It's All Relative... HumidityAdded : Tuesday at 11:30 The key word for this discussion is relative. Remember that and all will become clear by the end of it.
As we enter the middle part of Summer, you will find that some days and nights are rather muggy and oppressive, whilst others tend to feel comfortable, even though perhaps the temperature of the air could well be the same.
The other thing you might notice in your forecasts on the site is that the RH (or relative humidity) of the air always increases overnight, but why is that?
If we take a look at the current chart :-
You can see high pressure building across the South of the UK and to the West of the high we start to draw in winds from the Southwest. The dew point of this air is higher than air to the East of the high. It's the difference between the air temperature and the dew point temperature which allows us to calculate the relative humidity of the air and in turn figure out how comfortable the temperature will actually feel.
Warmer air will always be able to hold more moisture than colder air, that's one of the basic laws of physics. This means that overnight when the temperatures cool, the relative humidity of the air will increase (as the air temperature drops but the dew point remains the same).
Into this weekend and we see the winds pumped in from the Southwest :-
This will bring higher dew point temperatures, but the air temperature will be similar to that which we have seen at the start of this week. The result? Well, it will feel a little more humid for many.
At present, the dew point across the UK is around 10-12c which means that if the daytime temperature reaches 22c which will result in a relative humidity of 50% which will feel rather nice.
But, if at the weekend we get temperatures of 22c but a dew point of 18c then the relative humidity will be 78% which will feel a bit muggy, even more so when overnight the temperature falls to 19c and the dew point stays the same at 18c which gives a relative humidity of 94%.
So, if you have trouble sleeping on warm, muggy nights then don't blame the air temperature, blame the dew point temperature instead.
METEOROLOGIST : MARSH |