I have used these on fires and at official NWS surface weather stations. They are very accurate, and kinda fun to use. The wet bulb is just a thermometer covered with a bit of cloth. When you wet this cloth and spin it around for a bit, you find the lowest temperature that you can get from evaporating water into that particular atmosphere. When the relative humidity is high water does not evaporate off of the wet bulb as much and the difference between the actual temperature and the wet-bulb temperature is not as high.
Since WWII there has been a huge push to record weather observations across the globe. The best way to do this was the sling psychrometer and an anemometer. For the last 30 years ASOS (Automated Surface Observing System) has been being implemented. ASOS works even better and are better than humans in every aspect but ceilings and cloud types. Until the power goes out, then we break out the old sling. Or, the Kestrel...
These are not not approved for official surface observations, but I can tell you that is what is being used and recorded in the 'official' database.
Kestrels are awesome! They record the temperature, dewpoint, wind speed, and pressure. They have software that will find altimeter settings, and other readings of interest. The Kestrel even records readings into a database that you can use with other software in you home. These are the coolest things ever! They are a little pricey, but I believe they are worth the money, and work better than any other personal weather stations.
No matter how cool Kestrels are (and you should buy one for yourself and one for me), it goes to show that surface observations are not being nor have they ever been taken in a scientific manner. When the power to ASOS goes out and the weather observer goes outside with his/her Kestrel, do you think they are going to walk all the way to the ASOS site in order to take temperature in the same area? I can tell you they walk right out the front door into the parking lot and take a weather reading. Which place is going to be hotter?
The placement of ASOS has never been done in a scientific way either. ASOS equipment is expensive, and it is expensive to man it. Therefore when an aiport needs weather data, and an ASOS site was set up a few miles away. It costs a lot less to move the site next to the runway than to set up an entirely new site. This happens all the time. They move these sites to more populated, WARMER areas and still use these readings in global warming studies.
That's fine, but please use satellite or ice cores or something else when trying to convince me of global warming.
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