Relative Humidity (Evaporation)

What is relative humidity? The percentage of moisture the air holds compared to the amount of moisture it could hold at a certain temperature

How does moisture enter the air? Through evaporation Oceans are the largest source of moisture For the planet.

What temperatures hold more water vapor? Higher temperatures hold more water vapor because the “air expands” allowing more room for water vapor.

What device is used to measure relative humidity? A sling psychrometer, it contains a dry bulb and a wet bulb temperature

What is the relationship between the wet bulb and the dry bulb and the amount of humidity in the air? As the difference of the wet bulb and dry bulb increases, the humidity decreases, and vice versa. If the wet bulb and dry bulb are close together, the humidity is high.

What are three factors that affect the rate of evaporation? Energy- temperature – the higher the temperature, the higher the rate of evaporation Surface area- as surface area increases, the rate of evaporation increases. Moisture content of the air- As moisture content (RH) of the air decreases, the rate of evaporation increases.

Cooling the Surface…. Because water needs energy (heat) to evaporate into a gas, it gets this energy from the surface it was on. Once the water leaves, it takes this energy (heat) with it, leaving the surface feeling cooler. This is why humans sweat. Sweating makes us feel cooler in temperature. This why the temp of wet bulb is always colder.