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Dewpoint vs Relative Humidity....spread?


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OK, newbie question here.

 

I am in the middle of my weather ground training. I keep going back to dewpoint and relative humidity. I've read al about it in "Aviation Weather". I've studied the diagrams. I just don't get it.

I DO understand that the atmosphere can hold water (duh).

I guess what I need help in, is how that applies to a/c....how does it affect performance? WHY is this important to me?

 

 

 

 

 

Confused.

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Hi there,

 

Here's a brief answer for you; think of air as being made up of air molecules, dust, water, bugs, whatever. As an airfoil moves through the air, it creates lift by reacting with the air molecules flow over it...it stands to reason that a certain amount of air molecules creates a certain amount of lift.

 

Now if the relative humidity (water vapour) increases in the air, it pushes the air molecules aside, and when the airfoil comes zooming along and finds there's more water than it's used to, or less air, then it has to work harder to get the same amount of lift.

 

So an increase in humidity in the air means a decrease in performance of the airfoil, and an increase in power demand.

 

Also as a result of the increase in water vapour in the air, in an R22 or reciprocating engine, there are less air molecules entering the carburettor. This also decreases performance in the engine.

 

Further, if you don't lean the fuel mixture, then the ratio of air/fuel changes to a 'rich' condition (more fuel and less air), which can have a tendency to foul the spark plugs...which could cause an engine failure and zero performance!

 

Hope that helps, tattooed one.

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I forgot to mention that dewpoint and temperature spread are good indicators of ice formation, both structural and intake, and whether you can expect fog or clouds to occur.

 

Usually if the dewpoint is 2ºC or less (5ºF), you can expect fog and/or low clouds. If other conditions are conducive to icing, then you can expect ice to form on or in the heli.

 

Ice on the airfoil changes the shape of the airfoil and increases drag, which of course lessens performance.

 

I think I've answered the question, now, right?

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Yes, that does help alot, and makes perfect sense.

 

(Now I have to edit my post since your reply posted faster than mine!).

 

 

Yes, all very helpful and exactly what I needed to understand. Makes perfect since. I just could not figure out how this applied to real life.

Now after reading your explaination.......I am now saying to myself...."Well, DUH!!!!"

 

Thanks!

 

 

Any and all info greatly appreciated!!!

Edited by tattooed
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Usually if the dewpoint is 2ºC or less (5ºF), you can expect fog and/or low clouds.

 

Thats the answer. Expect fog when you have that low (2 or 3degrees) of a dewpoint. I usually describe this situation as thinking of air in a bottle. More humidity or water means less room for air.

 

Great explanations michael ...

 

Goldy

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Yes, that does help alot, and makes perfect sense.

 

Now, where does dewpoint come in? ATIS gives dewpoint and "spread". How shalt I use such glorious information?

 

Dewpoint is the point or Temp at which the current level of humidity needs to drop to/reach for the water vapor to condensate on a surface or around a nuclei. IE is it is 65f and the dewpoint is 45f the temp would need to drop to 45 before the vapor would become liquid and either drop dew on the ground, collect on a surface or form clouds fog etc on any form of condensation nuclei. The point spread is a concern in carb'd engines, like an R22, in that if the dewpoint and the current temp is with in 15 degrees there is a HIGH risk of carburetor ice to form if you do not apply carb heat.

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Thats the answer. Expect fog when you have that low (2 or 3degrees) of a dewpoint. I usually describe this situation as thinking of air in a bottle. More humidity or water means less room for air.

 

Clarification req'd....

 

When you say "2 or 3 degrees of a dewpoint"....do you mean the OAT being 2-3 degrees, or do you mean 2-3 degress "spread" from the dewpoint?

 

 

(ain't it interestin' what concepts your brain will and won't "wrap around"? This ain't one of dem)

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Ok, so you know that the atmosphere can hold water. The amount of water it can hold varies with temperature. Cold air is a small glass, warm air is a big glass. Relative humidity and dewpoint are both ways of looking at how much water is in the air.

