The Psychrometric Chart is fascinating summary of the physics of an air-water mixture. As you probably noticed, the wet-bulb contours are also enthalpy contours. It's nice to be able to show students direct simple applications of abstract quantities. Another chart that your comments on contours and axes would apply nicely to is the Stuve diagram. It shares many of the qualities of the Psychrometric chart but can be applied at various pressures and altitudes through the atmosphere. It gives adiabatic contours for parcels of rising air for situations where condensation has occurred or not. By following the adiabat for clear air and its intersection with the contour of constant moisture, one can predict where cloud base will form.
A quick way to get the wet bulb temperature is to place a thermometer in a moist paper towel and set the combination on a counter top for about 5 minutes. Natural convection will bring the temperature of the towel to the wet bulb temperature. The Psychrometric chart will then give the dew point and relative humidity.
Bob at PC
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From: phys-l-bounces@carnot.physics.buffalo.edu on behalf of John Denker
Sent: Wed 7/9/2008 11:28 PM
To: Forum for Physics Educators
Subject: [Phys-l] the physics of muggy weather ... and the evil of axes
My not-so-fond memory of this is that when I was 11, I could not figure out
how to use the psychrometric chart for myself. My father understood it, but
I did not.
With the benefit of a few decades of hindsight, I can identify a big part
of the problem: Axes.
I had been taught to understand charts in terms of the X-axis and the Y-axis.
That's a problem, because the psychrometric chart doesn't have a relative
humidity axis or a wet-bulb temperature axis. (It does have a dry-bulb
temperature axis and an absolute humidity axis, but that doesn't solve the
problem. There are four or five sets of contours, and only two axes.)
Axes are bogus. You don't ever need an X-axis. In contrast, you always
need the contours of constant X. These are hinted at by the tick marks on
the X axis ... but the axis itself is red herring, i.e. an unhelpful distraction.
The idea that there is an "X direction" is sometimes true, but sometimes
not true, and never necessary.
This is a never-ending source of confusion. In common parlance we speak
of the X axis and the X direction all the time ... even though that is not
the smart way to think about things.
The psychrometric chart has contours of constant relative humidity, but it
does not have (and cannot have) an axis representing "the" direction of
increasing relative humidity.
When you teach kids how to plot stuff on a chart, emphasize the
_contours of constant X_ and the _contours of constant Y_. Use graph paper.
Do not abstract away the grid too soon. This approach is simultaneously
simpler and more sophisticated. It is simultaneously easier, more powerful,
and more /physical/ than fussing with axes.
Teaching kids how to read the contours on a topo map never hurts, either ...
although you need to choose the map carefully, since the contours on some
maps are not very clearly drawn.
Take-home messages:
1) Wet-bulb temperature is a nifty concept
1a) It correlates with how comfortable you will be running around outside.
1b) It correlates with how comfortable you will be in a house with a
swamp cooler.
2) You can figure out a lot of good stuff using a psychrometric chart.
3) Axes are evil. Emphasize the _contours of constant X_ and the
_contours of constant Y_.