The Sling Psychomotor & Psychometric Calculator

2012-05-10 20:52:00.000 – Roger Pushor,  Weather Observer/IT Specialist

Sling Psychrometer/Psychrometric Wheel Calculator

The first Cog trains for the season were up this past weekend with plenty of tourists who want to visit the highest point East of the Mississippi and North of the Carolinas. With those visitors will come one very common question of the Observers on the deck when they’re doing their observations – What are you doing with that thing you’re spinning around in your hand???

The ‘thing’ is a sling psychrometer which is made up of two thermometers one with the bulb covered with a muslin wick that has been wetted in distilled water before the Observer brings it out onto the deck and starts spinning, or ‘slinging’, it.

So you ask – Do Observers do this ‘slinging’ just as a form of exercise for their hands and arms or is there a purpose to coming out once an hour and spinning this thing around in the air??? Well there is a purpose – As the sling psychrometer travels through the air, water is evaporated from the wick on the ‘wet bulb’ thermometer; and just like when you get out of the shower and feel cool as the water evaporates off of your body, the thermometer is cooled by the evaporation that is caused as the air passes over it. After a few minutes, the Observer notes the temperature reading of both thermometers and goes back inside to record this information as part of the hourly observation that’s sent to the National Weather Service and made available to the public once an hour.

From these two temperatures, we know the outside air temperature and we can calculate the dew point and humidity using either a psychrometric wheel calculator or a computer program. Because it’s so much faster and easier, we almost always choose the computer program. Over the past couple of days, I’ve learned to use the psychrometric wheel calculator to validate the output from a new version of one of the programs we use to do the calculation we use here on the summit.

If you can make it to the summit this year, please visit the Mount Washington Museum where you’ll see this and other interesting weather instruments along with information on the natural surroundings. If you’re a member of the Mount Washington Observatory, ask one of the Museum Attendants about getting a free, behind the scenes tour of the Observatory, including a chance to get one of the best views on the summit on a clear day – at the top of the instrument tower.


Roger Pushor,  Weather Observer/IT Specialist

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