Humidity -vs relative humidity in laymans' terms?

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Sailingsaves

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I am ashamed to admit I have forgotten all I knew about this.

My Dad has a machine that removes moisture from his garage and wants to know if his garage is dry.

He asked his scientist son who now asks you.

Relative humidity is given as a % (see below for Google help)

My best instrument that I gave him was two thermometers, one dipped in muslim in water. No difference in mercury levels means environment is saturated and a big difference means air is drier and accepting evaporation from muslim.

But his dehumidifier machine just states 70% or whatever.

Can anyone explain in it idiot terms for me to understand and then relay to him?

Is my explanation at very end correct?

Thanks

PS how can he lower the % humidity? His mate's garage is 30%.His is 68%. My bedroom is 58%, lounge is often 70% (all different digital instruments).

Warmer air holds more water vapour, so simply heat garage with his woodburning stove whilst using dehumidifier on max power? Then allow garage to cool and then turn machine off once cool?

To get condensation out of my car I run it with heater on full (and air con) and at end of trip, crack open windows to suck moisture laden air out.
cold air holds very little moisture.

Google says:

What's the difference between Humidity and Relative Humidity?

Submitted by uk.sci.weather on Wed, 17/01/2007 - 12:50pm.
Absolute Humidity, often just referred to as 'the humidity', is a measure of the actual amount of water vapour in a particular sample of air: measured as a partial pressure (vapour pressure/hPa or millibars); a mixing ratio (gm water vapour/kg of dry air), dew point etc.

Relative Humidity - expressed commonly as a percentage value, is the ratio of the actual amount of water vapour present in a sample (the Absolute Humidity) to that amount that would be needed to saturate that particular sample.

The two terms are not interchangeable and can lead to confusion; e.g. on a cold, raw winter's day close to the east coast of England, the dew point might be 1 degC and an air temperature of just 2 degC. This would give a RH of 93%; a 'high' Relative Humidity, yet few would refer to such conditions as 'humid'. Conversely, on a hot summer's day, with a dew point of 18 degC, and an afternoon temperature of 30 degC, that's a RH of 49%; a 'low' Relative Humidity, but high Absolute Humidity.



__________________________________________________________________________________________________________________________________

Relative humidity (abbreviated RH) is the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at the same temperature. Relative humidity depends on temperature and the pressure of the system of interest.

Humidity is the amount of water vapor in the air. Water vapor is the gaseous state of water and is invisible.[1] Humidity indicates the likelihood of precipitation, dew, or fog. Higher humidity reduces the effectiveness of sweating in cooling the body by reducing the rate ofevaporation of moisture from the skin. This effect is calculated in a heat index table or humidex, used during summer weather.

There are three main measurements of humidity: absolute, relative and specific. Absolute humidity is the water content of air.[2]Relative humidity, expressed as a percent, measures the current absolute humidity relative to the maximum for that temperature.Specific humidity is a ratio of the water vapor content of the mixture to the total air content on a mass basis.

__________________________________________________________________________
Me again:
"Relative Humidity - expressed commonly as a percentage value, is the ratio of the actual amount of water vapour present in a sample (the Absolute Humidity) to that amount that would be needed to saturate that particular sample."

My understanding of this relative humidity (as machines seem to give) would be:

70% rel hum means 7:10 actual amount of water vap present:amount needed to saturate

so 70% means 7 units of water vapour in air compared to 10 units needed to create saturation.

so 60% is drier air than 70% air.

Conclusion; lower the % number means drier the air?
 
Have you not just answered the question yourself?

Maybe give him an example. Warm summer air would be more humid than cold winter air, but it would have a much lower relative humidity, so would feel dry, whereas the winter air feels damp. The way to demonstrate this is to seal up your boat in August and come back in December.

When the RH gets high, you get mist/dew/condensation.

On the other hand, there's far more water in the air in summer, so a product like Sikaflex that relies on that sets quicker than it would in winter.
 
lpdsn said:
Have you not just answered the question yourself?

Maybe give him an example. Warm summer air would be more humid than cold winter air, but it would have a much lower relative humidity, so would feel dry, whereas the winter air feels damp.
Click to expand...

Went over my head that one.

warm summer air would have more h2o vapour in it, yes.

cold winter air would have less h2o vapour in it, yes.

