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See also Electronics notes/humidity sensing

Humidity and Relative Humidity

This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

Humidity refers to water vapour present in a volume. That much was fairly clear already.

Absolute humidity is (usually) mass per volume of air (e.g. grams per cubic meter), or (sometimes) mass per mass of air (e.g. grams per kilogram)

Absolute humidity feels like a fairly direct measurement, but at the same time is not the most helpful to practical questions like "is there mist" or "how well can I sweat" or "is it likely to rain".

...largely because answers to those also depend on details like how much mass the air could contain right now, which also varies with other things - largely temperature, and also pressure.

Relative Humidity (RH) relates the humidity to the dew point - the point at which the air can hold no more water.

This is more practical, but is also little more involved.

The dew point is basically 'how much water can the air contain right now' - the saturation point of water in air, at which mist/clouds will happen, and at which condensation onto things happens very easily.

...which means RH is now a function of both amount of vapour and temperature (and sometimes hides a 'assuming the temperature does not change').

The dew point itself varies with temperature, because it affects the amount of water moving in and out of the air, but in the context of weather, in any one region, this variation is usually considered to be relatively negligible.

So around things like weather forecasts we simplify things and report dew point as just a temperature.

This is not correct to physics, and actively confusing if you want to understand the intricacies (like how dew relates to dawn), yet it's practical when we only care about the temperature below which air can no longer hold all the vapour so a lot of condensation happens.

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
this is where the "yeah, but it's a dry heat" comes from, and why it makes sense
  • moist cold feels colder than dry cold, largely because condensing moisture conducts out heat faster
different mechanics, similar result
  • 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 seas and near coastlines, in relatively wet climates, rainforests
    • 80-100% on islands and near seas, and in some very wet areas
    • in any one place, seasonal differences is easily 10-20%, varying a little per region and climate type (verify)
    • this is simplified - local climate types and some other influences may have large effects
  • RH varies throughout the day. How much differs per climate, but can easily be 50%

Controlling humidity

This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.
  • 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)
(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)
  • Zinc chloride
  • potassium hydroxide
  • sodium hydroxide

See also Air_machines#Dehumidifier

See also

On climate types:

Also related: