Pool Evaporation in Florida: How Much, Why It Matters, and How to Manage It

A Florida pool loses more water to evaporation in August than most Northern pools lose all summer. That evaporation isn't just a bill from the autofill — it's the single largest energy loss from any heated pool, and it reshapes the chemistry you add every week. Understanding it quantitatively is the difference between chasing numbers and managing them.

How much water actually evaporates

For a typical South Florida summer (warm water, warm air, modest wind), a pool loses roughly 1/4 to 1/2 inch of water per day to evaporation. On a hot, breezy day with low humidity, it can hit 3/4 inch. For context, a 15,000-gallon rectangular pool with 450 sq ft of surface loses about 80–140 gallons per day in mid-summer before any splashout or backwash.

The PHTA's reference: evaporation of one gallon of pool water removes roughly 8,730 BTUs of heat energy. Evaporation is responsible for about 50% of all pool heat loss — more than radiation, convection, and conduction combined.

The four variables that drive evaporation

• Water temperature vs. air temperature. The bigger the gap (hot pool, cool night), the faster evaporation runs.
• Relative humidity. Low humidity = fast evaporation. This is why Phoenix pools evaporate faster than Miami pools at the same temperature.
• Wind speed. Air moving across the water surface removes saturated air and replaces it with drier air. A screened enclosure dramatically cuts evaporation.
• Surface area. Linear with pool size. Larger pool = more evaporation, proportionally.

Chemistry consequences

Every gallon of pool water that evaporates leaves behind every dissolved solid it contained— calcium, salt, CYA, whatever. Water exits as pure vapor; the chemistry stays in the pool and concentrates. Over a summer, you can see:

• Calcium hardness drift up, especially with hard Florida fill water topping off the autofill.
• Salt levels climb above the cell's rated range in chlorine-generator pools.
• CYA creep upward, then crash back down when a heavy rain dilutes it.
• Total dissolved solids (TDS) build to levels that trigger salt-like taste, reduced sanitizer efficacy, and scale.

Managing evaporation on a service route

1. Pool covers when practical.A bubble or liquid cover can reduce evaporation 50–70%. Practical for rarely-used pools; impractical for residential daily-use pools.
2. Water drop > 2 inches in a week with no cover— not evaporation. That's a leak. Run a bucket test.
3. Check CH, salt, and TDS monthly in summer. Partial drain-and-refill is the reset if numbers climb out of range.
4. Adjust autofill high-limitso it doesn't run for hours on a skimmer-deep pool during a heavy-evaporation week.

Indoor-pool considerations

Indoor commercial pools evaporate into the building, which is why natatorium humidity control is its own engineering discipline. Relative humidity should sit at 50–60% for occupant comfort and building protection; air temperature is maintained 2–4°F above water temperature to discourage condensation and slow evaporation. A natatorium without a proper dehumidifier has a short life — the roof structure corrodes from water vapor in a matter of years.

Evaporation is physics, not a problem to be solved. The goal is to understand it well enough that the chemistry, the water bill, and the heater schedule are planned around it instead of fighting it.