But you say the summer air would have a LOWER rel. humidity? - Lost me there. Can you explain it in even more dumbed down ways, because I would have thought the summer air would have a HIGHER rel. humidity.

thanks
 
Sailingsaves said:
Went over my head that one.

warm summer air would have more h2o vapour in it, yes.

cold winter air would have less h2o vapour in it, yes.

But you say the summer air would have a LOWER rel. humidity? - Lost me there. Can you explain it in even more dumbed down ways, because I would have thought the summer air would have a HIGHER rel. humidity.

thanks
Click to expand...

(These numbers are just made up to illustrate an example but hopefully they'll illustrate the concept).

Say summer air at 25 degs C can hold five times the water vapour at saturation as winter air at 5 degs C.

Say the summer air has a RH of 20%. i.e. it is holding only a fifth of the water it could when it is saturated at that temperature. It feels very dry to a human.

You cool it down to 5 degs without changing the amount of water it contains. It is now saturated and has a RH of 100%. It feels very damp to a human, but the amount of water in the air has not changed.
 
My understanding is that as the air temperature increases the amount of moisture it can hold also increases ergo for any given amount of water vapour in the air the percentage of max it can hold reduces as the temp increases. As such if you have a kilo of water as vapour, with the warmer air the relative humidity will be lower because the percentage out of the maximum it could hold as vapour will be lower.

Heating the garage will reduce the relative humidity but won't make any difference to the actual amount of water 'held' in the air. In fact in reality it might increase it as the higher temp will allow some to evaporate off damp surfaces thereby increasing the volume of water in the air.
 
Thanks,
But in your first post,would the Winter air not have the lower rel Humidity as it held less water vapour. If not then I still have not understood it yet.

ALSO, can anyone explain (1) better:
Absolute humidity is the mass of water vapor divided by the mass of dry air in a volume of air at a given temperature.

Got it.

(1) "Relative Humidity - expressed commonly as a percentage value, is the ratio of the actual amount of water vapour present in a sample (the Absolute Humidity) to that amount that would be needed to saturate that particular sample." IN THE LATTER PART, IS THAT THE AMOUNT OF WATER NEEDED TO FILL THE REMAINING 30% OR THE AMOUNT OF WATER VAPOUR TO FILL A DRY CUBIC METRE.

OR IN OTHER WORDS: Take 1 cubic metre of air at constant temp. 70% rel hum means 7/10 of the cubic metre of air is wet, compared to "the amount needed to saturate the cubic metre" - so is that compared to 100/100 of moisture needed to saturate 1 cubic metre or compared to the 30/100 left of dry air in the cubic metre that needs to be wet in order to reach saturation?

Giving myslef a headache here and feeling very inadequate.
 
Keith-i said:
My understanding is that as the air temperature increases the amount of moisture it can hold also increases ergo for any given amount of water vapour in the air the percentage of max it can hold reduces as the temp increases. As such if you have a kilo of water as vapour, with the warmer air the relative humidity will be lower because the percentage out of the maximum it could hold as vapour will be lower.

Heating the garage will reduce the relative humidity but won't make any difference to the actual amount of water 'held' in the air. In fact in reality it might increase it as the higher temp will allow some to evaporate off damp surfaces thereby increasing the volume of water in the air.
Click to expand...

Yep, got that. Heating the garage will definitely allow more vapour to be held in suspension (hence why I heat my car and then expel the air to dry it)
"Heating the air will reduce the rel humid" - let me think - same amount of water available, but greater capacity for air to hold more vapour -
"Relative Humidity - expressed commonly as a percentage value, is the ratio of the actual amount of water vapour present in a sample (the Absolute Humidity) to that amount that would be needed to saturate that particular sample."
so as air is hotter, the amount needed to saturate it has increased, so one part of ratio has increased, so from 4:10 we have gone to 4:20 (say) simplifies to 2:10 = 20% rel humidity -
yep, increasing heat of garage lowers rel. humidity from 40% to 20%,

so my Dad's garage could be at a temp of 20degrees C with a rel hum of 70% (and only 1kg of water in the air say), but his mates garage could be 25 degrees C with a LOWER rel hum but still have 1 kg of water in the air?

Am I getting there?
 
Sailingsaves said:
Thanks,
But in your first post,would the Winter air not have the lower rel Humidity as it held less water vapour. If not then I still have not understood it yet.
Click to expand...

Winter air can hold less water vapour. So the same amount of water vapour per m3 in winter gives a much higher relative humidity.

