Dew Point question

[Lump Jockey]

Hello all, just one question, why is the dew point temp so important to know?
Thanks
LJ.

[saudipc-9]

Lump Jockey,
The dew point is important to know for a few reasons
1. A big dew point/temp spread can be a good indication of future CB's etc ie unstable weather

2. A narrow dew point/temp spread can give an indication of possible saturation and therefore fog/mist etc

3. A high dew point can tell you of a high density altitude which will decrease aircraft performance or increase the likely hood of carb ice if you are flying a piston aircraft due to a high moisture content.

Hope this helps

[Bullet Tooth Tony]

Agree with saudi-pc9 on points 2 and 3, but not necessarily point 1.

For CB's to form the air needs to be deeply unstable and then be given a kick - a trigger action - as well as having the above mentioned high water content. Without all 3 of these conditions CB's will not form.

For what it's worth, I would say the fog/mist/visibility aspect is the biggest factor effecting pilots regarding high dew points.

[saudipc-9]

BTT,
Agree with you about the CB's but it is just an indicator not really saying that CB's will develop. A big temp/dew point spread can point to an unstable airmass and therefore cumulus type clouds.
METAR's are reall a good way of seeing if the weather guesser has got his forecast even close:D
cheers

[mutt]

saudipc9,

Do you find that the temp/dew point spread gives you an indication of temperature inversions in your part of the desert?

Mutt.

[saudipc-9]

mutt,
Been in Saudi for 3 1/2 years and I don't think I have ever seen a temperature inversion here.
Saw alot when I was in Canada and it was -40C:p

cheers

[mutt]

Strange.....:confused: :confused: :confused:

Your local international airport suffers from summer inversions in the mid-teens, as do most of the Gulf Airports.

The MEPA in RUH lauch twice daily weather balloons to monitor temperatures and issue forecast and actual inversions.

I guess that they picked an excellent location for your base. :)


Mutt.

[saudipc-9]

Mutt,

I spend most of my time at 500' or less so I guess I don't pay much attention to if there is a temp inversion. Tell you what, I'll look out for it and let you know next time I see one:p

Mind you, I don't know where the base gets their TAFs from but they do make us laugh at times they are so screwed up. I think the Wx guessers at KKIA have more of an idea as to what is going on.

Cheers

[Maxflyer]

My Instructor (I'm trying to get my PPL) told me that you can multiply the difference between temp and dew point by 400 to get a rough idea of the cloud base.

Sorry if I,m posting to the wrong forum.

TJ

[unwiseowl]

saudipc-9: you say a high dew-point is indicative of a high density altitude. If I want to know density alt I'll enter a chart with ambient temp and pressure altitude ie, there's no need to consider dew point.

[Hippy]

tmybr

That's true, what your instructor says. Works very well on a typical cumulus producing day. I used that formula quite often when I was a Met observer to check the dodgey old CBR I was given to use. :D

[saudipc-9]

unwiseowl,
Ok I just said it is an indicator. Answer me this
1. temp 30 dew point 5
2 temp 30 dew point 20

Which one has the higher density altitude? ie less performance for your aircraft type?

[MightyGem]

Not sure that I get the connection either. I obtain DALT by using a chart and by knowing the temp and pressure alt. Humidity doesn't come into it. 30/20 tells me that the air is more humid than 30/5 but the DALT will be the same. Granted that the high humidity may effect engine performance.:confused:

[saudipc-9]

Granted that this is an odd website for the answer( ie about motorcycles) but you will notice that "Relative Humidity" which is indicated by a dew point ie high dew point=high relative humidity
is part of the equation to calculate air density.

Here's another site goes into alot of detail and is aviation orientated. You will notice that the calculator specifically says Dew Point.

Let me know what you think!

Added the sites again
cheers.[URL]http://wahiduddin.net/calc/density_altitude.htm (http://www.nightrider.com/biketech/calc_airdensity.htm

[URL)

I think that does it.

[MightyGem]

As I type this I’m still trying to figure out: a. What have I found out, and b. Is it relevant.:confused:

Yes, the humidity does make a difference to the DALT. I put various figures into the calculators on the web pages (your links need editing, they bring up a page http:/// ) using standard atmosphere values and came up with the following. Using RH values of 5% and 95% respectivly the humidity changed the DALT as follows:
At 2000ft OAT 11deg c, 5% = 2100, 95%= 2267
At 10000 OAT –5 deg c, 5% = 10002, 95%= 10067
At 35000 OAT –55deg c, 5%= 35011, 95%= 35012

The differences in the Relative Air Density varies from 0 at 35000, 0.1% at 10000 and 0.4% at 2000. Even at 30deg c at 2000 the difference is only 1.5%.

Still not sure whether it makes a difference or not, especially as in aviation we use a graph based on the formula DA=PA+/-120T which makes no mention of humidity. The change of lift on an aerofoil is probably negligable. If you want to squeeze max power out of a piston engine then the humidity will make a difference, as humid air increases the weight of the charge drawn into the cylinder and increases the heat efficiency of the engine (I think!!). Both of which give an increase in power (I think!!!).

Phew, now I need to go and lie down for a while.
:confused:

[Tinstaafl]

No. Humid air will decrease performance.

It may increase the weight of air (or it may not. Can't remember. I *think* it decreases it...) but the problem is that it replaces some proportion of oxygen. The less O2, the less fuel that can be burnt to release energy.

I suspect you're thinking of Water or Water/Methanol injection. This works because the fluid is introduced in liquid form & then evaporates. In doing so the water extracts heat from the charge, cooling it. Because the air is now cooler it's more dense ie the same volume of air contain more oxygen.

Still not as good as an equivalent temp. charge of dry air since some of the mass is now water and not O2.

[saudipc-9]

I've flown the T-38 a few times while I was in the US and a one degree temp change could mean to difference between stopping on a aborted take off or going for a X-country drive off the end of the runway. ie accelerate/stop distances. Guys who had flown for a while would also note the dew point, as they knew that it is relevent and that it does effect your aircraft performance.
I'm sure that with most aircraft the difference is neligible. However, the Dew Point does effect density altitude and does effect aircraft performance. Just ask any old 104 drivers out there if they thought about it. I'm sure they will say it is an "indicator" of decreased density altitude just as I have mentioned while trying to answer Lump Jockey's question.

cheers

Ps here's a better URL to go to:p http://members.aol.com/matt999h/beer.htm