 

Relative humidity is a measure of how much water is in the air vs how much it can hold (how full is the glass?). It is a measure of saturation--air that is saturated (full glass) has an RH of 100%. Put more water in, and the glass overflows (you get precipitation). You care about RH because it tells you how close you are to getting condensation in the form of icing, fog/clouds, T-storms. Low RH means not too much water in the air, and low risk for condensation. It also tells you what you get for a given chunk of air: high RH means you get more water and less oxygen than the same chunk of air at low RH.

 

Dewpoint is how much you'd have to cool the air to make it 100% saturated. The amount of water air can hold varies with temperature, and colder air holds less water than warmer air. Dewpoint is how much you'd have to shrink the glass (cool the air) to get it to overflow (precipitate).

 

Both RH and dewpoint are different ways of looking at the same thing, but each tells you something slightly different. High RH can be an indication that the air is less dense (water displaces some of the oxygen), which means piston engines will suffer. Maybe not so good an indication on a below freezing day, when the air density is already high and the amount of water the air can hold is low already. Dewpoint tells you how much cooling you need for condensation--that cooling can come from air rising, surface cooling, air flowing over your airfoil, or air getting sucked into the carb. When temp and dewpoint are close, you don't need much cooling to occur before you get condensation, in the form of clouds, fog, precipitation, structural icing, or carb icing.

 

ATIS usually gives you the temp and dewpoint. If it's warm and the temp-dewpoint spread is close (I don't know what this would be since I'm flying where it's bone dry), you know you have a big glass that's pretty full, so you can expect a decrease in engine performance (you're pouring more water into the engine and less oxygen). When there's a small temp-dewpoint spread (11 degrees in the R-22/44 POH), you are at risk for carb ice and need to follow Frank's recommendations for dealing with that. When the temp-dewpoint spread is small (

 

"if you don't lean the fuel mixture" --> in the -22/-44, you don't want to be leaning the mixture in flight (altho this is what you'd do in other a/c).

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I GOT IT, I GOT IT!!!! DING DING DING. (BIG lightbulb goes on over my head).

 

 

Thanks SOOOO much for all the explainations. I cannot explain why it just wasn't sinking in. Got it now. Makes perfect sense.

 

 

Phew. Now I can relax and go back to fregin' clouds.

Yah know, I actually took a semester of meteorology in HS. I obviously forgot it all.

 

 

KODOZ, I feel like Ralphie: "Dewpoints taste good."

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Relative humidity is a measure of how much water is in the air vs how much it can hold (how full is the glass?). It is a measure of saturation--air that is saturated (full glass) has an RH of 100%. Put more water in, and the glass overflows (you get precipitation). You care about RH because it tells you how close you are to getting condensation in the form of icing, fog/clouds, T-storms. Low RH means not too much water in the air, and low risk for condensation. It also tells you what you get for a given chunk of air: high RH means you get more water and less oxygen than the same chunk of air at low RH.

 

That's a great example, and the one I use with my students. not to confuse anyone more but for those that would like to know how they come up with Relative Humidity.

 

It is the ratio of Water vapor actually in the air to the maximum amount of water vapor required for saturation at that temperature and pressure.

 

RH= Water vapor content / Water vapor capacity

 

in terms of Vapor pressure, we'll say the air holds 100% pressure, (which it does). So

 

RH = (actual vapor pressure / Saturation vapor pressure) X 100%

 

It can also be expressed in Mixing ratios but I'll save you all from that.

 

EXTREME SCIENCE CONTENT, NOT FOR THE FAINT OF HEART BELOW THIS LINE!!! Please don't read unless you want to be really confused.

 

Now for the reason performance is degraded, the main reason is that Humid air is less dense. Here's why.

Givin the same temperature and pressure of two air masses, the one with more water vapor weighs less because. Thinking about atoms, for all intensive purposes the weight is located in the nucleus where the protons and neutrons are. Electrons don't weigh anything in this example so we will trow them out. To get weight of an atom we add the weight of protons and neutrons. So Hydrogen has 1 proton and no neutrons, so it's atomic weight is 1. Nitrogen weighs 14, Oxygen 16.

