The low desert year, month by month

There is a way of describing the Phoenix year using the four seasons, and it does not fit. The desert year here has a cool season that is also the real growing season, a spring that is over almost as soon as it starts, a five-month hot season, and a monsoon window with dates of its own that ignores all of it. This page walks the year in twelve parts and names what each part asks of a household.

Everything below is a climate normal: the 1991–2020 average recorded at Phoenix Sky Harbor International Airport, station USW00023183. A normal describes a typical month, not a forecast, and in any given year these dates can sit a week or more either side of where the averages put them.

The cool season: November to March

Five months of the year ask almost nothing of a household in the way of heat management.

December has a normal high of 66.2 °F and a normal low of 45.3 °F. January is barely different at 67.6 °F and 46.0 °F, and February reaches 70.8 °F. March, at 78.1 °F, is the last month before the ramp, and it is the month the last of the winter visitors leave and the patio is comfortable at lunchtime again.

This is the season for everything that needs to be done outdoors. It is also the season with the year’s only real cold risk, and it is smaller than people expect. The normal number of nights below 40 °F is 6.8 in December, 4.7 in January and 1.7 in February. Below freezing, the normals give 0.1 nights in December, 0.4 in January and 0.1 in February, which is to say that a normal year at Sky Harbor does not freeze at all, but the record back to 1933 shows that individual winters do, and a tender plant is a plant with a December plan. The Frost & Freeze Date Explorer works from the record rather than from the normals, which is the difference between “rare” and “this has happened”.

Rainfall in the cool season is modest and steady: 0.87 inches in January, 0.87 in February, 0.83 in March. The month that breaks the pattern is April, which drops to 0.22.

The ramp: April and May

The interesting part of the Phoenix year is not July. It is the speed of the two months before it.

March has a normal high of 78.1 °F. April jumps to 85.5 °F. May jumps again to 94.5 °F (nine degrees in thirty days) and by 31 May the normal high has already crossed 100 °F. Autumn reverses the same distance much more slowly, which is why the year feels asymmetric: it climbs fast and it comes down gradually.

The other number that moves is the low. May’s normal low is 69.5 °F, already warm enough that an evening stays outside weather until late. The first nine days at or above 100 °F arrive in May, and April contributes one. If the yard has work that needs doing before the heat, this is the six-week window it has to happen in, and the Planting Calendar is built around exactly that compression.

The hot season: late May to late September

Counted from the normals, the hot season is a run of 111 days.

The normal high first reaches 100 °F on 31 May and last reaches it on 18 September. Inside that run there is a hotter core: the normal high reaches 105 °F on 18 June and falls back below it on 22 August, a stretch of 66 days. July is the hottest month at a normal high of 106.5 °F, but the margin over August, at 105.1 °F, is small.

The figure that governs the household is not the high. It is the low. July’s normal low is 84.5 °F and August’s is 83.6 °F, so the coolest hour of a typical July night is still in the mid-eighties. The normals put 84 nights a year at or above 80 °F, running 20 June to 11 September, which is the stretch when a house gets no relief from open windows and the cooling system runs around the clock. That is the month the Home Cooling Task Scheduler is written for, and the reason July electricity bills here are what they are.

One thing that does not change with the season is the shape of the day. On the hourly normals, the warmest hour is 4 p.m. in every month of the year. Only its height changes: 65.4 °F on a normal December afternoon, 104.8 °F on a normal July one.

The monsoon window: 15 June to 30 September

The National Weather Service gives the Arizona monsoon fixed dates (15 June to 30 September, 108 days) so that forecasts and warnings share one calendar. It is a planning season rather than a description of the weather, and the Monsoon Season Clock tracks where today sits inside it.

The season has three acts. The build-up runs from 15 June to about 10 July: the driest stretch, when storms are isolated and high-based and can deliver dust and lightning without much rain. The wet core runs from 11 July to 31 August and carries the season’s rain: July averages 0.91 inches and August 0.93, about a quarter of the year’s 7.22 inches between them, against June’s 0.02. The wind-down runs through September, when storm frequency falls away but the ground is still wet and the washes are still primed.

The dates and the humidity do not agree, and it is worth knowing that before the first forecast of the year confuses you. This site’s count from the hourly dew point normals puts the daily mean dew point at 55 °F on 16 July (a month after the window opens) and back below it on 8 September, three weeks before the window closes. The window is what the forecast uses. The dew point is what the weather is doing.

The two growing seasons

The Valley’s gardening year is not one season but two, split by the heat rather than by the cold.

The autumn and winter window is the main one: soil that is cool rather than frozen, a normal high in the sixties and seventies, and no normal frost to work around. The spring window is short and has to be started early, because the same nine-degree jump that carries April into May will carry a young plant into 100 °F before it has finished establishing. Midsummer is not a planting season at all for most things, and the second window reopens as the high falls back through the eighties in October.

What can be sown, transplanted and picked in each month is the subject of the Planting Calendar rather than of this page. What matters here is the shape: two openings, a long closure between them, and a frost risk in the second window that is rare but not zero.

The year at a glance

The other windows worth holding in your head are these. The normal high is at or above 100 °F for 111 days, from 31 May to 18 September. It is at or above 105 °F for 66 days, from 18 June to 22 August. The normal low is at or above 70 °F for 141 nights, 18 May to 5 October, the stretch when an evaporative cooler still works overnight. And the hottest single day of the normal year is 4 July, at 106.9 °F, with the coldest night on 20 December at 44.4 °F.

What the year asks, in one line

Read as a calendar of demands rather than of temperatures, the Phoenix year has a long cool season for doing things, a five-week spring in which to do the rest, a hot season in which the household’s job is to keep the building cool, and a monsoon window that adds water and wind to the list.

If you are planning something outdoors, the Heat-Safe Hours Planner will take the month and the activity and tell you which hours are workable, rather than leaving you to read it off a monthly average. If you have just arrived and want the short version of what changes, What the climate asks of a new resident is the next page.