Short Wave Trough MasterclassAdded : Friday at 9:46 Right, give us a virtual �5 and if you don't understand completely what short wave troughs are by the end of this masterclass then we will give it back to you.
What is a short wave trough?
If you take a look at the chart above you'll notice a slight kink in the isobars. It's not a frontal system and it's not a weather system. It is in fact a short wave trough and these things can be seen on pressure charts pretty much every day. In fact, if we pick another chart for next week you can see another example of one :-
How are they created?
A very good question.. The answer is down to the chaotic make up of the atmosphere and the huge variations in temperature. When you look at charts like the one above you may think that the air coming in from the Northwest will become gradually cooler. Which in essence is true, but if you take a look at the upper air temperatures you can see this :-
Can you see the very slight change in temperature where the short wave trough is? It is these tiny changes in temperature which causes the thickness or density of the air to change. When this happens it means that the air will move at slightly different speeds, only slightly, but enough to create a short wave trough.
If the air was moving along these isobars at a steady 20mph then everything would be lovely, but if there was a slightly warmer area then the thickness of the air increases, whilst if it was slightly cooler then it would decrease. You can see this in action when you are in a plane and descending through clouds. You (and your G&T) will notice that the plane appears to drop slightly quickly when it descends through each cloud then stays constant when it doesn't. The reason is simply due to the plane flying through cooler air (the clouds) and the thickness of the air being a little less.
This is important, as it means air flowing along the isobars will encounter this same effect, but if the air is slightly warmer (thicker) then it moves a tad slower and if it's cooler then a tad faster. The problem is that air doesn't like this variation in speed as it results in faster moving air piling up behind slower moving air.
When this happens, the air has to rise. As it does so it cools, condenses and starts to spin anticlockwise slightly.
When this happens, a short wave trough is born. Not just that, but as the process continues, so the short wave trough starts to become stronger and so the processes feeds back into itself.
So what happens in these troughs?
Well, we mentioned that the air rises within them. As this happens, the air condenses and cools which results in rain or sleet or snow depending on the temperature. You can see this on the chart below :-
You can see how the model develops an organised band of showers in the short wave trough.
These features are extremely difficult to predict by numerical weather models, but the key is finding regions within an airmass which appears to have a long straight fetch but has tiny variations in temperature within it. If you can identify those areas then you can expect short wave troughs to develop within them.
A classic example of this happening is when we get a strong Northerly wind across the UK. All of a sudden we may see a trough develop in the North Sea and a band of showers become far more active than any numerical weather model expected. The reason? Purely down to the fact that there was some slight variation in temperature in the airmass which the models didn't see.
So, next time you hear a weather forecaster say that showers could merge to produce longer spells of rain or a band of showers push across your region that wasn't forecast then in fact it's more likely that a short wave trough is actually passing through.
Still don't understand them? Here is your virtual fiver back then...
METEOROLOGIST : MARSH |