Morning fog

Problem

Early-morning fog often dissolves as the Sun warms the air. Explain in five lines the energy balance in terms of latent heat and the liquid-to-vapour transition.

Solution

What fog is. It is a suspension of tiny liquid water droplets in the air. It makes the atmosphere opaque because the droplets scatter light.

Role of the Sun. At dawn the Sun warms the air and the droplets. To turn from liquid into vapour, each droplet must absorb the latent heat of vaporisation: energy that reaches it from solar radiation.

Effect. The droplets EVAPORATE (liquid→vapour transition), the droplet density drops and visibility increases: the fog “dissolves”. The water does not disappear, it turns into invisible vapour.

Energy balance. The solar energy is split between evaporating the droplets and heating the air:

$Q_\text{Sole}=Q_\text{evaporazione}+Q_\text{riscaldamento aria}$

$\ev{\text{the droplets absorb latent heat and evaporate}}$

Links

Topics: Thermodynamics Concepts: Latent heat and phase changes Skills: Analysis of limiting cases and science-fiction physics