Water loss from a swimming pool is a concern year-round. Pool water evaporates in cold weather, and the rate can be surprisingly high. This phenomenon often confuses homeowners who associate evaporation only with high summer heat. Understanding the physics behind water loss in cooler temperatures is the first step toward managing it and protecting the pool structure.
Why Water Loss Continues in Low Temperatures
The primary driver of accelerated water loss in cold weather is the temperature differential between the pool water and the surrounding air. When a pool is warmer than the atmosphere, the rate of evaporation increases significantly, even if the air temperature is low. This difference creates a powerful energy transfer as the pool acts as a heat source, rapidly turning surface water molecules into vapor.
The concept of vapor pressure is central to this mechanism, as warmer water has a higher vapor pressure, meaning its molecules are more eager to escape into the air. Cold air naturally holds less moisture than warm air, leading to lower relative humidity. This dry, cold air readily accepts the moisture evaporating from the warmer pool surface, sustaining a high rate of loss. The visible cloud of steam or fog often seen rising from a pool on a cold morning is a direct result of this rapid evaporation. Wind significantly magnifies this effect by constantly moving the saturated air layer away from the water surface, exposing it to fresh, dry air.
Is It Evaporation or a Leak?
For a pool owner, the primary concern is distinguishing between normal evaporative loss and a structural leak. A simple diagnostic tool called the bucket test can reliably differentiate between the two. To perform this test, ensure the pool is filled to its normal operating level and that any automatic water fillers are turned off.
Next, place a weighted bucket on a pool step so that the container is partially submerged in the pool water. Fill the bucket with pool water until the water level inside is approximately the same as the water level outside. Use a permanent marker or tape to mark the water level inside the bucket and the pool level on the outside.
After 24 hours, compare the two marks; the water in the bucket and the pool should both have dropped due to evaporation. If the pool water level outside the bucket has dropped noticeably more than the water level inside, the excess loss indicates a leak in the pool structure or plumbing. If the levels have dropped by the same amount, the loss is due to natural causes. Other indicators of a leak can include saturated ground near the pool equipment or the need to add water consistently even when the pool is covered.
Methods to Reduce Winter Water Loss
Pool owners can employ several strategies to mitigate evaporative loss during cold weather. The most effective method is using a pool cover, which creates a physical barrier between the water and the cold, dry air. Both solid winter covers and solar covers are effective, with some designs reducing evaporation by up to 95%.
Another option is to utilize a liquid pool cover, which is a chemical compound that forms a single-molecule-thick, invisible barrier on the water’s surface. This layer hinders the escape of water molecules and heat without affecting the pool’s chemistry or use. Installing windbreaks, such as solid fencing or dense landscaping, around the pool perimeter can reduce loss by blocking air currents that accelerate evaporation. Finally, if the pool is heated during cooler months, lowering the water temperature will reduce the temperature differential and thus slow the evaporation rate.