Psychrometry WTF? #2 - Absolute Humidty

This is the second post in our 
Psychrometry wtf ! - "without the fear" series

This post is about "wetness" - which is otherwise known as humidity. Air can be bone dry. Or not.

To recap the first post Dry Bulb Temperature is summarised very simply as follows: As we move from left to right it gets hotter

Click to Enlarge

Summer = Hot = Right Side of graph
Winter = cold = left side of graph


Now onto wetness. How wet is wet air and how dry is dry air?


Suppose for every bit of air we had a bit of water - would that make it 50% wet ? - As a humidity ratio it would depend how we measured.


If we take a subject close to my heart - so close in fact that we need to show a picture... - You might imagine Simon Theakston offering you a pint of Theakstons Old Peculiar (well I did say that I would try to make this as painless as possible).

http://www.beer-pages.com/stories/theakston.htm 

Now that the beer has reverted back to family ownership the beer has been made more peculiar yet - it is 3.8% Alcohol By Volume


That means that if you had a swimming pool full of it, the alcohol would fill up a paddling pool of 3.8% of the volume of the swimming pool.


That would be a silly thing to do, because actually it is best to drink it.


However, another way of saying how strong the beer was would be to discuss Alcohol By Weight.  Since a volume of water weighs more than a similar volume of alcohol the ABW figure would be slightly less than the ABV value. 


However, the crazy guys who do meteorology for a living decided to measure wetness of air in terms of Water in Air By Weight.  There is a good reason for this - while water and alcohol are quite incompressible air can be squashed into a small space.  If you try squashing Beer it makes the place you put it bigger - though it happens over years:)


So how wet is air - we talk in ratios = e.g 38 grams water per 1000 grams of air  or 0.038


It is the absolute humidity ratio


If you click on the graph above you will see the horizontal blue lines are each marked with a certain wetness - with the amount increasing as you go up the chart.


So now we can think of the chart as follows (ignore all the other lines for now):

COLD & WET	HOT & WET - Google for images - Not Safe For Work !
COLD & DRY	HOT & DRY

Psychrometry WTF? #1 - Dry-bulb temperature

OK - we have the "terrifying" psychrometric chart in all of its' glory.


Put it out of mind right now ! (click on it for a gory detail reminder if you must)


I have promised to explain

Psychrometry wtf-without-fear series
and make it crystal clear.



I am going to do this by playing with some simple ideas about household objects.  These may also help you remember things.


We start off with the humble thermometer - these are not frightening - mothers and doctors stick them in the mouths (and elsewhere) of babies and sick people so they cannot be all that bad.


So here is a thermometer :  


As it gets hotter the red liquid expands and moves up into a vacuum from the "bulb" or reserve at the bottom.  


So the longer the red marker the hotter it is.


The bulb is important !


Generally if you stick your finger in your mouth so its wet, and then stick it in the air (no picture :) it will feel colder than if you stick it in the air while it is dry.  However, if it is raining or foggy (very humid) it does not feel colder.


So how it feels depends on how wet it is.


Normal temperatures are taken with a "dry bulb" -
i.e. stick your finger in the air (not in your mouth).

I asked a friend of mine to demonstrate - but he said he gets fed up with them - so here is someone clearly chosen entirely at random doing it for you.


So even politicians know that summer is warm and winter is cold. 


My argument is that if a politician can understand this issue you can.


All you need to know about the dry-bulb temperature and a psychrometic chart is that it gets hotter from left to right !


You can imagine the thermometer being laid on its side so the red line rises from left to right.



In this diagram you will see vertical green lines drawn from the bottom where they are marked with the dry-bulb temperature (how hot it is - rising left to right )

Click to Enlarge