Dew Point and FogAdded : Monday at 13:36 Right, it's over to you tonight to do a little bit of nowcasting for your back garden (if you have one) if you are up for the challenge?
The issue tonight across much of England and Wales is mist, drizzle, low cloud and fog. Here is the reason...
If you take a look at the chart for midday today :-
You can see a rather brisk Easterly wind across many parts of the UK. Nothing too out of the ordinary there. But by tonight we have this :-
You can see a pool of milder air coming in from the Southeast as well as the isobars slackening which allows the winds to fall lighter. With this milder air also comes milder dew point temperatures (or the wet bulb temperature if you have a weather station outside).
The wet bulb temperature is basically the temperature at which the air will become totally saturated and can no longer hold any more moisture (i.e a cloud). When you have colder air in situ across the UK and milder air coming in from the South this results in two things :-
- The milder air becomes saturated as it comes into contact with the colder air (a bit like the moisture forming on your bathroom mirror in the morning)
- Slight low level turbulence develops when the process is fairly rapid (as in the case tonight).
This turbulence is important and it explains why we aren't expecting widespread dense fog. Had the process been a little slower then yep, dense fog on the cards. But this is only an issue really for Western areas overnight and perhaps Central England for a time.
For other places, the process happens too quickly. When this happens, the water molecules bump into each other and you end up with drizzle. An obviously, when you have drizzle, this means water molecules falling through the sky and bumping into other ones en route to the ground which lowers the risk of fog.
Now, back to your homework. If your weather station has wet and dry bulb thermometers, then try charting the rise of both of them tonight. You'll notice the wet bulb rising quicker at first, but then the dry bulb rising as energy is released from this condensation process. Every so often, when the wet bulb gets close to the dry bulb temperature take a look in the garden and you'll see either drizzle or mist forming.
What you are seeing, is basically nature opening the door after a hot shower and then wandering over to the cold mirror for a shave and wondering why it has steamed up...
Isn't meteorology just fabulous... even if a bit unshaven tonight.
METEOROLOGIST : MARSH |