In this article we will look at a phenomenon which skydivers, paragliders and pilots are well acquainted with.
These are ‘bubbles’ or pillars of rising warm air that can reach high altitudes. Let’s see how they appear.
We’ve already mentioned that warm air is lighter and less dense. It always tends to rise and cold air replaces it. A thermal is an ‘accumulation’ of air, in other words. It rises as it’s lighter than the surrounding air.
The surface of the Earth is warmed up by the Sun during the day and the ground heats the air above it.
If the process is slow the warm air will rise in a continuous flow.
If the heating is fast bubbles may occur. They stay on the surface, grow for some time and suddenly tear away.
Let’s take a pot of water as an example. If it heats up slowly we’ll see a pillar of rising warm water at the bottom. If the heating is intense bubbles are formed.
The surface of the Earth is patchy - the air above the ground will heat up differently at different spots.
The more contrast in the weather (cold air - hot sun) - the stronger the thermal columns. In windless weather thermals are almost vertical. If there’s wind the thermal leans in its direction.
Thermals are quite dangerous close to the ground as they cause turbulence for skydivers.