Topic Options
#1451520 - 04/02/2018 14:24 humidity and dew point
liberator Offline
Weather Freak

Registered: 28/11/2010
Posts: 309
Loc: Kyabram
Ok sorry if this seems like a basic question to most of you knowlegable folks.

I've checked the glossary and beleive I do understand the difference between dew point and humidity.

I'd just like some input into how each can impact a persons level of comfort. I know in high humidity summer days "sultry" can be quite uncomfortable and this is due to high levels of moisture in the atmosphere that limits your ability to cool through sweating. I've seen dew point mentioned a lot on here so whats the relationship between these two parameters and how cool we do or dont feel againt temperature?

I've searched the forum but too many hits to find a direct answer to my query.

Top
#1451527 - 04/02/2018 15:33 Re: humidity and dew point [Re: liberator]
Blair Trewin Offline
Weatherzone Addict

Registered: 13/07/2001
Posts: 3741
Loc: Melbourne, Australia
The basic starting point here is that the amount of moisture the air can hold gets greater the higher the temperature of the air is.

From that point:

- relative humidity is the amount of moisture in the air, as a percentage of the maximum possible moisture at that temperature

- dew point is the temperature, with the current moisture content of the air, that you would need to cool the air to for dew to start to form - it has a one-to-one relationship with the total moisture content.

What this means is that, if the moisture content of the air stays the same but the temperature rises, the relative humidity will drop (because you're dividing by a larger number). This also means that 20 degrees with 50% humidity feels reasonably dry, but 35 degrees with 50% humidity is seriously humid - in the latter case, the total moisture content has gone up dramatically. (As for the idea of 30 degrees with 90% humidity, that almost never happens in Australia, even in the far northern tropics).

Top
#1451529 - 04/02/2018 15:46 Re: humidity and dew point [Re: liberator]
liberator Offline
Weather Freak

Registered: 28/11/2010
Posts: 309
Loc: Kyabram
thanks Blair - well explained

Top
#1451567 - 04/02/2018 20:18 Re: humidity and dew point [Re: liberator]
Surly Bond Offline
Weatherzone Addict

Registered: 23/08/2003
Posts: 2085
Loc: Manilla, near Tamworth NSW
There was extensive discussion in an earlier thread about how human comfort and stress relates to relative humidity and dew point. Blair contributed to it.
The relationship is not nearly as simple as thought.

http://forum.weatherzone.com.au/ubbthreads.php/topics/899168/The_use_and_abuse_of_relative_humidity
_________________________
Data are cheap; information is expensive!

Top
#1451627 - 05/02/2018 09:03 Re: humidity and dew point [Re: liberator]
retired weather man Offline
Weatherzone Addict

Registered: 01/07/2007
Posts: 4417
Loc: Wynnum
Basically, the higher the dew point, the more discomfort it causes. A dew point above 25C starts to cause issues.

In Townsville while I was on duty in Feb 1991 ( or 2 ) Townsville reached a dew point of 29 deg C. twice in one afternoon on the electronic instruments. The temp was 32C. I went and checked the wet and dry thermometers and the reading was correct. Cairns and Willis Island had dew points of 28C the same day.
_________________________
Wyn Nth 2018-Jan12.2(158),Feb72.0(146)YTD84.2(304),

Top
#1451632 - 05/02/2018 09:21 Re: humidity and dew point [Re: liberator]
Brett Guy Offline
Meteorological Motor Mouth

Registered: 05/10/2010
Posts: 5009
Loc: Bently Park, Cairns
Just wondering if anybody knows the actual amount of water that the air can hold in any given temperature Say mls or litres per square metre?