Speaking about atmospheric gasses, Oxygen (O2) weighs 32 (two Oxygen molecule's). Nitrogen (N2) 28.

 

Assuming dry air being 99% Oxygen (21%) and Nitrogen (78%)

 

Oxygen Weighs 16 X 2 atoms = 32

32 X 21% = 7

Nitrogen Weighs 14 X 2 atoms = 28

28 X 78% = 22

 

7 + 22 = Molecular weight of dry air = 29

 

Now water vapor has 2 atoms of Hydrogen and one Oxygen. 2 + 16 = Molecular weight of water vapor = 18

 

So if we had two air masses both the same volume, and both the same temperature/pressure. Let's say both masses have 50 molecules of dry air.

 

50 X 29 = Molecular weight of our mass = 1450

 

But now let's replace 30 of those dry air molecules in the second mass with water vapor.

30 X 18 = Molecular weight of our water vapor in the mass = 540

20 X 29 = Molecular weight of our dry air in the mas = 580

Water vapor 540 + Dry air 580 = Molecular weight of moist air mass = 1120

 

1450 - 1120 = difference in weight between air mass's = 330

 

END OF SCIENCE CONTENT

 

Now I realize that was really in depth and way beyond anything anyone would possibly ever want to know, but just in case you ever woundered why. There you have it.

 

I'm really not that much of a dork, I swear!

Edited by AngelFire_91
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I tried to give a disclamer... Maybe it needs to include something about accidental explosions of the brain. Sorry if I really confused you more. This isn't how I teach usually, unless someone really wants to know exactly why, but I figured it was good information if someone DID want to know.

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Air is made up of many different gases. Nitrogen is the primary gas, and oxygen, carbon dioxide, and others are also components. Water vapor is another gas, just like the others, so it's just another part of the air. Water vapor, however, is lighter than most of the other gases, because hydrogen is the lightest element that exists, and 2/3 of a water molecule is hydrogen. Thus, the more water is in the air, the lighter and less dense it is. Less density means less lift is produced for the same airfoil at the same angle of attack. Increasing altitude also means less density, and less lift. This relationship is known as density altitude - the altitude at which dry air would produce the same lift as the air that exists (temperature is also a factor, of course, but ignored for this discussion). The other thing that dewpoint affects is visibility. Air can only hold so much water vapor at a given temperature, and at some point the water changes from vapor to liquid. This occurs near the dewpoint, but not necessarily at exactly the dewpoint, depending on many factors. The things to remember is that wet air is generally bad - it produces less lift, and is likely to reduce visibility, either through haze or even dense fog. Humid air also means baseballs fly further, and curve balls break less. Humid air in Denver results in a lot of home runs.

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At times that seems backward. If you look at it, it would seem that if the air was more humid-more water-then the blade would work better because it's going through a liquid that's denser than the air, kinda like a boat prop.

 

Counterintuitive huh.

 

Later

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For the students who remind me of Ralphie, I have to use the "sponge" example when talking about dew point. Think of the sponge as a parcel of air. Relative humidity is how wet the sponge is. Your hand squeezing the sponge is cold contracting the air/sponge. A dry sponge sitting on the top of the sink for a day needs to be squeezed a lot to get any visible water out of it. A wet one that's just been used to clean your ramen noodles bowl doesn't need to be squeezed much at all to get water out of it.

 

Over simplified, but sometimes that's what it takes!

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At times that seems backward. If you look at it, it would seem that if the air was more humid-more water-then the blade would work better because it's going through a liquid that's denser than the air, kinda like a boat prop.

 

Counterintuitive huh.

 

Later

I've always thought it was backwords, it seems exactly like you said, you would think water vapor would make it thicker, and eaiser to grab like a boat prop. But that's how I keep it straight, I just have to remember that it is backwords and it's not that way.

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Water vapor is not liquid water, and doesn't have the same properties. Water vapor is a gas, and acts like a (rather light) gas. Try throwing a ball through water, or even walking through water. It's not nearly the same as in air, and even if it's raining hard, it's still mostly air, not liquid.

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