radiation, or the air is cooled by adiabatic expansion (rising). This can happen

• along warm and cold fronts (frontal lift),
• where air flows up the side of a mountain and cools as it rises higher into the atmosphere (orographic lift),
• by the convection caused by the warming of a surface by insolation (diurnal heating),
• when warm air blows over a colder surface such as a cool body of water.

2. Cloud can be formed when two airmasses below saturation point mix. Examples are breath on a cold day, aircraft contrails, and Artic.

3. The air stays the same temperature but absorbs more water vapour into it until it reaches saturation.

The water in a typical cloud can have a mass of up to several million tonnes. However, the volume of a cloud is correspondingly high, and the net density of water vapor is actually low enough that air currents below and within the cloud are capable of keeping small droplets suspended. As well, conditions inside a cloud are not static: water droplets are constantly forming and re-evaporating. A typical cloud droplet has a radius on the order of 1 x 10^-5 m and a terminal velocity of about 1-2 cm/s. This give these droplets plenty of time to re-evaporate as they fall into in the warmer air beneath the cloud.

Most water droplets are formed when water vapor condenses around a condensation nucleus, a tiny particle of smoke, dust, ash, or salt. In supersaturated conditions, water droplets may act as condensation nuclei.

Water droplets large enough to fall to the ground are produced in two ways. The most important is through the Bergeron Process, theorized by Tor Bergeron, in which supercooled water droplets and ice crystals in a cloud interact to produce the rapid growth of ice crystals, which precipitate from the cloud and melt as they fall. This process typically takes place in clouds with tops cooler than -15°C. The second most important process is the collision and wake capture process, occurring in clouds with warmer tops, in which the collision of rising and falling water droplets produces larger and larger droplets, which are eventually heavy enough to overcome air currents in the cloud and the updraft beneath it and fall as rain. As a droplet falls through the smaller droplets which surround it, it produces a "wake" which draws some of the smaller droplets into collisions, thus perpetuating the process. This method of raindrop production is the primary mechanism in low stratiform clouds and small cumulus clouds in tradewind and tropical regions, and produces raindrops of several millimetres diameter.