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

Registered: 28/11/2010
Posts: 319
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.

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

Registered: 13/07/2001
Posts: 3835
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).

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

Registered: 28/11/2010
Posts: 319
Loc: Kyabram
thanks Blair - well explained

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

Registered: 23/08/2003
Posts: 2144
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
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#1451627 - 05/02/2018 09:03 Re: humidity and dew point [Re: liberator]
retired weather man Offline
Weatherzone Addict

Registered: 01/07/2007
Posts: 4831
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.
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#1451632 - 05/02/2018 09:21 Re: humidity and dew point [Re: liberator]
Brett Guy Offline
Meteorological Motor Mouth

Registered: 05/10/2010
Posts: 5118
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)

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

Registered: 13/07/2007
Posts: 3436
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)

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

Registered: 27/08/2003
Posts: 7637
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)

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

Registered: 13/07/2007
Posts: 3436
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.

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

Registered: 05/10/2010
Posts: 5118
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

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

Registered: 21/01/2002
Posts: 410
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.

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#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: 5118
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?

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

Registered: 13/07/2007
Posts: 3436
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.

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

Registered: 27/08/2003
Posts: 7637
Loc: Adelaide Hills
1 mm of liquid water distributed evenly over a square metre has a weight of 1 kg.

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#1473901 - 17/10/2018 00:13 Re: humidity and dew point [Re: liberator]
karlw Offline
Cloud Gazer

Registered: 08/10/2018
Posts: 1
I'd like to chime in on this. I've been studying these concepts for a while and have come to my own conclusion that relative humidity AND dew point influence comfort.

Hypothetical situation 1 - Hold the relative humidity at a constant 50%.
scenario #1 - temp is 20 deg C (which means a dew point of 9.26 which rates as fairly dry)
scenario #2 - temp is 40 deg C (which means a dew point of 27.6 which rates as uncomfortably "muggy")
In scenario #2 there is far more water vapour in the air (higher dew point) yet the relative humidity is the same as in scenario #1.
We probably expect to feel less comfortable in scenario #2 because of the higher dew point. There is more water vapour in the air (too much to be comfortable) so this makes sense.
This means that dew point influences comfort.


Hypothetical situation 2 - Hold dew point at 9.3 (thereabouts), which rates as fairly dry.
scenario #1 - temp is 20 deg C (which means relative humidity of 50%)
scenario #2 - temp is 40 deg C (which means relative humidity of about 16%)
In scenario #2 the relative humidity is much lower yet the dew point is the same as in scenario #1.
Dew point (water vapour in the air) in both scenarios is the same but you would think that scenario #2 is going to feel drier (because evaporation from body is more effective with lower relative humidity, yes?).
This means that relative humidity influences comfort.

So both dew point AND relative humidity (along with other factors like wind, etc) influence comfort, not one OR the other.

When we heat our house in winter, we are heating air that generally has a low dew point.
When the temp inside the house rises, the relative humidity theoretically falls.
Does it become less comfortable (too dry) because of this (lower relative humidity), even thought the dew point hasn't changed?

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#1473949 - 17/10/2018 18:46 Re: humidity and dew point [Re: liberator]
Petros Offline
Meteorological Motor Mouth

Registered: 30/12/2002
Posts: 7540
Loc: Maffra, Central Gippsland, Vi...
KarlW, google yourself up an AIRAH Psychrometric chart and print it out. Invaluable.

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#1474019 - 18/10/2018 08:42 Re: humidity and dew point [Re: Petros]
retired weather man Offline
Weatherzone Addict

Registered: 01/07/2007
Posts: 4831
Loc: Wynnum
Originally Posted By: Petros
KarlW, google yourself up an AIRAH Psychrometric chart and print it out. Invaluable.


Or look up https://www.kwangu.com.work/psychrometric.htm - and fill in temp and RH ( or DP ) and the rest is displayed for you.


Edited by retired weather man (18/10/2018 08:46)
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#1474322 - 21/10/2018 12:02 Re: humidity and dew point [Re: retired weather man]
Petros Offline
Meteorological Motor Mouth

Registered: 30/12/2002
Posts: 7540
Loc: Maffra, Central Gippsland, Vi...
Originally Posted By: retired weather man
Originally Posted By: Petros
KarlW, google yourself up an AIRAH Psychrometric chart and print it out. Invaluable.


Or look up https://www.kwangu.com.work/psychrometric.htm - and fill in temp and RH ( or DP ) and the rest is displayed for you.


I presume that the calculator assumes MSLP of 1015 hPa?


Edited by Petros (21/10/2018 12:02)

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#1474401 - 21/10/2018 17:33 Re: humidity and dew point [Re: Petros]
retired weather man Offline
Weatherzone Addict

Registered: 01/07/2007
Posts: 4831
Loc: Wynnum
Originally Posted By: Petros
Originally Posted By: retired weather man
Originally Posted By: Petros
KarlW, google yourself up an AIRAH Psychrometric chart and print it out. Invaluable.


Or look up https://www.kwangu.com.work/psychrometric.htm - and fill in temp and RH ( or DP ) and the rest is displayed for you.


I presume that the calculator assumes MSLP of 1015 hPa?


Don't know Petros. I only use this site to get the wet bulb temp as one site I put my obs onto wants this, despite me adding RH and DP anyway. I also put in my height above MSL ( 60 ft ).

BTW the so called world MSL average is 1013.2.
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#1474411 - 21/10/2018 18:27 Re: humidity and dew point [Re: liberator]
Petros Offline
Meteorological Motor Mouth

Registered: 30/12/2002
Posts: 7540
Loc: Maffra, Central Gippsland, Vi...
Thanks RWM, yep my Psychrometric Chart does have 101.325 kPa as the reference point (my laziness/bad memory in previous post).

I have no idea how much impact a baro pressure of say, 1025 hPa would place on the accuracy of my chart/your site calculator. Maybe something to churn over in the head if I have trouble sleeping tonight
wink

edit: I miss my wetbulb thermometer (got broke), have now a hand held electronic meter (cheap, $60 delivered from Ebay/China), very accurate, but far less fun.


Edited by Petros (21/10/2018 18:36)

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