![]() ![]() I don’t know how you’d ever estimate that speed, but that was the goal. For example, some instruments had a small fan inside that would blow water across this wick, or more commonly, two temperature sensors were attached on a rotating handle, so they could spin them in the air at about one meter per second (or two miles an hour). Once that wick is saturated with water, the water begins to evaporate, and they would use wind to enhance that evaporation. This was just a fabric with water dripped onto it. Researchers made this wet bulb temperature by putting a cotton wick around the bulb of the thermometer. To calculate vapor pressure from our dew point temperature, we’ll call vapor pressure of the air, e a which is equal to the saturation vapor pressure (e s) at the dew point temperature (T d) (Equation 1). The beautiful thing about dew point temperature is that if you know this value, you can easily calculate vapor pressure and even go on to calculate relative humidity, as I talked about in another lecture. We call this T d or dew point temperature. So we can say that the dew point temperature is the point at which the air is saturated and water begins to condense out. But it was a good approximation and a great way to demonstrate what dew point temperature is. This experiment wasn’t perfect because there is certainly a temperature difference between the inside of our glass where we’re stirring with the thermometer and the outer surface of the glass. At the point the film began to form, we looked at the temperature to get the dew point temperature, which means exactly what it says: the point at which dew begins to form. So we watched the temperature go down, and at some point, we observed a thin film form onto that glass. The thermometers were rotating around in the glass, and our job was to look carefully and find out when a thin film of dew began to form around on the glass. The professor had us take a beaker of water and a thermometer and put ice in the beaker and start to stir it. ![]()
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