Atmosphere Question...

[redozbris]

I dont want to doubt someone elses work, but to me this seems like an incorrect statement Pg 19 Annex A Fly Better - Noel Kruse

 

"The International Standard Atmosphere starts at sea level with a temperature of

 

 

15ºC (Celsius) and a pressure of 1013.2Hp (Hectopascals) or 29.94 Inches of

 

 

 

 

 



Mercury in the USA. A cubic foot of air under these conditions weighs 0.0765

 

 

 

 

 



lb and has a Density of .002376 ‘Slugs’ per cubic foot. The amount of water

 

 

 

 

 



vapor in the atmosphere causes a slight variation to this weight, ‘damp’ air being a little lighter than ‘dry’ air, but we are going to ignore this difference as it has no significant effect on aircraft performance."

 

 

 

 

 

 

If the air is dry, therefor its free of vapor (H20) and therefor should be lighter, than damp air? Or am I missing something basic... (I know I am..)

 

 

 

Thanks All :bad computer:

 

 

 

Red

 

 

 

[motzartmerv]

Howdy,

 

Humid or moist air is lighter then dry air because a water molecule is smaller than a nitrogen or oxygen molecule(which is what the 'air' is made of)

 

cheers.

 

 

[redozbris]

Really... I honestly thought it would be the other way around.... Fare enough, but that makes sense, if you have a larger molecule, displacing other molecules, then it would be lighter... it just seems counter intuitive...

 

Thanks gents...

 

 

[Ultralights]

Hot and humid = bad, cold and dry = good.

 

 

[Guest Crezzi]

Its been a while since my chemistry days but the relative dimensions of the molecules isn't a factor - there are lots of gaps between them in the atmosphere anyway. A water molecule is lighter because it has an atomic mass of 18 (16+1+1) compared to an oxygen molecule of 32 (16+16) or nitrogen at 28 (14+14). Hence a higher proportion of water reduces the weight.

 

Although O2 is heavier than N2 it has a smaller effective molecular radius - a tyre filled with air (containing oxygen) will go flat faster than if filled with nitrogen as the smaller molecules leak out easier.

 

Cheers

 

John

 

 

[facthunter]

Universal gas equation.

 

" Equal volumes of all gases contain the same number of molecules , under the same conditions of temperature and pressure". This will be the way of arriving at the density, by taking molecular weight into account. Water is in gaseous (vapour) form in this instance. Nev

 

 

[motzartmerv]

Crezzi, relative dimensions isn't a factor??...then in the same post "Although O2 is heavier than N2 it has a smaller effective molecular radius"....????.... Confused much??...

 

 

[horsefeathers]

Basically, just refer to Avagadros law.... (warning - science content ahead)

 

Avogadro's law (sometimes referred to as Avogadro's hypothesis or Avogadro's principle) is a gas law named after Amedeo Avogadro who, in 1811, hypothesized that two given samples of an ideal gas, at the same temperature, pressure and volume, contain the same number of molecules. Thus, the number of molecules or atoms in a specific volume of gas is independent of their size or the molar mass of the gas.

 

 

 

For example, 1 litre of molecular hydrogen and 1 litre of nitrogen contain the same number of molecules when they are at the same temperature and pressure.

 

 

[kaz3g]

Water (H2O) has a molecular weight of 18

 

Nitrogen (N2) has a MW of 28

 

Oxygen (O2) has a MW of 32

 

Water vapour is invisible and "floats" or rises in air.

 

Clouds are accumulation of minute water droplets or ice crystals - the vapour has condensed or has undergone sublimation.

 

If the drops/crystals aggregate (deposition) they become heavier than air and fall as rain, sleet, snow.

 

kaz

 

(Personal explanation:

 

1. I'm old

 

2. I started my working life as a chemist

 

3. I later became a biologist

 

4. I became a lawyer even later still and remain one

 

5. In between messing about with professions and more study I went farming and ran pastoral properties)

 

 

[Guest Crezzi]

Crezzi, relative dimensions isn't a factor??...then in the same post "Although O2 is heavier than N2 it has a smaller effective molecular radius"....????.... Confused much??...

Water molecule isn't lighter because its smaller.

 

Water is lighter because it has a lower molecular weight.

 

Size & weight aren't the same thing .

 

A smaller molecule can weigh more than a larger one (eg O2 and N2).

 

Hopefully reduced your confusion ?

 

Cheers

 

John

 

 

[motzartmerv]

Lol, yep, thankyou..

 

 

[Guest Kevin the Penniless]

basically, just refer to avagadros law.... (warning - science content ahead)avogadro's law (sometimes referred to as avogadro's hypothesis or avogadro's principle) is a gas law named after amedeo avogadro who, in 1811, hypothesized that two given samples of an ideal gas, at the same temperature, pressure and volume, contain the same number of molecules. Thus, the number of molecules or atoms in a specific volume of gas is independent of their size or the molar mass of the gas.

 

 

 

For example, 1 litre of molecular hydrogen and 1 litre of nitrogen contain the same number of molecules when they are at the same temperature and pressure.

my brain just exploded!

 

 

[facthunter]

Thought I said that in my post #7 ( in simpler terms). Nev

 

 

[Methusala]

Isn't that what they call a mole?

 

 

[horsefeathers]

Isn't that what they call a mole?

Nup

 

A mole is the amount of pure substance containing the same number of chemical units as there are atoms in exactly 12 grams of carbon-12 (i.e., 6.023 X 1023 - ie avagardos number)

 

 

[Guest Andys@coffs]

Hmmm so there is the scientific explaination as has been covered in the last pages of post....and then there is the observation thing.... Clouds are up high. If water vapour was heavier than air then I guess they'd all be down at ground level.... And that would make VFR truely a bugger!

 

Andy

 

 

[Methusala]

And, with humble respect (adding that you are quite correct Horsefeathers) I believe that the concept of molar quantities is so abstract and bears so little relevance to flying, everyday life, why clouds are up there and not down here (thanks Andy for that) and my prospects for a good night's sleep that I'm going to forget the whole thing right now! Good night all, Don

 

 

[redozbris]

Thanks all... I think I've got my mind around this one :) More questions to follow....

 

 

[facthunter]

Just a small point with Andy's. Water vapour is invisible. Clouds are water drops of various sizes having condensed after 100% relative humidity is reached.( or ice crystals, in the case of cirrus clouds). Nev..

 

 

[motzartmerv]

Lol, fair dinks fellas. KISS principle.... Students will be running for the hills reading this thread.

 

 

[facthunter]

Maybe they will Merv, but it is right on the thread. The initial question was exactly what we are on about now, so there is a need to answer it correctly. It's not compulsory and there are other than students on the forum. (Though we are ALL students). Perhaps not ab-initio. Nev

 

 

[Guest Andys@coffs]

Nev

 

Accept that, however it doesnt change the fact that in general clouds are high and if they come from water vapour then it must be up there otherwise the clouds wouldnt be, unless they formed on the ground and then rose up...which observation generally doesnt support

 

Andy

 

 

[facthunter]

Water vapour can be anywhere in the atmosphere. As the air mass gets colder (environmental lapse rate), with increasing altitude condensation may occurr there. The base of the cloud represents the point where the vapour condenses forming water droplets, and becomes visible. The change of state from vapour to water gives up heat ( Heating the parcel of air so it rises) When water goes to vapour ( evaporates) heat is used up. ie. The opposite happens Like when we sweat, and become cooler. Cloud forming at ground level is called fog. In that case the heat is lost by radiation to the atmosphere.to reach dew point.(most common method.) Nev