Edited by Brett Guy (05/02/2018 09:21)

Top
#1451643 - 05/02/2018 10:12 Re: humidity and dew point [Re: liberator]
Mike Hauber Offline
Weatherzone Addict

Registered: 13/07/2007
Posts: 2930
Loc: Buderim
See Wikipedia Article

Chart:



Air is about 1.2g per meter cubed, so if you call this close enough to 1, then the above chart will tell you how many grams of water per square meter for the lowest 100 meters of atmosphere (i.e. up to about 3 grams around 30 deg C). Temperature goes down as you go up, so its hard to get a good idea of the total amount in the atmosphere. I'd guess equivelant of maybe 3 kilometers or so, which would be 90 grams per meter squared at 30 degrees. Thats 0.09 mm of rain equivelant, so I I've lost track of a few decimal points somewhere (Highly saturated atmosphere can be 70mm or more)

Top
#1451699 - 05/02/2018 20:43 Re: humidity and dew point [Re: liberator]
Seira Offline
Meteorological Motor Mouth

Registered: 27/08/2003
Posts: 7311
Loc: Adelaide Hills.
My understanding is the change in saturation vapour pressure of the troposphere with change in temperature (de[s]/dT) is called the Clausius-Clapeyron Equation/Equality. It can be written in many ways, but the principle is the same. The higher the air temperature, the more water-vapour can be sustained in a given volume of air. However, this does not mean sustained necessarily equals the actual vapour pressure.


Edited by Seira (05/02/2018 20:50)
_________________________
Silence can be beautiful because it is an avenue to peace of mind.

Top
#1451701 - 05/02/2018 20:54 Re: humidity and dew point [Re: liberator]
Mike Hauber Offline
Weatherzone Addict

Registered: 13/07/2007
Posts: 2930
Loc: Buderim
Found the lost decimal points. Air is 1.2 kg per meter cubed, not 1.2 g. So 3% = roughly 3kg per meter cubed, and 30 degrees through 3km of atmosphere would be 90mm of rain.

Top
#1451705 - 05/02/2018 21:19 Re: humidity and dew point [Re: liberator]
Brett Guy Offline
Meteorological Motor Mouth

Registered: 05/10/2010
Posts: 5009
Loc: Bently Park, Cairns
Thank you very much Mike and Sierra. Hard to fathom 3 litres(kg) of water in 1m3 of air. No wonder DP's of 28-29deg feel so oppressive. You are nearly reaching that amount of water suspended in the atmosphere

Top
#1451721 - 06/02/2018 08:02 Re: humidity and dew point [Re: liberator]
Graham M Offline
Weather Freak

Registered: 21/01/2002
Posts: 392
Loc: Boambee East near Coffs Harbou...
I personally prefer absolute humidity as an indicator of comfort. Nice when it's under ten grams per cubic metre, pretty awful when it's around twenty (as it often is in summer here). I've written software for my AWS that displays relative and absolute humidities.

Top
#1451725 - 06/02/2018 08:54 Re: humidity and dew point [Re: Brett Guy]
Brett Guy Offline
Meteorological Motor Mouth

Registered: 05/10/2010
Posts: 5009
Loc: Bently Park, Cairns
Originally Posted By: Brett Guy
Thank you very much Mike and Sierra. Hard to fathom 3 litres(kg) of water in 1m3 of air. No wonder DP's of 28-29deg feel so oppressive. You are nearly reaching that amount of water suspended in the atmosphere


No. 3kg doesn't sound right. I think we are talking 30grams or maybe 300 grams?

Top
#1451731 - 06/02/2018 09:58 Re: humidity and dew point [Re: liberator]
Mike Hauber Offline
Weatherzone Addict

Registered: 13/07/2007
Posts: 2930
Loc: Buderim
lol. Lose 3 decimal points. Then when I find them I add another two in....

Should be 30 grams. 3kg/m2 for lowest 100 meters.

Top
#1451856 - 07/02/2018 20:41 Re: humidity and dew point [Re: liberator]
Seira Offline
Meteorological Motor Mouth

Registered: 27/08/2003
Posts: 7311
Loc: Adelaide Hills.
1 mm of liquid water distributed evenly over a square metre has a weight of 1 kg.
_________________________
Silence can be beautiful because it is an avenue to peace of mind.

Top


Who's Online
9 registered (slipperyfish, Raindammit, menthurae, Willraja, Un_stable, morts, ashestoashes, Weather_Viewer, 1 invisible), 345 Guests and 4 Spiders online.
Key: Admin, Global Mod, Mod
Today's Birthdays
Jinda, Melkizadec, Silver Lining
Forum Stats
29495 Members
32 Forums
23819 Topics
1480873 Posts

Max Online: 2925 @ 02/02/2011 22:23
Satellite Image