Evaporation is the primary source of water loss in swimming pools, and the rate increases dramatically under high-temperature conditions. When the air temperature is around 100 degrees Fahrenheit, pool owners often see a noticeable drop in water level, leading to concerns about leaks. This significant water loss is a direct consequence of evaporation being accelerated by intense heat and other environmental factors. Understanding this process helps distinguish natural evaporation from a structural problem.
The Physics of Pool Evaporation
Evaporation is a process where water molecules transition from a liquid state to a gaseous state. This transition requires energy, known as the latent heat of vaporization, which the water molecules absorb from the surrounding environment, primarily from the pool water itself. The molecules that gain enough energy to break free from the liquid’s surface tension become airborne water vapor.
The rate of this molecular escape is directly related to the water’s vapor pressure. As the pool water temperature rises, more molecules possess the necessary kinetic energy to escape, increasing the vapor pressure above the water surface. This physical principle explains why a heated pool or a pool under intense sunlight experiences higher rates of water loss.
Environmental Variables That Accelerate Water Loss
A 100-degree air temperature acts as a powerful catalyst for pool water loss, but it is not the only factor. The difference between the pool water temperature and the ambient air temperature, known as the temperature differential, is a key element. Evaporation is often highest when warm water meets cool, dry air, as the water quickly gives up its latent heat and turns to vapor.
Low relative humidity is another significant variable that accelerates water loss. Dry air acts like a sponge, readily absorbing the water vapor molecules that escape from the pool surface. In contrast, when the air is humid, it is already saturated with moisture, which decreases the concentration gradient and slows the rate at which new vapor can be absorbed.
Wind speed is the most forceful environmental factor that dictates the evaporation rate. A layer of saturated, humid air naturally forms above the water’s surface, slowing the escape of water molecules. Wind constantly sweeps this saturated layer away, replacing it with drier air that can absorb more moisture, accelerating evaporation. Wind speeds as low as seven miles per hour can increase evaporation losses by 300% or more.
Estimating Expected Water Loss and Identifying Leaks
Under typical conditions, a pool might lose about a quarter of an inch of water per day to evaporation. However, when air temperatures reach 100 degrees Fahrenheit, especially when combined with low humidity and high wind, the daily water loss can easily exceed one-half inch. In extreme, hot, and dry environments, loss can approach an inch or more daily.
To determine if excessive water loss is due to accelerated evaporation or a structural leak, pool owners can perform a simple “Bucket Test.” Fill a bucket with pool water and place it on a pool step so the water level inside the bucket is identical to the pool level. Mark both the inside and outside water levels on the bucket.
After 24 hours, compare the two marks. The bucket water level drops only due to evaporation, while the pool level drops due to both evaporation and any potential leak. If the pool level has dropped significantly more than the bucket level, this indicates a structural leak in the pool or its plumbing. If both levels drop by the same amount, the water loss is only due to natural evaporation.
Strategies for Reducing Evaporation
The most effective way to reduce evaporation is by placing a physical barrier between the water and the air. A pool cover, such as a solar blanket or an automatic safety cover, can reduce water loss by up to 95%. These covers trap the water vapor underneath, which saturates the air layer and reduces evaporation.
Installing strategic windbreaks around the pool area is another practical method. Fences, landscaping, or dense shrubs positioned at a right angle to the prevailing winds can cut the wind speed over the pool surface, minimizing the sweeping away of the humid air layer. These barriers can reduce wind speed by up to 70% in the protected area.
Pool owners can also use liquid solar covers, chemical evaporation inhibitors that form an invisible, ultra-thin molecular barrier on the water surface. While less effective than a physical cover, this layer disrupts the water molecules’ ability to escape into the air. Finally, lowering the water temperature slightly, if comfortable, decreases the water’s vapor pressure and reduces the temperature differential, naturally slowing the rate of evaporation.