Humidity

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This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

See also Electronics notes/humidity sensing

Humidity and Relative Humidity

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

Humidity refers to water vapour present in a volume.


Specific humidity is the mass of vapour in a mass of air.

It seems like a more direct measurement, yet is not that helpful to quesions like "is there mist" or "how well can I sweat" or most others, because that depends on further details - primarily how much mass of vapour air can contain right now.


Relative Humidity (RH) is more useful, but also more complex.

It relates the humidity to the dew point. The dew point is the saturation point of water in air - basically the point at which mist/clouds will happen and condensation onto things almost automatic.

Put another way, the RH depends on both amount of vapour and temperature.

If you vary amount of water, you get proportionally higher/lower RH
if you change temperature, the amount of water the air can hold changes
...which is basically the same thing as saying the dew point changes


Technically the dewpoint also varies with temperature, but its variation is usually considered to be constant enough to be negligible, at least in weather summaries.

This also means that dew point is often reported as a temperature (a practical simplification, if physically not entirely correct) which can be slightly confusing.



On relative humidity values:

  • Below perhaps 30% RH air is dry enough that moisture is easily moved into the air
    • clothes dry quickly
    • mold has little chance
    • chapped lips, dry mouth, dry skin, sore eyes, and nosebleeds are more likely
    • static electricity builds up more easily -- because it won't discharge in smaller amounts as easily as in humid environments
  • Books and other paper materials are often stored in at somewhere between 30% and 50% RH, at a lowish temperature.
Lower RH is too dry for paper (verify)
  • Above 50% you start to see easier condensation onto cold surfaces
  • moist heat feels warmer than dry heat largely because it makes sweating less effective
  • moist cold feels colder than dry cold largely because condensing moisture conducts out heat faster
  • Throughout the world, average RH is roughly (verify)
    • 0-40% in desert(-like) regions
    • 40-50% in dry inland regions
    • 60-70% in moderate regions
    • 70-90% in above and near seas, in relatively wet climates, wet regions, rainforests
    • 80-100% on islands and near seas, and in some very wet areas
    • seasonal differences is easily 10-20%, varying a little per region and climate type (verify)
    • Note that local climate types and some other influences have large effects
  • RH varies throughout the day. How much differs per climate, but can easily be 50%


Dew point

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

The dew point is a saturation point: the point at which the volume is saturated with water vapour that it can hold at current conditions.

In terms of RH, that describes 100%.

In terms of actual conditions, this depends on pressure and temperature, but is usually reported as a temperature (under the assumption of typical barometric pressure levels).


Note that all this does not (directly) depend on the presence of air.

While the saturation point does vary with what the mexture as a whole is (...and despite that dew point is also referred to as 'full saturation level' and similar terms...), the saturation refers to volume, not to the mixture; there is no direct interaction, and you can just as easily have a relative humidity of water in a vacuum, as water in air.

Controlling humidity

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)
  • Calcium Chloride (CaCl2) (chemically a salt)
deliquesces[1] (when there is a lot of water)
can be reused by dehydrating (e.g. exposing maximum surface area to dry air, or with gentle heat)
http://en.wikipedia.org/wiki/Calcium_chloride
(verify): Usually contains CaO, so is basic. So not the best choice when preserving acidic compounds
  • Silica Gel (SiO2)
reusable a bunch of times, in that you can remove the moisture with (gentle) heat
itself clear and nontoxic; if blue or pink, it has a moisture indicator added which is less healthy (if ingested) (verify)
http://en.wikipedia.org/wiki/Silica_gel
  • Zinc chloride
http://en.wikipedia.org/wiki/Zinc_chloride
  • potassium hydroxide
http://en.wikipedia.org/wiki/Potassium_hydroxide
  • sodium hydroxide
http://en.wikipedia.org/wiki/Sodium_hydroxide



See also Air_machines#Dehumidifier

See also


On climate types:

Also related: