Condensation
Condensation is the opposite of evaporation. It’s when a gas changes into a liquid. This happens when the particles of a gas, which are are very far apart, get cold and stick closer together. Condensation is very important because it is responsible for cloud formation. Condensation near the ground is called fog. We can see condensation on people who wear glasses, on coasters for cups, and on windows.
Why is it colder higher up?
Have you ever wondered why it is colder up a mountain than on the ground? The higher up we go, the colder the temperature gets. This is all because of air pressure.
The more pressure, the more heat. Think of a bicycle wheel. If you pump and pump and pump the wheel so much, the pressure gets high and the molecules inside rub against each other. The wheel will become hotter.
The higher up we go in the sky, the less pressure there is. This means the molecules are not rubbing together so much and the temperature is cooler. The closer we get to the ground, the more air pressure there is, and the higher the temperatures.
Condensation near the ground
Condensation can happen on the ground, forming fog. To make fog the gas does not need to rise. Fog forms when humid air comes into contact with a colder surface and cools down (condenses), creating low-level clouds.
Fog that is formed when warm air moves over a cold surface is called advection fog.
Radiation fog forms at night when the temperature is lower. If the air is still, the water vapour doesn't mix with the air above it, and forms fog that is very low to the ground.
Condensation in the air
Even if we cannot see clouds in the sky there is still water vapour present, the water droplets are just too small to be seen. Droplets of water vapour in the air can mix with particles in the air like dust, soot, or smoke, and together they form clouds. These tiny particles are called aerosols and may or may not be visible.
Cloud droplets can vary from 10 microns (millionths of a metre) to up to 5mm in size!
Clouds act as blankets, they are very important for the climate. They regulate the amount of heat that comes to the surface of the Earth by reflecting solar radiation back to space.
