Low pressure weather systems are as common as muck across our part of the world as we live right next to a huge body of water in the form of the Atlantic.
However, when we have an area of low pressure developing to the West of us it needs three main things to take place in order for the area of low pressure to continue to develop and deepen.
Let's take a look at one system developing and see how this works :-
For low pressure to develop the first thing we need to see is upper level divergence. This is the process of literally sucking air out of the top of the system which in turn lowers the air pressure of the column of air. This is a little like putting a ping pong ball in a glass tube and blowing across the top of it. The faster air at the top removes air from the top of the tube and lowers the air pressure which forces the ball to rise.
This happens in the atmosphere due to the jet stream. If we take a look at the corresponding jet stream :-
Here, we are can see the jet stream sitting over the top of the system which will cause upper air divergence.
The other thing we need for the system to develop is lower level convergence. This is where the air near the surface starts to blow towards the centre of the area of low pressure. As it does this it is forced to rise (as it can't go down into the sea/land).
The wind charts will show us whether this is happening :-
Here you can see the wind barbs starting to point in towards each other which shows that low level convergence is taking place.
The third ingredient is the release of latent heat from the system. This happens as moist air starts to rise and condenses into clouds/rain. This may sound obvious, but the process of the low pressure releasing this energy causes the air within the system to be more buoyant and start to cool. As it cools, so it continues to rise which in turn develops the area of low pressure further.
Well developed systems will have a mixture of all three of these processes happening :-
- Upper level divergence
- Low level convergence
- Release of latent heat.
When the upper level convergence is greater than the amount of air coming in to the surface then the low will continue to deepen.
This happens with our system here :-
However, once the system moves away from the jet stream, the amount of upper air divergence falls and the amount of air coming in at the surface increases. Once this happens, the low starts to weaken and fill :-