## Section4.2Cloud formation processes

The behavior of water with changing temperature is key to understanding cloud formation:

• as air cools, the relative humidity increases. If cooled sufficiently, the dewpoint is reached and $$\phi=100\%\text{,}$$ driving condensation

• the curvature effect makes it easier for water molecules to evaporate from a small droplet

Droplets form around particles via nucleation.

• homogeneous nucleation: formed only by water molecules (need very low temperature and high humidities)

• heterogeneous nucleation: particles & aerosol serve as condensation nuclei for water droplets

• ice nuclei are particles around which ice crystals form, which occurs with specific molecular orientation

Note that fog can be thought of as a surface cloud, formed by condensation out of saturated atmosphere. There are different ways this can occur:

• radiation fog forms following nights where significantly surface cooling has occurred, raising the relative humidity of the air to saturation (can occur w/inversion)

• advection fog occurs when warm, humid air flows over a colder surface, causing the air to cool to saturation.

• evaporation or steam fog occurs when warm water evaporates into the air

But what about cloud formation? Generally need a mechanism for air to ascend. There are different ways this might occur:

• orographic lifting (upslope flow)

• frontal lifting

• thermal convection

• convergence

The extent of lifting and the possibility of cloud formation depend upon the temperature and relative humidity of the rising air, and upon the environmental conditions of the atmosphere for that time and location.