Dew Point

[Ou Man]

Please help me to understand Dew point.
What I know is that it is the point where moister will condensate (form cloud or fog).
I further know that temperature and relative humidity is the main culprits .
Now….as I understand it, at a high relative humidity level, dew point will be at a relative low temperature and visa versa. So if I don’t now the exact relative humidity level. I won’t be able to calculate the dew point temp.
Why is it then that in the altitude calculator (Pilot friend) it says that they use the dew point because it is more stable than relative humidity and they use the figure of 67° F.

[Morph]

From Wikipedia, http://en.wikipedia.org/wiki/Dew_point

The dew point (sometimes spelled dewpoint) is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water. The condensed water is called dew. The dew point is a saturation point.

The dew point is associated with relative humidity. A high relative humidity indicates that the dew point is closer to the current air temperature. Relative humidity of 100% indicates that the dew point is equal to the current temperature (and the air is maximally saturated with water). When the dew point stays constant and temperature increases, relative humidity will decrease.

The dew point is an important statistic for general aviation pilots, as it is used to calculate the likelihood of carburetor icing and fog, and estimate the height of the cloud base.

http://www.weathersa.co.za will give you the dew point and the relative humidity

Wednesday, 6 August 2008 11:00 SAST
Temperature 17.7 °C
Dew Point: 12.8 °C
Humidity: 73 %

Wind: N, 6.0 km/h
(3 Kts)

Pressure (QNH): 1020 hPa

[RudiGreyling]

ditto,

Here is a rough rule of thumb if I remember it correctly :
If the dew point and actual temp is the same or close together it is very likley you will have (Cloud) fog forming at ground level.
For every 1Degree C difference you can estimate that cloudbase will likely be 400-500ft of the ground.
So Dew point 4dC and actual temp 6dC cloudbase likely be +/- 800-1000ft agl.

Regards
Rudi

[Trikenut]

Very simply, dew point is the temp at which air condenses and turns to dew.
The temp changes depending on the season, humidity and a few other factors...
Correct me if I'm wrong, Guys.

[Learjet]

Hope this isn't a dumb question (or confirmation that I was asleep during my met lectures!) but is there a dew point "average" that one can use for density altitude calculations when you happen to be in some out of the way place without the specific regional met data on hand. The reason I ask it that there are some nice downloadable DA calculators available on the net but they require the dew point data to be inputted as part of the calculation.

[Wargames]

Hope this isn't a dumb question (or confirmation that I was asleep during my met lectures!) but is there a dew point "average" that one can use for DE altitude calculations when you happen to be in some out of the way place without the specific regional met data on hand. The reason I ask it that there are some nice downloadable DA calculators available on the net but they require the dew point data to be inputted as part of the calculation.

To be on the safe side, I would suggest that you use current temp as dewpoint. That would in essence put your density altitude higher than would be the case, but it would make you err on the safe side, than other way round.

My 2c.

[Learjet]

Thanks Wargames!

[Tumbleweed]

http://www.avcom.co.za/phpBB3/viewtopic ... 13&t=36334

[Morph]

just go online or call the weather office and they can give you the dew point

[RudiGreyling]

just go online or call the weather office and they can give you the dew point

Yes I do it early morning to decide if I am going out to the airfield, and if we are going to have fog or enough cloud base clearance to fly.
If Dewpoint = +/- Actual Temp or higher, then I wait a bit for the diffs to get bigger.'

Rudi