Pool water evaporates at night, and the rate of water loss can often be surprisingly high during the hours of darkness. Evaporation is the natural process where liquid water turns into invisible water vapor and escapes into the atmosphere. This continuous phase change means water levels constantly drop, regardless of the time of day or the presence of direct sunlight. Understanding the physical principles that govern this process is the first step toward managing pool water levels and distinguishing normal loss from a potential leak.
The Physics of Water Vaporization
Evaporation occurs because water molecules are constantly in motion, possessing kinetic energy proportional to the water’s temperature. While the bulk temperature may be far below boiling, the kinetic energy of individual molecules varies. A small fraction of molecules at the water’s surface will always be moving fast enough to overcome the cohesive forces that hold the liquid together.
When these high-energy molecules escape into the air, they transition into water vapor. This transition requires a significant amount of energy, referred to as the latent heat of vaporization, which is drawn from the remaining water, causing a slight cooling effect. The tendency of water to evaporate is related to its vapor pressure, the pressure exerted by the escaped molecules above the liquid surface. A higher water temperature increases the vapor pressure, meaning more molecules exit the surface layer.
Environmental Factors Influencing Nighttime Rates
Nighttime conditions can sometimes accelerate evaporation rates compared to the daytime. The most influential factor is the temperature differential between the pool water and the surrounding air. When a pool is warmed during the day and the air temperature drops significantly after sunset, the warm water surface rapidly transfers heat to the cooler air.
This large temperature difference creates a powerful driving force for evaporation, often visible as a mist rising from the water’s surface in the early morning. Wind speed is another major variable, as air movement constantly sweeps away the thin, saturated layer of humid air that forms just above the pool surface. Replacing this boundary layer with drier air allows the evaporation process to continue at an accelerated pace, increasing water loss overnight.
Humidity levels in the nighttime air also play a direct role. When the air is dry, it has a greater capacity to absorb water vapor, pulling more molecules from the pool surface. Conversely, on a humid night, the air is already near saturation, which slows the rate at which water molecules can escape the liquid phase. The highest evaporation rates occur on clear, cool, and windy nights following a warm day, particularly when the pool is heated.
Distinguishing Evaporation from Other Water Loss
Pool owners need to determine if their water level drop is normal evaporation, which typically ranges from a quarter-inch to a half-inch per day, or a more serious issue like a structural leak. The most practical diagnostic tool is a simple comparison method known as the bucket test. This test isolates the rate of evaporation by comparing the water loss in the pool to the water loss in a container floating in the pool.
To conduct the test, fill a bucket with pool water and place it on a pool step so the water level inside the bucket is equal to the pool water level outside. Both water levels are marked, and after a 24-hour period, the two marks are compared. Because the bucket’s water temperature and exposure are nearly identical to the pool’s, the water level drop inside the bucket represents the loss due to evaporation alone.
If the pool water level outside the bucket has dropped significantly more than the water level inside, the difference indicates water loss from a leak, not just evaporation. Other non-evaporative causes of water loss include splash-out from use, filter backwashing, or leaks in the plumbing lines or equipment. The bucket test provides a baseline measurement of natural water loss, offering clear evidence if the loss rate exceeds what atmospheric conditions can explain.