Humidity -vs relative humidity in laymans' terms?

[William_H]

In practical terms humidity affects us and what we do or want in various ways.
In one way when our body is hot we perspire to cool the body. The evaporation and cooling effect depends on temperature of the air and relative humidity. So hot dry air cools us well while hot humid air means moisture can not avaporate easily so we stay hot. Likewise in cold air the relative humidity may be quite high meaning perspiration can not evaporate so we stay damp even though the cold air may cool the body.
In the case of the garage we are concerned that damp things will dry by evaporation and this requires a low relative humidity. Cold air may have a high relative humidity which means it will not take up much moisture so everything stays damp.
Also in the garage if warm air comes in carrying some moisture then when this air cools say at night time the cold air can not carry the moisture and it falls out like dew so things get damp. (from the cold we might imagine)
So to reduce these latter 2 effects in the garage we need to either raise the air temperature which will reduce relative humidity so drying wet things or retaining moisture in the air. However it can be inefficient trying to keep a garage warm when outside is very cold. Better is to remove the moisture from the air. A previous poster has explained how dehumidifiers work. To remove the moisture means the air can be colder but still tend to dry things or at least not drop moisture out of the air.

None of which is much of a problem here with 29 max today 35 tomorrow and low humidity. But I feel for you in UK olewill

 

[danielefua]

I think perhaps you mean water volume reaches a minimum, or density reaches a maximum, and actually at 4 degrees C. But I think you're right about Daydream's misconception.

Of course!!!! My stupid error, I wrote "minimum" but I meant a "maximum" in the density curve vs temperature! ...and of course, you are right about the point being around 4°C.

Daniel

 

[cpedw]

Hoping not to confuse what seems to be an answered question, this http://www.fao.org/docrep/s1250e/S1250EEV.GIF allows humidity and relative humidity to be figured from wet (damp muslin wrapped) and dry bulb temperatures.

Plot the dry bulb temperature on the bottom horizontal axis, with a line going up vertically and the wet bulb temperature on the top left curved line, with a line going horizontally to the right. The intersection gives the relative humidity using the lines curving up from the bottom left. The "absolute humidity" is the vertical axis on the right hand side, labeled Moisture Content g/kg of dry air. It depends only on the wet bulb temperature.

You can see that Moisture Content increases as wet bulb temperature increases - Saturation or 100%RH is wetter at high air temperature.

There's a lot more on the chart but I sense the onset of too much information.

Derek

 

[danielefua]

Hoping not to confuse what seems to be an answered question, this http://www.fao.org/docrep/s1250e/S1250EEV.GIF allows humidity and relative humidity to be figured from wet (damp muslin wrapped) and dry bulb temperatures.

Plot the dry bulb temperature on the bottom horizontal axis, with a line going up vertically and the wet bulb temperature on the top left curved line, with a line going horizontally to the right. The intersection gives the relative humidity using the lines curving up from the bottom left. The "absolute humidity" is the vertical axis on the right hand side, labeled Moisture Content g/kg of dry air. It depends only on the wet bulb temperature.

You can see that Moisture Content increases as wet bulb temperature increases - Saturation or 100%RH is wetter at high air temperature.

There's a lot more on the chart but I sense the onset of too much information.

Derek

Sorry but there are some inaccuracies.
1) the lines of Wet Bulb Temperature are sloping down from left to right.
2) the Water Mixing Ratio (aka Moisture Content g/kg of dry air), plotted with horizontal lines, does not depend on Wet Bulb Temperature only.

Daniel

 

[cpedw]

Sorry but there are some inaccuracies.
1) the lines of Wet Bulb Temperature are sloping down from left to right.
2) the Water Mixing Ratio (aka Moisture Content g/kg of dry air), plotted with horizontal lines, does not depend on Wet Bulb Temperature only.

Daniel

Don't apologize. I started out a bit uncertain but as I looked at the chart it all seemed to become clearer. It's been a long time since I used it in earnest...
Derek

 

[ghostlymoron]

It's like density and relative density. One is absolute and one a value compared with a standard. Nothing to do with the time taken to make your granddads garage drier.