Cloud physics

Cloud physics is the study of how clouds are formed in the atmosphere. Clouds can be droplets of water, ice crystals or both, along with dust or smoke particles onto which water or ice can condense.^[1]

In the 17th century, a German scientist called Otto von Guericke suggested that clouds were made of water bubbles.^[2] Modern cloud physics started in the 19th century.^[3]

As water evaporates from the Earth's surface, it makes the air moist. Moist air is lighter than dry air, so it rises into the atmosphere. It doesn't mix with the surrounding air and forms clouds.^[4]

Water vapour is invisible, but as the air rises, it cools to its dew point through a process called adiabatic cooling.^[5] Atmospheric pressure decreases with altitude, so the rising air expands and cools, causing water vapour to condense into clouds.^[6] Water vapour is attracted to dust or smoke particles in the air to form small droplets that may collide to form larger droplets, which fall as precipitation when they are heavy enough.^[7]

In addition to adiabatic cooling, other processes that cause air to rise and cool to its dew point include frontal and cyclonic lift,^[8] convective lift,^[9] and orographic lift.^[9]

Other mechanisms that lower the temperature of the air to its dew point occur near the surface and cause fog to form.^[10] Conductive cooling happens when warmer air comes into contact with a colder surface, as when warmer air from the sea moves across a colder land area. Radiational cooling is caused by infrared radiation and is common during the night when the sky is clear.^[11] Evaporative cooling happens when moisture is added to the air through evaporation.

The Bergeron process states that the amount of water vapour that air can hold depends on what the vapour is interacting with and that it is lower for ice than for water.^[12]