Why pool water gets too warm in a Phoenix summer

A pool does not have a temperature of its own. It has the temperature the air, the sun and the sky give it. Every hour the water is warmer than the air around it, it loses heat; every hour the air is warmer, it gains it. In a Phoenix summer the second case runs almost all day and most of the night, and the water climbs until something other than the atmosphere starts removing heat.

That is the whole answer to the question people ask in August, when the pool is too warm to be refreshing and too expensive to drain. The pool is not malfunctioning. It is sitting in the middle of a very large, very slow oven, and the numbers that describe that oven are all in the NOAA normals rather than in the pool equipment.

Everything below is a climate normal: the average of thirty years, 1991 to 2020, recorded at Phoenix Sky Harbor International Airport, station USW00023183.

The night that never gets cool

The normal low for July is 84.5 °F. That single figure explains most of what a Valley pool does in midsummer. The coolest hour of a typical July night is warmer than a comfortable swimming pool, which means the water cannot shed heat to the air at any point in the twenty-four hours. It can only shed heat by evaporating, by radiating to a clear sky, or by being physically drained and replaced.

Counted from the daily normals rather than the monthly ones, the stretch on which the normal overnight low stays at or above 80 °F runs from 20 June to 11 September: 84 days, comfortably more than a quarter of the year. For those twelve weeks the pool’s night-time cooling is not slow. It is absent.

The afternoon that does the damage

The nights are what make the problem permanent, but the afternoons are what start it.

In a normal July day at Sky Harbor, the temperature falls to its minimum of 85.6 °F at 6 a.m. and reaches its maximum of 104.8 °F at 4 p.m. Between those two points it is above 90 °F for seventeen of the twenty-four hours, and at or above 100 °F for eight of them: a run that begins at 1 p.m. and does not end until 9 p.m.

Hours here are the hour beginning at the time shown, and they are local standard time: Arizona is on MST all year and does not move its clocks.

Every count in that table is this publication’s arithmetic on the hourly normals, not a figure NOAA publishes. The sky adds to it. At Phoenix’s latitude on a July afternoon the sun is high enough that direct radiation is landing on the water all day, and a pool’s surface absorbs most of what reaches it. The result is the pattern most Valley pool owners recognise: the water is at its most bearable at seven in the morning, tolerable at noon, and by five in the afternoon it is warmer than the air, which is a strange thing to feel with your hand.

What actually moves the number

A pool has four ways out and one way in, and knowing which is which is the difference between fighting the climate and working with it.

• Evaporation is the largest loss in a dry climate, and it is the one a cover removes. In June, when the normal 4 p.m. dew point is 34.8 °F, the air is dry enough to take water at a high rate. In August, when the normal 4 p.m. dew point is 54.8 °F, the same pool evaporates less and cools less.
• Radiation to the sky is a real loss on a clear night, and it is why an uncovered pool drops further overnight than a covered one. It is also why a pool’s temperature can fall below the air temperature on a cool, clear night in October: the water is radiating to something much colder than the air.
• Conduction and convection to the air are the losses that disappear in July and August, because the air is warmer than the water for most of the day.
• Make-up water is an input, not a loss: in midsummer, refilling a pool from a hose delivers water at whatever temperature the hose and the ground have reached, which in July is warm to the touch.
• Sun on the surface is the only large gain, and it is also the one a homeowner can change most easily. That is what the Sun Path & Shade Finder exists for: it gives the sun’s altitude and bearing through the day at this latitude, which is what you need in order to know whether a shade structure, a sail or a tree will actually intercept the afternoon sun or miss it by the width of the yard.

What this page does not do is print a pool temperature. No agency measures one for Phoenix. Arizona’s pool water is not a climate record, and the site will not estimate a figure and present it as measured. What is published (the air temperature by hour, the low by month, the dew point) is enough to explain the behaviour, and it is where the explanation stops.

The shoulder months, and why September feels different

The pool’s year turns about three weeks before the weather’s does.

Through September the normal high is still 100.4 °F, which is hot by any measure, but the normal low has fallen to 78.1 °F. Below about 80 °F the pool spends several hours a day warmer than the air, so the direction of heat flow reverses for part of every night and the water stops climbing. By October the normal low is 65.6 °F against a normal high of 89.2 °F, and a pool that was unswimmable in August is refreshing again.

The same arithmetic runs in reverse in spring, and it is why late May can feel worse than the numbers suggest. The normal low on a May night is 69.5 °F (cool enough for the pool to lose heat) but the normal high is 94.5 °F, and the normal count of days at or above 100 °F in May is nine. The pool is being heated hard in the afternoon and cooled modestly at night, and which of the two wins depends on how much shade the yard gets, how much wind moves across the surface, and how long the pump runs. Those are the variables the Pool Month-by-Month Planner puts on a calendar, because they are the ones that change month by month rather than once.

Where the heat finally goes

There is no trick that makes a Phoenix pool cold in July. There is a set of decisions that change how warm it gets, and most of them are about shade and evaporation rather than about equipment.

What is worth knowing is that the climate is doing the same thing to the water that it does to the house. Phoenix nights staying hot is the single largest reason July electricity bills here look the way they do, and it is the same fact that governs the pool: the desert does not cool down at night in midsummer, and neither does anything sitting in it.