Pool care through a Phoenix year

A Phoenix pool has two lives and the calendar splits them roughly in half. From October to May it is a body of water sitting in a mild, dry climate that is kind to it. From June to September it is a body of water sitting in air that is often hotter than the water itself, and everything the owner does is a negotiation with that.

This article walks the year in order, using the hourly and monthly normals for Phoenix Sky Harbor. It is about the schedule rather than the chemistry: this page does not print doses, and it does not estimate a bill. For the specific question of why August water is warm, why pool water gets too warm is the article to read next.

The year in one figure

The single most useful number for a pool owner here is not a temperature. It is how many hours of the day the air is above 80 °F, because that is the air the water is trying to lose heat to.

Two dates fall out of it. The normal low last drops below 80 °F on 19 June and does not do so again until 12 September, which is 84 consecutive nights in the eighties. And the last night at or above 70 °F is 5 October, after which the water finally has cool air to work with.

January to March: the pool nobody swims in

These are the three months when the pool is not a swimming pool so much as a piece of outdoor equipment with water in it.

The normal highs run 67.6 °F, 70.8 °F and 78.1 °F, and the normal lows 46.0 °F, 49.0 °F and 54.5 °F. Not one hour of a normal January, February or March day reaches 80 °F, which means the air is doing the pool a favour for the entire season: the water loses heat to it at night and gains very little back in the afternoon.

This is also the only stretch of the year with a freeze question in it. The normals are gentle (0.4 nights below 32 °F in January, 0.1 in December and February) but the station record back to 1933 is not: 212 January nights at or below freezing, 140 in December, 35 in November, and a January low of 17.1 °F. What that means for a particular pool is a question for the equipment’s own manual, which is where the instruction for your pump and controller belongs.

April and May: the switch-on

April is the first month with any hours above 80 °F at all (seven of them, in the afternoon) and May takes it to fifteen. The normal high goes from 85.5 °F to 94.5 °F: warm enough to swim in, and still with nights in the 60s and 70s to cool the water down.

That combination is why the local trade treats this as the opening season. The water is comfortable, the overnight cooling still works, and the equipment is being asked to run daily for the first time since October. Whatever the system needs before it runs every day (the filter, the pump, the timer) belongs in this window rather than in June.

The dates to hold in mind are 18 May and 31 May: the first is the start of the 141 nights at or above 70 °F, and the second is the day the normal high first reaches 100 °F.

June: the month the nights stop helping

June is the transition month, and it is easy to miss.

The normal high reaches 104.2 °F, the normal low goes to 78.6 °F, and twenty-three of the day’s twenty-four hours are at or above 80 °F. On 20 June the normal low stops dropping below 80 °F altogether, and the pool enters the stretch that why pool water gets too warm deals with directly: with no cool night air and no wind to speak of, a pool loses heat only by evaporation, which is a slow process and one that goes slower as the humidity rises.

July and August: the monsoon months

July and August are the only two months in which all twenty-four hours of a normal day sit at 80 °F or above. The normal July low is 84.5 °F, the normal high 106.5 °F, and August is a degree behind at 83.6 °F and 105.1 °F.

They are also the two wettest months of the year in the normals: 0.91 inches in July and 0.93 in August, against 0.02 in June. Almost all of that arrives in a handful of large monsoon days, and it arrives with dust, with leaf litter from a yard that has been shedding since the first hot week, and occasionally with so much of it at once that a pool is visibly different the next morning.

Two things follow. The first is the chemistry check after a storm, which belongs on the same afternoon as the clearing up rather than the following weekend. The second is where the water goes: if the level has to be brought down, the official guidance on draining a pool directs it to the sanitary sewer rather than the street or a storm drain, and the county is the authority on that rule rather than this page.

September and October: the second turn

The water starts cooling again on 12 September, when the normal low drops back below 80 °F for the first time in twelve weeks. By the end of the month the normal low is 78.1 °F, the normal high is 100.4 °F, and twenty-two hours of the day are still above 80 °F, cooling, but not much of it.

October is the month that changes. The normal high falls to 89.2 °F, the normal low to 65.6 °F, and the count of hours at or above 80 °F drops from twenty-two to ten. On 5 October the normal low goes below 70 °F and stays there until 18 May. A pool that was uncomfortably warm in August is usually comfortable by the middle of October without anything being done to it.

November and December: the short season

The last two months have no hours above 80 °F at all. The normal high is 76.5 °F in November and 66.2 °F in December, and the normal low is 53.7 °F and then 45.3 °F.

This is the cheapest stretch of the pool year in every sense except one, and the exception is the same one as in January: December is the month with 140 nights at or below freezing in the station record, and November has 35. A covered pool in a mild winter needs very little; an uncovered pool in the same month loses its heat every night.

The equipment calendar behind the water calendar

Three equipment decisions run alongside the temperature year.

The cover is the simplest. The EPA’s WaterSense guidance treats a pool cover as one of the basic ways to reduce evaporation and heat loss, and in a city where the water a pool loses mostly goes up rather than out, that is the intervention with two effects: it saves water and it keeps some heat in the water at the same time. It also keeps dust and monsoon debris out of the pool, which is why the local trade recommends one regardless of the water argument.

The pump schedule is where the pool meets the electricity question. Both Valley utilities publish time-of-use plans whose on-peak windows fall on weekday afternoons and evenings, the same hours the pool’s filter most often runs on a factory default. The hours differ by plan and change from time to time, so the peak-hours planner links to the utilities’ current plan pages rather than describing a window of its own. This page makes no claim about what any schedule costs.

The winter setting is the last one, and it is the one to decide in October rather than December: how much the system runs in a month when nothing is swimming. Whatever the answer is, it is a better decision made in the shoulder than during the first cold night.

Safety is a separate subject

Nothing in this article is pool safety guidance, and it is deliberately not mixed in with the seasonal schedule. The pool safety around children article covers that ground, and the CDC publishes the national guidance on supervision, barriers and water quality. If a pool is going to be open through a summer, those two sources belong together on the same shelf as this page.

What this article does not print

It does not print a water temperature for a Phoenix pool, and it does not print chemical doses, run times or costs. All four depend on things this page cannot know: the volume and surface area of the pool, whether it is covered, how much of the day it is in shade, what the pump and filter are, and how the household uses it. What the normals can do is date the year, and they do that precisely enough to plan around.