The Physics of Evaporation: Dalton's Equation
Water evaporation is not simply driven by air temperature; it is governed by the difference in vapor pressure between the saturated boundary layer of air directly above the water surface and the ambient air mass:
Dalton's Evaporation Formula: E = (pw - pa) x (A + B x Wind Speed)
Where 'pw' is saturation vapor pressure at water temp, 'pa' is ambient air vapor pressure, and wind speed provides convective mass transfer.
The four environmental variables that dictate your daily evaporation rate include:
- Relative Humidity: Dry desert air (10% to 20% humidity in Nevada or Arizona) absorbs water vapor rapidly, tripling evaporation compared to humid climates (Florida or Georgia at 80% humidity).
- Wind Velocity: Wind strips away the moist micro-boundary layer of air resting on the water surface, replacing it with dry air that draws out more moisture.
- Water-to-Air Temperature Gap: When warm water (84 F) meets cool nighttime air (58 F), vapor pressure differential peaks, causing visible steam and massive overnight water loss.
- Pool Surface Area: Evaporation scales directly with surface square footage, regardless of how deep the pool is.
Normal Daily and Weekly Water Loss Benchmarks
Expected Water Loss by Environmental Conditions
| Weather Conditions | Daily Water Loss | Weekly Water Loss | Gallons Lost (18x36 Pool) |
|---|---|---|---|
| Humid & Calm (75% RH, <5 mph wind) | 1/8 Inch (0.125 in) | 7/8 Inch (0.875 in) | 350 Gallons / Week |
| Moderate Summer (50% RH, 5-10 mph wind) | 1/4 Inch (0.250 in) | 1.75 Inches | 700 Gallons / Week |
| Arid & Windy (20% RH, 15-20 mph wind) | 3/8 to 1/2 Inch | 2.5 to 3.5 Inches | 1,400 Gallons / Week |
| Active Leak (Plumbing / Shell Crack) | 1/2 to 1+ Inch | 4.0+ Inches | 2,000+ Gallons / Week |
How to Reduce Evaporative Water Loss
In drought-prone regions where municipal water rates are tiered, excess evaporation can add hundreds of dollars to monthly utility bills. You can slash evaporation using simple mitigation tactics:
- Use a Solar Blanket: A physical bubble cover cuts evaporation by 95 percent, saving thousands of gallons of water per month.
- Liquid Solar Blankets: Invisible mono-molecular alcohol layers spread across the pool surface to suppress vapor release, reducing evaporation by 30% to 40% without handling plastic covers.
- Windbreak Landscaping: Installing perimeter privacy fences, hedges, or retaining walls blocks surface cross-breezes, dramatically reducing convective moisture loss.
- Turn Off Waterfalls and Fountains at Night: Aerating water into mist exponentially increases surface contact area with cool dry air, skyrocketing evaporation rates.
pool water evaporation FAQs
How many gallons of water are in one inch of pool water?
In an average 20,000-gallon pool with 450 square feet of surface area, one inch of depth equals approximately 280 gallons of water (Square Feet x 0.623 = Gallons per inch).
Why does my pool lose more water at night than during the day?
Nighttime water loss is often higher because pool water retains daytime warmth (say 82 F) while air temperatures drop into the 60s. This wide temperature differential creates high vapor pressure, pulling moisture out of the water rapidly.
Do infinity pools evaporate faster than standard pools?
Yes. Infinity edge (vanishing edge) pools constantly cascade water over a weir wall into a catch basin. This cascading waterfall sheet dramatically increases water surface area exposed to ambient air and wind, boosting evaporation by up to 50 percent.
When should I suspect a leak instead of evaporation?
If your pool consistently loses more than 1/4 inch of water per day in humid weather, or if water loss continues at high rates while the pool is covered with a solar blanket, perform the bucket test to confirm a physical leak.
More pool guides
- Pool Bucket Test: How to Detect Leaks vs Evaporation
Differentiate normal water evaporation from structural leaks with the DIY bucket test.
- Do Pool Solar Covers Work? Heat Gain, Physics, and Care Guide
The thermal science behind solar bubble blankets: heat retention, savings, and proper use.
- Signs Your Pool is Leaking: Detection, Warning Signs, and Dye Testing
Identify early structural and plumbing leak signs before soil erosion damages decking.