What is evaporation?
As we know from boiling water and seeing it turn into steam, a simple definition of evaporation is: the process whereby liquid water becomes a gas and dissipates as the water temperature is increased.
Eventually, a saucepan of water will ‘boil dry’ as all the liquid becomes a gas and disappears into the atmosphere.
Calculating evaporation rates is a very complicated process, with many variables such as water surface area, water temperature, air temperature, air pressure, air density, wind speed, and humidity – among others – all affecting evaporation.

Evaporation from swimming pools
The main factors that affect evaporation rates from domestic outdoor pools are:

1. Pool surface area: The bigger the pool, the more surface area, therefore, a greater evaporation volume.

2. The temperature of the water and air: The highest evaporation rates occur when the differences between water and air temperatures are the greatest. This may not be in the middle of a hot day when the pool is in use. At this point the water and air temperatures may be quite close. Later at night the pool water may remain warm, but the air temperature has fallen substantially – that means a greater temperature difference between the air and water – and greater evaporation.

3. Humidity: The drier the air is, the greater the evaporation rate. In very humid conditions less evaporation occurs.

4. Wind: The final and very significant factor for home pools is wind. A breeze of just a gentle 7 mph can more than double the evaporation rate by removing the insulating layer of warm, moist air directly above the pool surface.

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