Humidity

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.

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

Specific humidity is the mass of vapour in a mass of air, e.g. grams of water, per kilogram of air.

Absolute and specific humidity feel like fairly direct measurement, yet turn out to not be very 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 varies with other things - largely temperature, but 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 meaningful, but also a little more complex to determine exactly.

The dew point is basically that mentioned '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.

This means RH is a function of both amount of vapour and temperature.

The dew point itself also varies with temperature (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 fairly negligible.

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

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 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.