Jet Stream MasterclassAdded : Friday at 12:53 Pretty much every dog, man and woman has heard about the jet stream. Though you'll be surprised how many get it confused with the Gulf Stream which is an oceanic feature bringing warmer waters North towards the UK across the Atlantic.
The jet stream, is a ribbon of air at around 30,000ft which circumvents both hemispheres. In each hemisphere there are three jet streams, the sub tropical jet, the polar jet and the arctic jet. For this masterclass we will concentrate on the polar jet which is the main jet stream which affects the UK and Irish weather.
This is the jet stream today :-
It should come as no surprise that the polar jet stream divides tropical air to the South and polar air to the North. You can see this by looking at upper air temperature charts for today too :-
The jet stream will usually be in one of two phases. Zonal is when the jet stream is pretty much flatlining across the Atlantic like this :-
When the jet stream is like this then you can expect wet and windy weather to rapidly push in from the West. The other phase is meridional and looks like this :-
Note the accentuated ridges and troughs within the jet as it meanders around the Atlantic. These are really important for surface meteorology as they dictate how much vorticity (or spin) is created around the jet stream and mid and lower levels.
You will probably see the term positive vorticity advection (PVA) used in the storm discussions on Metcheck. The opposite is negative vorticity advection or NVA. Wherever you see a jet stream chart, the two things will be happening.
PVA is cyclonic spin and occurs at the bottom of a trough and to the West of the jet stream i.e here :-
If you get the combination of this PVA, moisture and warmth coming in from the South then that's the recipe for intense and rapid developing thunderstorms and low pressure areas.
The opposite is NVA which occurs at the top of a ridge and to the area West of this ridge i.e here :-
In these areas we have anti cyclonic spin as well as descending air. This has the opposite effect of aiding development and instead we see calmer weather as well as any surface features like low pressure systems decay rapidly if they are under them.
A good example of this happens next week with remnants of Hurricane Maria and Lee which move in from the West, but an upper ridge pushes in quickly which decays these systems quickly.
So, if you wonder why some low pressure systems develop quickly and others don't then take a look at the jet stream images and you'll be able to see why.
For rapidly developing systems we also look at the left entrance, right entrance and exits of the jet stream, but that is another discussion for another day and we've taken up enough of your lunch break already.
METEOROLOGIST : MARSH |