The year in Arizona, month by month
The figures come from three stations, 1991–2020 series: Phoenix (337 metres, 183.4 millimetres a year over 33.4 days), Tucson (728 metres, 269.5 millimetres) and Flagstaff (2,135 metres, 521.2 millimetres and 229 centimetres of snow). The scoring assumes you can choose your altitude, and crosses heat, rain and how usable the ground is.
The Arizona desert has a reputation for being dry and it is: 183.4 millimetres of rain a year in Phoenix over 33.4 days. What almost nobody expects is when that rain falls.
01 A monsoon in the middle of summer
From late June the prevailing wind turns and starts bringing humid air from the south. That change of regime, the North American monsoon, produces near-daily afternoon storms over a territory that a month earlier was completely dry.
The jump is best measured in Tucson: from 5.8 millimetres in June to 56.1 in July, almost ten times more, and July is the wettest month of the year there. In Phoenix, from 0.5 to 23.1 millimetres; in Flagstaff, from 7.6 to 66.3, and August climbs higher still, to 77.2.
The odd part is not the amount, but the coincidence. The months when it rains are exactly the months when it is hottest: Phoenix averages highs of 41.4 degrees in July and 40.6 in August. And relative humidity, at 19.4 per cent in June, rises to 36.2 in August.
From 5.8 millimetres in June to 56.1 in July: the month that brings the water also brings the forty degrees.
02 June, which is the genuinely hard month
Of the whole calendar, the month to avoid above all others is neither the wettest nor the busiest: it is June. Phoenix records average highs of 40.1 degrees, 0.5 millimetres of rain over 0.5 days and humidity of 19.4 per cent, the year's lowest.
It is the worst possible combination: maximum heat with no cloud to break it and no humidity to soften the radiation. The city's absolute record, 50.0 degrees, was recorded in June.
When the monsoon arrives in July, the heat does not drop but it does change character: afternoon storms cover the sun for a few hours and bring the temperature down abruptly. It is more humid and uncomfortable in another way, but it stops being a continuous oven.
03 What makes that rain dangerous
Twenty-three millimetres a month sounds like nothing, and on average it is. The problem is that it does not fall evenly: it falls on 4.6 days, and often most of it in less than an hour.
On desert ground, baked dry and thinly vegetated, that water does not soak in: it runs. The dry washes that cross the roads of the southern state can go from sand to torrent in minutes, and the storm that fills them can be twenty kilometres away and invisible from where you are.
Hence the two rules that govern summer here. You do not cross a wash with water in it, however little it looks, and you do not camp on the floor of a canyon between July and September. These are operational warnings, not rhetoric.
The other consequence of the monsoon is electrical. The storms produce a great deal of lightning, which rules out ridges, exposed viewpoints and summits from midday onwards in July and August.
04 Up top the calendar is a different one
Everything above describes the lower half of the state. On the plateau, above 2,000 metres, the same monsoon produces a very different effect: it makes July and August the months of humid, green forest.
Flagstaff receives 66.3 and 77.2 millimetres in those two months, over 12.0 and 13.3 days, almost twice as many rainy days as any other month. And its highs stay at 27.8 and 26.3 degrees.
Hence a rule that solves much of the summer: when it is forty degrees below, it is twenty-seven above. Climbing the step of the Rim is, in July, a thirteen-degree decision.
05 And winter, which is the desert's season
The other half of the annual rain arrives in winter and by another mechanism: fronts from the Pacific, gentler, longer and far less violent. In Phoenix they leave between 18.8 and 22.1 millimetres a month from December to March.
For the traveller, that is the desert's best moment. Phoenix's highs run from 19.0 to 25.6 degrees between December and March, you can walk at any hour, and the sun, with 3,871 hours a year, is still guaranteed.
The price is that the other half of the state closes. Flagstaff averages −1.1 degrees in December and −0.8 in January, accumulates 18.5 and 53.1 centimetres of snow in those two months, and some roads and forest tracks stop being reliable.
06 How to choose the month
If you are only going to one half of the state, the answer is easy. For the desert, November to March. For the plateau and the forest, May to September, accepting the afternoon storms of July and August.
If you are going to cross the step, which is what makes the trip worthwhile, two months work on both sides: April and October. In April, Phoenix gives 29.7 degrees and Flagstaff 15.1; in October, 31.8 and 17.6. Neither has monsoon or snow.
And the timetable rule applies throughout the hot half of the year: the morning is the reliable part of the day. In summer, because from two o'clock the storm builds; in spring and autumn, because at midday the desert still bites. In Arizona the calendar matters, but the clock matters more.
The year's four stretches
Highs of 19.0 to 21.6 degrees in Phoenix, with 18.8 to 22.1 millimetres a month. Below it is the best season of the year; above, Flagstaff averages between −1.1 and 0.4 degrees and gets most of its 229 centimetres of snow.
Rain falls from 21.1 to 3.3 millimetres and Phoenix's highs climb from 25.6 to 34.7 degrees. April is the balance point: 29.7 below and 15.1 above, with snow retreating and the desert still workable.
June gives 40.1 degrees with 0.5 millimetres; July and August, 41.4 and 40.6 degrees with 23.1 and 23.6. Tucson goes from 5.8 millimetres in June to 56.1 in July. The two wettest months are the two hottest.
Phoenix's highs drop from 38.0 to 24.7 degrees and rain settles between 14.2 and 14.5 millimetres. October is the only month when both halves of the state work well at once.
