So we know that given the right conditions (temperature, humidity etc.) rising air will expand and cool down, eventually reaching its dew point. This causes the water vapour in the air to condensate into visible water droplets, forming clouds.
There are many events that can force air to rise, starting this process that leads to cloud development:
Convection is the main process we have looked at so far, where cloud formation is the result of air rising when it is warmer than its surroundings. This rising air eventually reaches its dew point, condensation occurs and clouds are formed.
However, heat is not always required to cause air to rise. If humid air is forced over a mountain range, say as a result of a pressure gradient (remember that from the lesson on air pressure?), it will still expand and cool as it rises. Cloud will form when this air cools to the dew point.
This process is called orographic uplift.
Where two air masses with different temperatures meet, the lighter warm air mass will be forced to rise over the heavier cool air mass.
The boundary between the air masses is called a front – we will learn about fronts in detail in the next section.
When two air flows meet head on, there is no where for them to go but up. For example, when a sea breeze forms in the afternoon it can meet a steady wind that has been blowing from inland all morning.
A convergence typically forms more widespread cloud as the air rises slower and over a larger area.
Air flowing over the surface encounters friction with every obstacle it comes into contact with, creating updraughts, downdraughts, and varying wind speeds. The air is expanded and compressed as it moves up and down. Cloud develops when the vertical motion is high enough to reach the dew point and as the same air sinks below the dew point, the cloud evaporates.