How hot Phoenix gets, month by month

There is a version of Phoenix that exists on a calendar and a different one that exists in a thermometer, and the second is the one that governs how people live here. The calendar says summer runs from late June to late September. The normal high says the hot season starts in May and ends in October, and that the two months either side of it are not summer at all but something closer to a second spring.

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

The five-month hot season

Counted from the normals, the year splits cleanly at 100 °F.

The normal high first reaches 100 °F on 31 May and last reaches it on 18 September. That is a run of 111 days (just over a third of the year) during which the normal afternoon temperature is at or above the century mark. Not the hot afternoons. The normal one. The annual figure says the same thing from the other direction: Phoenix averages 111.3 days a year at or above 100 °F, which is the highest count of any major city in the United States.

Then there is a hotter core inside it. The normal high reaches 105 °F on 18 June and falls back below it on 22 August: 66 days, just over nine weeks. July and August are the months people here build their lives around without quite saying so: the errands that get done before nine in the morning, the yard work that stops for the summer, the evening walks that move indoors.

What the spring actually does

The interesting part of the Phoenix year is not July. It is the speed of April and May.

March has a normal high of 78.1 °F, genuinely pleasant, the month the last of the winter visitors are still here and the patio is usable at lunchtime. April jumps to 85.5 °F. May jumps again to 94.5 °F, a full nine degrees in one month, and by the end of May the normal high has already crossed 100 °F.

Nine degrees in thirty days is a change most temperate cities make over a season. It is why the local advice about planting, about putting shade cloth over a vegetable bed, and about when to stop pruning is all compressed into a few weeks: spring here does not arrive, it passes through on its way to summer. The planting calendar exists because of exactly this.

Why the nights matter more than the highs

Look at the low column and the shape of the problem changes.

The normal low for July is 84.5 °F. The coolest hour of a typical July night in Phoenix is not cool at all, and because of how the urban heat island works over a city of this size, the outlying parts of the Valley are only a few degrees better. There is no part of a July day that is comfortable without air conditioning, and there is no part of a July night that is either.

That is what makes the heat here a residential problem rather than an outdoor-recreation one. In a mountain town, the afternoon is hot and the night is a relief; a house can be opened at nine and closed at eight. Here, the house never gets the relief, which is why July electricity bills in the Valley are among the highest residential bills in the country and why cooling system maintenance is a summer-critical task rather than a spring chore.

The two shoulder months behave completely differently, and this is the part that surprises people who move here. By mid-October the normal low has dropped to 65.6 °F while the normal high is still 89.2 °F, a thirty-degree swing in a single day. November and December are wider still. A coat in the morning and a shirt at lunch is normal Phoenix behaviour from October through March, and it is not an affectation.

The months that are not hot at all

From November to March, heat is not the constraint on anything.

The normal high runs 66.2 °F in December, 67.6 °F in January and 70.8 °F in February, a stretch that would be an excellent spring in most of the country and is, here, simply the cool season. March at 78.1 °F is the last comfortable month before the ramp.

Those five months are when everything that requires being outdoors gets done. Trail closures that protect hikers in July are irrelevant in January. The Heat-Safe Hours Planner reports that all twenty-four hours of a January or December day sit below any of its activity ceilings, because the heat index cannot reach the first National Weather Service category at 80 °F, and the normal January high does not get there either.

What the cool season does bring is a risk most newcomers do not expect. The normal number of nights at or below freezing in January is 0.4; in December and February it is 0.1. Those are small numbers, and they are averages over thirty years, which is to say the normals describe a season where freezing is not normal, but the station record back to 1933 shows it happening often enough that any tender plant here is a plant that needs a plan for a December cold snap. The Frost & Freeze Date Explorer works from that longer record rather than from the normals, which is the difference between “this is rare” and “this has happened 212 times in January”.

Where the year turns

The single hottest day of the normal year is 4 July, at a normal high of 106.9 °F. The coldest night is 20 December, at a normal low of 44.4 °F.

Those two dates are a useful pair to hold in mind, because the distance between them is the whole story of living here. The gap between the hottest normal afternoon and the coldest normal night is 62.5 °F, a big number, but not the number anyone expects. What surprises people is where the extremes sit: the hottest day lands in the middle of the monsoon, when the humidity is at its highest, and the coldest night lands four days before Christmas, when nobody is thinking about the yard.

If you are planning something outdoors, the Outdoor Event Heat Check will tell you how often your specific date has actually reached a temperature, from the station record rather than from these averages, which is the more honest question when you only get one attempt at the day.