dew point and weather

dswanson · Post by **dswanson** » 08 Nov 2006, 19:19

Hi,

Planning some flying around the equator and wondered if changing the air temp and dew point settings made any difference to cloud formation etc.

I've always left the temps as default before - is it worth fiddling or not ?

Charlie Bravo · Post by **Charlie Bravo** » 08 Nov 2006, 23:27

When temp and dew point meet, you usually get fog.

DispatchDragon · Post by **DispatchDragon** » 08 Nov 2006, 23:43

If Possible I would use something along the lines of ASv6 and use real time
For the last two weeks Its been giving me some VERY interesting fly weather in the Solomon islands

Leif

TobyV · Post by **TobyV** » 08 Nov 2006, 23:57

Dew point is (for those that havent guessed!) the temperature at which water vapour held in the air condenses out. This is why with air conditioning, the system drops the temperature of the air flowing through to below the dew point and the result is cool, dry air leaves the system and fills you room/car/etc.

cstorey · Post by **cstorey** » 10 Nov 2006, 22:56

As someone has pointed out , the dew point is the temperature at which the air becomes 100% saturated. Thus if you get the temp and dewpoint close together, the probability of fog is high. It is why, in humid areas, it is always vital to examine the temp and dp to ensure that they are at least 4 or 5 C apart,, particularly on winter afternoons, because as the temperature drops with sundown , if the two coincide , fog is inevitable, and one can suddenly find the runway rolled up and put away even as you are on finals! Incidentally, I have experienced fog in Barbados, so it is not just restricted to temperate zones

Tom Clayton · Post by **Tom Clayton** » 11 Nov 2006, 21:50

You also need to be careful of icing if the humidity is high and the temp is low. As the air moves more quickly over the wings, it loses pressure, and therefore temperature. If you find that air at a temp below freezing AND below the dewpoint, that condensing water vapor will condense in the form of ice!

Chris Trott · Post by **Chris Trott** » 12 Nov 2006, 03:26

Actually, a temp below freezing and below dewpoint at the surface means that you have virtually no chance of icing as the so-called "icing level" is well below ground level. This is great weather to fly in because you have excellent engine performance (even though it does cost you slightly higher fuel consumption) without the need to pull the carb heat or engine anti-icing systems on and drain power.

Now, if the temperature is below freezing and the dewpoint is above your temperature but below freezing, then you have a potential problem.

cstorey · Post by **cstorey** » 12 Nov 2006, 12:00

Something is wrong with mr Trott's post : he has said the same thing twice, but with opposite conclusions, which I do not think was what he intended

Chris Trott · Post by **Chris Trott** » 12 Nov 2006, 16:19

You're correct. That's what I get for trying to type at 2am....

A temp below freezing and dewpoint below that will cause concern for icing. A dewpoint below freezing and a dewpoint below that causes no such concern.

Just remember this - if you imagine a thermometer as a line from left to right with 0 being in the middle, if the temperature is to the right of the dewpoint, you have a less-than-saturated air mass. The closer to dewpoint you get (moving left), the more likely that water will condense while flowing over your wings or through your air inlet to the carb or throttle body. Once the temperature gets to the left of the dewpoint, all of the water has condensed out, and there is no longer a concern.

Tom Clayton · Post by **Tom Clayton** » 13 Nov 2006, 02:04

I may not have made myself clear before, What I was refering to is something like an ambient temp of maybe 35Â°F and a dewpoint of maybe 28-29Â°F. As the air passes over the wing, wouldn't that produce a temp zone below that dewpoint?