Humidity measurement

[David]

I have several thermometer cum weather prediction (barometer) cum humidistats dotted around the place.

I did fancy something more accurate as the readings all seem suspiciously low (but then they are indoors).

Just taken the most long term reliable one outside to check. It was showing 30% humidity indoors and outside it has already dropped to 28%.

Meanwhile the Met Office local forecast is showing around 80% which seems quite high; I would expect it to be nearly turning misty if the air was that saturated. Probably wrong of me, though.

I looked on Amazon (as you do) and there are some nice big gauges but the reviews say they are accurate because they agree with the readings on their digital thermometers.

Are these humidity levels reasonable?

If not, can someone recommend an accurate humidity meter which is not horrendously expensive?

If I look up average humidity for March and get, for example:

this again shows highs around 90 and lows around 70.

These are measurements of relative humidity (so water condenses out of the air at 100%); is there a different measurement type?

All very puzzling as I would expect the weather to be reasonably heavy and clammy at 90% humidity, although the same amount of water in the air at

10C outside and 20C inside would give quite a different relative humidity.

Whatever, the humidistat outside has still only got up to 33%. I shall leave it for an hour and see what is changes to.

Cheers

Dave R

[harry]

There is only one accurate way to measure RELATIVE humidity. Wet and dry bulb thermometers. You can make one up yourself quite easily. You will need a chart to covert the temperature readings to relative humidi ty.

hermometer.29

The chart:-

(DP%2525252520%25252526%2525252520RH)%25252FESRT%2525252520page%252525252012%2525252520index.htm&source=iu&pf=m&fir=JDnGcJBop_TU1M%253A%252CBQk3P7CORsbFfM%252C_&usg=__W_IY7UJLE1m7YhOhbec5-8892zc%3D&biw=853&bih=596&ved=0ahUKEwjktL3f6s7SAhUBM8AKHdpND48QyjcIJQ&ei=siDEWOSTLYHmgAbam734CA #imgrc=JDnGcJBop_TU1M:

[Chris Hogg]

+1, just for a change. See also Also, on Amazon, and

[David]

Just brought it back in reading 58%.

Still nowhere near the 80-90% shown in the forecast.

Cheers

Dave R

[newshound]

• another 1

[Chris Hogg]

I wouldn't be surprised if the sensor is rather slow to react. You should leave it outside until you get two consecutive readings the same, or at least fairly close to each other.

[Roger Mills]

Therein lies the clue. The higher the air temperature, the more water it can hold. At 10 degrees it can't hold very much - which is why even a high relative humidity doesn't necessarily feel clammy. More likely to feel 'raw'.

[Harry Bloomfield]

David was thinking very hard :

Both values are unlikely. I have a weather station logging both values contantly. 40 to 45 is normal indoors, outdoors will be around the 70 to 95% most of the time at this time of year. It tends to dip in really cold weather, because the cold air cannot support much humidity.

[The Natural Philosopher]

It doesn't work like that.

What does tend to happen is that really cold weather freezes out the moisture at night and when it warms up its temporarily low humidity.

Today humidity at my local RAF airfield (Lakenheath) has been around 88%

- since dark its 100%.

Inside if you take 100% saturated air at say 5C and heat it to 20C its gonna be around 40% or less - roughly halves for every 10C

Winter internal humidity can be alarmingly low in frosty weather

No: mist forms at 100% by definition.

[Andrew Gabriel]

At 13:52 (the time of your post), it was 68.5%RH in East Finchley, (and 17.8C). Don't know if you were anywhere near there at the time you measured it.

Misty tends to be 100%, although 100% isn't always misty. Actually it can go over 100% as air can be super-saturated, and this forms mist on microscopic dust/polution particles, and condensation/dew on surfaces, although again, > 100% doesn't necessarily mean misty.

Yes, there's absolute humidity - this is specified as the temperature at which, for a fixed quantity of moisture in the air, the relative humidity is 100%. This is known as the dew point.

Can be some lag. It's also important it is measuring the air temperature, and not subject to significant radiative heating/cooling from the sun/sky, or other objects which might be being heated by the sun or cooled by the sky (look up a Stevenson Screen).

[Brian Gaff]

That link is longer than most web pages by the sound of it. How on earth do they manage to get such complexes addresses. Brian

[Chris Hogg]

Harry doesn't seem to know how to use TinyURL.

[The Natural Philosopher]

Its because he googled it and then cut and pasted what google used to identify it which is tagged with lots of google private tracking info.

[The Natural Philosopher]

Or google.

[T i m]

That takes me back ... we had a Stevenson Screen at primary school and for a while I was one of those whose responsibility it was to log the readings etc.

That was in the days when such things could sit outside and not get vandalised or the equipment stolen from. ;-(

Now days though you can get something as cheap (10 quid), small and low power consumption as a Raspberry Pi Zero W, install Linux and some free software on it (Domoticz in this case) and have it regularly scrape and log external weather stations (and other data sources, like my cousins solar panels / battery) giving a whole wealth of mildly interesting info. ;-)

This for example might be of interest to the OP:

I have no idea how accurate it is but my wireless outdoor thermometer seems to confirm the current temperature, +- a couple of degrees.

Cheers, T i m

p.s. This is the internal temperature log of the system when I changed from a RPi3b to the Zero W. Assuming the internal temperature sensors are accurate, it does seem to reflect how much lower power the Zero would be over the Pi3.

[Dave Liquorice]

Humidity varies greatly dependant on the local (very local) enviroment. North side of a building will tend to have a higher reading than the south side, if only due to the temperature difference. Nearby vegetation, ponds, rivers and relative wind direction will also affect it.

A centrally heated house at this time of year will be around 30% to

40%. If it's very close to, or below, 0 C outside for a day or three, the inside humidity may drop to 25 %.

[Harry Bloomfield]

Dave Liquorice explained :

This read a few minutes ago...

In 16.5C 48% Out 10.7C 79%

Doors have been open, nice day, no heating needed, but it will go on later. Outdoor sensors are near to rear end of back garden which is still very wet.

[Harry Bloomfield]

Dave Liquorice formulated on Monday :

If the humidity drops very low indoors in winter, I tend to suffer from dry lips etc. - then I add some humidity by placing trays of water on the radiators.