Another practical example: Imagine heating a room then letting it cool (it's winter so it'll eventually get pretty cold). At first, when still warm it is holding less water vapour than it can hold at that temperature, so it has a lower RH. As the air cools, that water vapour has nowhere to go so initially remains in the air.

Sooner or later as it continues to cool the amount of water vapour it can hold reduces and there will come a point where what it is holding is as much as it can hold (i.e. RH has increased to 100%) so after that the water has to condense out as it cools further.
 
Sailingsaves said:
so my Dad's garage could be at a temp of 20degrees C with a rel hum of 70% (and only 1kg of water in the air say), but his mates garage could be 25 degrees C with a LOWER rel hum but still have 1 kg of water in the air?

Am I getting there?
Click to expand...

Yes :)
 
What you have in your post seems about right to me.

To dry your Dad's garage out, is a more complex question, if you heat the air in the garage, water will evaporate from the contents of the garage and the %RH will rise. if you then slowly replace the warm wet air in the garage with cold air (with a lower absolute humidity) which should be heated up as it enters the garage, to make the %RH even lower, the garage will slowly dry-out. But it may take a long time, if the content of the garage is wet.

An alternative is to use a dehumidifier, which cools the air to below the dew point so that the water condenses out and then re-heats the air using the hot part of the refrigeration cycle. But these system only work well in relative high ambient temperatures, because when the air in the garage is cold it carries very little water and the refrigeration side of the de-humidifier often freezes up.

A further alternative is to use a, desiccant dehumidifier, the desiccant absorbs water and in a different part of the cycle, is then heated to drive the water out, the warm wet air is normally exhausted outside the space to be dried. Although desiccant dehumidifiers work better at low temperatures, they don't freeze up, it's better if the air in the garage is warm, simply because it can carry more water.

I'd be looking at placing a small fan heater in an area where it can draw cold air from the outside and warm it up, perhaps just inside a garage door and let the warm wet air exhaust through a partly opened skylight, or the garage may been naturally ventilating through cracks and holes. This is much the same way as a hot air diesel heater works on a boat, assuming that it's not recycling the cabin air.

Note: there is little point in operating the fan heater if it raining or misty, the cold air has to be reasonably dry and cold before a system like this will work.

I'd also be asking questions about whether the garage leaks, if there is any form of vapour barrier in the roof, walls and floors and how often damp or wet material is brought into the garage. It'll obviously be difficult to dry the garage out if water is getting in.
 
Just for the mix & for those that have not nodded off already
Has anyone remembered that whilst cold air holds less moisture than warm air there is a point where as the air gets colder it begins to hold more moisture
This transition happens around 2-3 degrees above freezing I seem to recall ------but grammar school is just a long distant memory & it is just a useless bit of info we were taught & at my age the hard drive gets a bit full & data drops out the bottom ( in more ways than one)
 
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Daydream believer said:
Just for the mix & for those that have not nodded off already
Has anyone remembered that whilst cold air holds less moisture than warm air there is a point where as the air gets colder it begins to hold more moisture
This transition happens around 2-3 degrees above freezing I seem to recall ---....
Click to expand...
Sounds wrong to me:
http://www.kayelaby.npl.co.uk/chemistry/3_4/3_4_2.html
Vapour pressure of water keeps increasing with temp.

Unless you are talking about clouds?
Clouds are water droplets, not water vapour as a gas. Maybe clouds at these temps can hold more water without raining?
Water does odd things at a few degC, it must do to get less dense from 4 to 0 degC.
 
Daydream believer said:
Just for the mix & for those that have not nodded off already
Has anyone remembered that whilst cold air holds less moisture than warm air there is a point where as the air gets colder it begins to hold more moisture
This transition happens around 2-3 degrees above freezing I seem to recall ------but grammar school is just a long distant memory & it is just a useless bit of info we were taught & at my age the hard drive gets a bit full & data drops out the bottom ( in more ways than one)
Click to expand...
I am sorry but I think you are confusing humidity with water density. Indeed water density reaches its minimum around 2 degrees.

Moreover I hope I am not making things more confusing for whoever thought having understood the problem but writing that "warm air is able to contain more humidity" is a misleading sentence from a physical point of view because that phenomenon depends intrinsically on the temperature dependence of the water vapor saturation pressure and has nothing to do with the presence of air in the same volume. For whoever is interested in reading a very well done book on the subject, I advise Craig F. Bohren's "Clouds in a Glass of Beer". Nice and easy reading and you will learn a lot of helpful things.

Daniel
 
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