Humidity: Difference between revisions
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See also [[Electronics notes/humidity sensing]] | See also [[Electronics notes/humidity sensing]] | ||
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Humidity refers to water vapour present in a volume. | Humidity refers to water vapour present in a volume. | ||
'''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) | |||
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Absolute humidity is often expressed as either | |||
'''mass of water vapor per volume of air''' (in grams per cubic meter) | |||
with a maximum around 30 g per cubic meter. | |||
This is practical for most 'weather outdoor' situations, | |||
but less valid when there may be significant temperature or pressure changes, | |||
so certain physics works in '''mass of water vapor per ''mass'' of air''' (usually in grams per kilogram). | |||
Now we have a bunch of terms, including ''humidity ratio'', ''mass mixing ratio'', ''specific humidity'', | |||
some of them have more specific definitions, and some of those will sometimes be called absolute humidity. | |||
You could argue that 'absolute humidity' is now fuzzy, and should be avoided whenever you want to be unambiguous. | |||
''' | |||
'''Specific humidity''' is the mass of vapour in a mass of air, e.g. grams of water, per kilogram of air. | '''Specific humidity''' is the mass of vapour in a mass of air, e.g. grams of water, per kilogram of air. | ||
Specific humidity is the ratio of water vapor mass to total moist air parcel mass. | |||
--> | |||
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. | |||
This is more | |||
The [[#Dew point|dew point]] is basically | The [[#Dew point|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'). | |||
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: If you vary amount of water, you get proportionally higher/lower RH | : 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 | : 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 | :: ...which is basically the same thing as saying the dew point changes | ||
More precisely, RH is 100 times | More precisely, RH is 100 times | ||
* the [[partial pressure]] of water vapor in the air | * the [[partial pressure]] of water vapor in the air | ||
* divided by the equilibrium saturation vapor pressure of water at the air temperature T | * divided by the equilibrium saturation vapor pressure of water at the air temperature T | ||
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But does the saturation pressure? | But does the saturation pressure? | ||
Turns out yes{{verify}} - but | Turns out yes{{verify}} - but | ||
: relatively negligibly within the few percent of barometric pressure{{verify}}. | : relatively negligibly within the few percent difference of barometric pressure that happens in our weather{{verify}}. | ||
: the temperature remains a much larger effect, | : the temperature remains a much larger effect, | ||
: the temperature and pressure also do not vary the same way, | : the temperature and pressure also do not vary the same way | ||
So we pretend it's not, unless you want to get into formulae - which you just ''wouldn't'' in weather reports. | |||
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Now, weather forecasts talk about low pressure (systems) leading to rain all the time. | Now, weather forecasts talk about low pressure (systems) leading to rain all the time. | ||
Barometers on your wall will label low pressure ''as'' impending bad weather. | |||
Low pressure basically implies that air will be rising in it, reach colder temperatures, | that is not a direct relation, but is definitely correlated - it has to do with rising and falling air. | ||
Low pressure basically implies that air will be rising in it, reach colder temperatures, so will end up transporting any moisture into the form of clouds - concentrations of water enough to ''want'' to fall and large enough to actually fall to the ground. | |||
...roughly, anyway. There are many more details to these mechanics, like | ...roughly, anyway. There are many more details to these mechanics, like | ||
what concentrations you'll need for it to become rain, | what concentrations you'll need for it to become rain, | ||
why moisture makes it further in some forms, | why moisture makes it further in some forms, | ||
what combination of effects (largely heat and pressure and a source of moisture) is more likely to give you more sustained rain, or thunderstorms. | what combination of effects (largely heat and pressure and a source of moisture) is more likely to give you more sustained rain, or thunderstorms. | ||
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But then, consider that the barometric pressure varies a few percent at most. | But then, consider that the barometric pressure varies a few percent at most. | ||
--> | |||
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. | |||
so a lot of condensation happens. | |||
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* Above 50% you start to see easier condensation onto cold surfaces | * 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 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 | * 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}} | * Throughout the world, ''average'' RH is '''''roughly''''' {{verify}} | ||
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* RH varies throughout the day. How much differs per climate, but can easily be 50% | * RH varies throughout the day. How much differs per climate, but can easily be 50% | ||
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'''Nerdier notes''' | |||
Technically, RH does not even (directly) depend on the presence of air. | |||
While the saturation point ''does'' depend on the mixture as a whole is {{comment|(...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. | |||
It's just that if that were happening out your window, we would have ''much'' bigger issues, so it's not really relevant, unless maybe you're doing space things. | |||
--> | |||
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* condensation on the inside of your window is likely to happen with lower temperatures, so if you want to avoid moist window panes it is recommended to try for lower humidity (perhaps the 30%-50% range). | * condensation on the inside of your window is likely to happen with lower temperatures, so if you want to avoid moist window panes it is recommended to try for lower humidity (perhaps the 30%-50% range). | ||
Latest revision as of 09:40, 26 April 2024
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.
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)
- 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
- potassium hydroxide
- sodium hydroxide
See also Air_machines#Dehumidifier
See also
On climate types:
- http://en.wikipedia.org/wiki/Climate
- ...and compare with: http://www.intellicast.com/Global/Humidity.aspx?location=default
Also related: