Condensation + Cloud Formation

Haze droplets:

The smallest liquid water droplets in the atmosphere; some can persist in unsaturated air.

Fog droplets:

Haze particles that grow large enough (~20 µm) to be classified as cloud liquid water droplets.

Mechanisms of Fog Formation

Cooling of Air; Addition of Water Vapor

Fog Types: Cooling of Air

Radiation Fog, Advection Fog, Upslope Fog

Fog Types: Addition of Water Vapor

Steam Fog, Frontal Fog

Radiation Fog

Cooling of Air; forms when the ground cools rapidly overnight

Advection Fog

Cooling of Air; forms when moist air moves over a cooler surface

Upslope Fog

Cooling of Air; forms when air is lifted to higher elevation, cooling the air

Steam Fog

Addition of Water; forms when cold, dry air moves over warm water

Frontal Fog

Addition of Water; warm rain falls through cold, dry air onto snow covered surface, adding moisture

What is the most common way clouds form?

Cooling of Air

Why does rising air cool?

As air rises, pressure decreases, causing expansion. Expansion requires energy, which is taken from the air’s kinetic energy, lowering its temperature.

Dry Adiabatic Lapse Rate

rate at which unsaturated air cools as it rises at a rate of 10c per km

adiabatic

no heat is exchanged with the surrounding environment.

Moist Adiabatic Lapse Rate:

rate at which saturated air cools as it rises at the rate of 6°C per km

Why does saturated air cool at a slower rate than unsaturated air?

Condensation releases latent heat, which offsets some of the cooling from expansion.

What happens if a rising air parcel starts unsaturated but becomes saturated?

It first cools at the dry rate, then at the moist rate once RH reaches 100%.

What happens when all the water vapor in a rising parcel has condensed?

cools again at the dry adiabatic rate if it continues to rise.

What drives cloud formation in the atmosphere?

The cooling of rising air, which leads to condensation when RH reaches 100%.