Relative Humidity is measured using a device called a sling psychrometer. This consists of 2 side by side conventional thermometers, one of which (the wet bulb thermometer) has a wetted wick placed at the bottom, the other (the dry bulb thermometer) has no wetted wick. When the psychrometer is swung around by its attached chain or handle, the air causes the water on the wick to evaporate which produces a lower temperature in the wet bulb thermometer. We now have a dry bulb and a wet bulb temperature which we can use to calculate relative humidity. The easiest way to do this is entering the dry and wet bulb temperatures into the calculator above and then click calculate. For example, if we have a dry bulb temperature of 20℃ and a wet bulb temperature of 15℃ what is the relative humidity? Using the calculator, we see that this is 58.4 %. Now, let's try using the above equation. (The equation is so complex, for ease of use, it has been split into 3 parts.) We have dry bulb temp (Td) = 20℃ and wet bulb temp (Tw) = 15℃ What is the relative humidity? Solving for ed ed= 6.112 ✱ 2.71828(17.502 ✱ 20 ➗ 240.97 ✚ 20) ed= 6.112 ✱ 2.71828(350.04 ➗ 260.97) ed= 6.112 ✱ 2.71828(1.3413) ed= 6.112 ✱ 3.8240 ed= 23.372 Solving for ew ew= 6.112 ✱ 2.71828(17.502 ✱ 15 ➗ 240.97 ✚ 15) ew= 6.112 ✱ 2.71828(262.53 ➗ 255.97) ew= 6.112 ✱ 2.71828(1.0256) ew= 6.112 ✱ 2.7888 ew= 17.045 Solving for relative humidity rh = ([ew ➖ .66875 ✱ (1 ✚ .00115 ✱ Tw) ✱ (Td ➖ Tw)]➗ ed) ✱100 rh = ([17.045 ➖ .66875 ✱ (1 ✚ .00115 ✱ 15) ✱ (20 ➖ 15)] ➗ 23.372) ✱100 rh = [17.045 ➖ .66875 ✱ 1.01725 ✱ 5] ➗ .23372 rh = [17.045 ➖3.4014] ➗ .23372 rh = 58.376 rh = 58.4%