Sling psychrometer:

A psychrometer is a simple device for measuring the relative humidity of the air. At any temperature, the air is able to absorb a certain amount of water vapor. The relative humidity is a measurement of the portion of the total possible water vapor that is currently in the air. When the air is nearly completely filled with vapor, it feels muggy if the air is hot or damp if it is cold.

Now let us understand what is evaporation and evaporative cooling?

Evaporation is a type of vaporization of a liquid that only occurs on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs within the entire mass of the liquid.

On average, the molecules in a glass of water do not have enough heat energy to escape from the liquid. With sufficient heat, the liquid would turn into vapor quickly . When the molecules collide, they transfer energy to each other in varying degrees, based on how they collide. Sometimes the transfer is so one-sided for a molecule near the surface that it ends up with enough energy to 'escape'

For molecules of a liquid to evaporate, they must be located near the surface, be moving in the proper direction, and have sufficient kinetic energy to overcome liquid-phase intermolecular forces. When only a small proportion of the molecules meet these criteria, the rate of evaporation is low. Since the kinetic energy of a molecule is proportional to its temperature, evaporation proceeds more quickly at higher temperatures. As the faster-moving molecules escape, the remaining molecules have lower average kinetic energy, and the temperature of the liquid decreases. This phenomenon is also called evaporative cooling. This is why evaporating sweat cools the human body. Evaporation also tends to proceed more quickly with higher flow rates between the gaseous and liquid phase and in liquids with higher vapor pressure. For example, laundry on a clothes line will dry (by evaporation) more rapidly on a windy day than on a still day. Three key parts to evaporation are heat, atmospheric pressure (determines the percent humidity) and air movement.

A sling psychrometer measures the humidity by measuring the cooling effect of evaporation. Water is constantly evaporating into the air and condensing out of it. When that water evaporates, it takes some of the heat from whatever it is evaporating off of with it, cooling that object down. That is why we sweat in the heat---to cool down. Water blowing past aids this evaporation, causing the object to cool more quickly when the air has a high relative humidity, however, water does not evaporate as easily. That is why humid days feel hotter than dry days---sweat doesn't evaporate off the body, and so the body doesn't cool down. The sling psychrometer calculates humidity by measuring how much evaporation contributes to cooling.

A sling psychrometer consists of two thermometers. One is left dry to record the true temperature, while the other is covered in water. The wet thermometer is then swung around, causing some of the water to evaporate. The lower the relative humidity, the more water will evaporate and the more the wet thermometer will cool down. By comparing the temperatures of the wet and dry thermometers to a chart called a psychometric chart, a scientist can determine how humid the air is