A slippery slope....

[Gaz]

to becoming a weather geek

Mark bought me a basic weather station for xmas, so i now have a couple of sensors in the garden. The main thing that interests me so far is being able to see the temps, but i can already sense that this will grow and grow!!!

Anyway humidity, how is it measured and what do the percentages actually mean?

[Petefree]

I'm not very clued up on this Gaz - and know we have a few people here who are...
I think what the digital weatherstations measure is "Relative Humidity" which is associated with air temperature (with warmer air having capacity to carry more water vapour than colder air).
I think Simon wrote a long post on it in the thread on "Frost Mechanisms"...
Pete

[Gaz]

I tend to understand the words and not always the meaning when Simon explains things

so its relative to what?

[Petefree]

Relative Humidity is the ratio of the amount of water in the air at a given temperature to the maximum amount it could hold at that temperature; expressed as a percentage.

So regardless of the air temperature (which I think dictates the amount of water vapour the air can carry) air at a given temperature will have a Relative Humidity between 0% and 100% (e.g. if the air has half the amount of water vapour it could potentially hold at that temperature its relative humidity will be 50%).

If it reaches 100% Relative Humidity this will be its 'Dew Point'.

Pete

[grub]

Is it one like this Gaz?http://www.tlc-direct.co.uk/Images/Prod ... KC8123.JPG

[stephenprudence]

Pete's pretty much spot on there. The relative humidity is the percentage of moisture, within an air particle. i.e. if an air particle holds 30% moisture per every particle, the relatively humidity is 30%

Where moisture relates to air temperature is determined by the dewpoint.

[Dave Brown]

Definately a slippery slope that goes hand in hand with what you can grow.

My version of explanation is Humidity is measured by the % of moisture the air is capable of holding at any given temperature..... so 100% is saturated (fog, or dew point) and 50% is half way to saturation. Generally as air warms it is capable of holding more moisture so the relative humidity % falls, and as air cools it can hold less moisture so the relative humidity % rises.

[simon]

I tend to understand the words and not always the meaning when Simon explains things

Thanks for the vote of confidence, Gaz I have always been told I was good at explaining things .

Pete and Dave have it spot on. I will just add that the dew point is when you have 100% humidity and the rule of thumb is that for every degree the dew point is below air temperature, the rH is 5% below saturation. So, if you have 75%rH in your garden the due point should be approximately 5C below the air temperature.

People often confuse this with absolute humidity, which is the actual amount of water in air by mass/volume. I can't quite make sense of what stephen has writen, but he seems to be making this mistake.