Dew Point

[Saitek]

I just don't understand it! Even Wiki couldn't get it through me. When ATIS says the dewpoint what am I supposed to take into account?

[Fozzer]

...a bit more technical info here...>>>>

"Dew Point"

http://en.wikipedia.org/wiki/Dew_point

The term is often used in the Air Conditioning Industry, (Refrigeration plants).

It involves the use of a wet and a dry thermometer reading, and reference to a chart to calculate the Dew Point.

Paul.... 8-)...!

"Dew Drop"....the drop of water on the end of your nose during cold weather... ...!

100% Humidity means that it is probably tipping down with rain outside... ...!
When you can no longer see your way around the Bathroom because of the steam, the Humidity is VERY CLOSE to 100%... ...!
20 Degrees Centigrade, and 50% Humidity is comfort conditions for Human beings (typical Office).....!

[Brett_Henderson]

What you want to heed.. is the difference between temp and dew-point. If they're within a few degrees C... that means low visibility is likely. If they're the same.. you can pretty much count on FOG  

[Brett_Henderson]

Here are three METARS I just grabbed.. for airports with progressively better visibility.

KCIN 291855Z AUTO 31022G35KT 1/2SM -SN BKN005 OVC011 M17/M19 A2964 RMK AO2

KCIN has 1/2 statute-mile visibility; with a 2 degree difference (-17/-19)

KCMH 291851Z 18010KT 5SM BR SCT008 BKN011 OVC015 10/06 A2950 RMK AO2 RAE01 SLP990 P0002 T00780061

KCMH has 5 statute-mile visibility; with a 4 degree difference (10/06)

KLAX 291850Z 05007KT 10SM SCT035 BKN050 13/04 A3026 RMK AO2 SLP245 T01330044

KLAX has 10 statute-mile visibility; with a 9 degree difference (13/04)

[Saitek]

Thanks, that helps a lot.

[expat]

What you want to heed.. is the difference between temp and dew-point. If they're within a few degrees C... that means low visibility is likely. If they're the same.. you can pretty much count on FOG  

In layman's terms, the temperature at which the grass is wet and it is not raining, but Bret also puts it nicely.

Matt

[beaky]

This explanation might seem more complicated, but bear with me, because it helps understand the effect of various dewpoint spreads very nicely, and helps you anticpate changes on a given day:

Air will hold water in suspension- that is, as vapor. Vapor is not mist or fog or cloud. Normally you can't see it.
The little water molecules are all mixed up with the molecules of various gases we call "air"... and this is possible because of temperature.
The warmer air is, the more its molecules jump and zoom around. They bump into each other, too... producing more heat, and also increasing the space between molecules.

Into these spaces, attracted by molecular forces, rush free water molecules, usually released from their liquid state by the same heat that warmed the air.

This is how stuff dries out.

Okay- now that you see how water can get mixed with air, you see what "humidity" is.

How humid it gets depends on temperature, even more than saturation with water- hence really "muggy" weather. The air can be hard to breathe... it seems thick, but in fact the air-the gaseous stuff- is thinner; its pressure has gone up and it's spreading out. And if there's water vapor in there, the ratio of water to gas in a given volume can get quite high... so there's less air to breathe.
 Your airplane engine notices it, too... although heat-induced air pressure is a greater factor.

But let's say that water-soaked mass of air cools. The air molecules start to settle down, getting closer to their comrades. The water molecules, having weaker attraction to the gas particles than to each other, start to clump together, forming mist or fog. Cloud and fog levels are determined by temperature at various levels, and this is why.

Water droplets form out of air when there is some particle or other solid thing for the water vapor to cling to, where it will condense more easily: a smoke or dust particle (at the heart of every raindrop and snowflake), or perhaps an airplane.

Now you see why dewpoint as well as temperature is an important indicator of icing conditions, also. Due to its higher velovity, slipstream air is usually cooler than the ambient temperature, so even innocent-looking mist can turn to water, then ice, on the surface of wings, etc. very quickly.

This is also why carb ice can occur, even when there is no visible moisture in the air or ice on the airframe. The air rushing into the carb inlet is at a much higher velocity, lower pressure, and thus lower temperature than the air just outside the engine.

Anyway, this point at which dew will form is the point where the temperature is low enough for the existing humidity level to cause formation of mist or droplets as that water is "squeezed" out. It will depend on how much water is present, which is why the dewpoint changes regularly in a given area.


So for flight planning, the closer the current temp. is to the estimated dewpoint temp., the things you watch out for are: poor visibilty and/or clouds, and  carb or airframe ice, including pitot and static ports, etc.

If the dewpoint is getting closer and closer to the ambient temp, all this gets worse. If it's spreading, that means better weather.

[Saitek]

Cheers for that. 8-)

[RitterKreuz]

as you know, humidity is expressed in a percentage such as 70% 100% 10% etc

Dewpoint is simply what the temperature must be in order for the air to reach 100% Humidity.

If the temperature is 20 and the dewpoint is 10 the humidity is 50%

If the temperature is 20 and the dewpoint is 20 the humidity is 100%

the smaller the "spread" between the numbers the greater the level of saturation.

A smaller temperature/ dewpoint spread often translates into rainy/foggy conditions accompanied by low ceilings.

a temp of 20 with a dewpoint of 5  usually equals fair weather

a temp of 20 with a dewpoint of 19 usually equals foggy weather

[Fozzer]

as you know, humidity is expressed in a percentage such as 70% 100% 10% etc

Dewpoint is simply what the temperature must be in order for the air to reach 100% Humidity.

If the temperature is 20 and the dewpoint is 10 the humidity is 50%

If the temperature is 20 and the dewpoint is 20 the humidity is 100%

the smaller the "spread" between the numbers the greater the level of saturation.

A smaller temperature/ dewpoint spread often translates into rainy/foggy conditions accompanied by low ceilings.

a temp of 20 with a dewpoint of 5  usually equals fair weather

a temp of 20 with a dewpoint of 19 usually equals foggy weather

Simplicity at its best... ..... [smiley=thumbsup.gif]...!

Paul...Ex Computer Room Air Conditioning Engineer... 8-)...!

[Saitek]

Yeah, 10/10. Now I know all about dew point!  ;D 8-)
I could have dived into several books at the library and it would have taken me ages to get the same result.