Monsoon rainfall normals in Phoenix, month by month

Phoenix is usually described as having a monsoon and a dry rest of the year. The rainfall normals do not quite support that. They describe two wet seasons, one of them famous and one of them quietly reliable, separated by four months in which almost nothing falls.

The figures below are climate normals, the average of the thirty years from 1991 to 2020, recorded at Phoenix Sky Harbor International Airport, station USW00023183. They describe a typical year rather than next year, and the annual total they add up to is 7.22 inches, which is roughly a sixth of what falls on Seattle.

The two months that carry the season

August and July are the wet months, in that order and by a margin that is almost nothing.

The two months together carry 1.84 inches of the 7.22-inch annual normal, which is very close to a quarter of the whole year’s rain falling in a ninth of it. Add September and it is 2.41 inches, or a third.

That is the “summer rainfall” figure people quote, and it is worth knowing which of the two numbers the quote refers to: a quarter of the annual rain falls in July and August alone, and a third falls in the three months from July to September. Both are true and they are not the same claim.

The rain-day counts say something the totals do not. August has 4.6 days with a trace of rain against July’s 3.9, and 1.2 days with at least a quarter of an inch against July’s 1.0. The season delivers its rainfall in a small number of afternoon and evening storms rather than spread across the month, which is why a wet monsoon year and a dry one can look identical on a calendar and completely different from a driveway.

June, the driest month of the year

The first half of the monsoon window is the driest stretch of the Phoenix year, and the numbers are stark about it.

June’s normal is 0.02 inches, and its median is zero: in the middle year of the thirty, no measurable rain fell at Sky Harbor in June at all. Its 75th percentile is also zero, which means three Junes in four produce nothing measurable. May manages 0.13 inches and April 0.22, and even those are lower than any month outside the spring.

So the calendar season begins in a drought and ends in one, or close to it: September’s 0.57 inches is only a little above November’s and October’s. The season’s rain sits in the middle. The Monsoon Season Clock marks the phases out (the build-up, the wet core and the wind-down) and the monthly numbers here are where those boundaries come from.

Why the average is not the typical month

Look at July’s mean and median side by side and the difference is 0.22 inches, a fifth of the whole month’s normal.

That gap is the signature of a place where rain is bursty rather than steady. A normal July is 0.91 inches, but half of all Julys deliver less than 0.69, and a quarter of them deliver more than 1.26. The mean is held up by the outstanding years, and the median describes the ordinary one.

This is not a reason to distrust the average. It is a reason to use the right one. For planning a month, the average is the honest figure for the season. For answering “what happened this July at my house”, the median is closer to what a person should expect.

The 75th percentile column is the most useful of the three numbers on the table above and the least quoted. It says what a wet July looks like: 1.26 inches in July, 1.29 in August. Those are the years the washes run, the ones the flash-flood warnings are written for.

The winter rain that gets forgotten

January and February are each 0.87 inches, which makes them the third and fourth wettest months of the Phoenix year: wetter than September, and wetter than October and November by half again.

That is a second wet season, fed by the storms that come down the coast in winter rather than by the summer thunderstorms, and it is why the annual total is as high as 7.22 inches for a desert city. The two seasons are close to equal in the middle of their own windows: January to March adds to 2.57 inches, which is more than July and August together.

The practical consequence is that a rain log kept only during the monsoon tells about half the story. The Rain Log on this site is a month-by-month record for exactly this reason, and the guide to keeping one worth keeping deals with the part that matters more than the totals: where the gauge sits, and what a trace actually means.

The limits of one gauge

Every figure on this page comes from a single station (Phoenix Sky Harbor International Airport, USW00023183) and that is both the strength and the weakness of it.

The strength is that the airport has a long, continuous, quality-controlled record and that it is the station every official Phoenix climate product refers to. When a forecast office or a news report says “normal July rainfall”, this is the number behind it, and using a different gauge would put this page out of step with every other published figure for the city.

The weakness is that a summer thunderstorm is a small thing. A monsoon cell can drop an inch on one side of a freeway and nothing at all on the other, and Sky Harbor records whichever of those it happens to sit under. Across thirty years those misses average out into a normal that is meaningful for the city as a whole and unreliable for any particular square mile of it. That is not a flaw in the data; it is what a point measurement of a patchy phenomenon is.

The practical reading is to treat these numbers as the reference line rather than as a prediction for a specific address. A rain gauge in a back yard will beat them for that purpose, which is what the Rain Log is for: it keeps a month-by-month record of what actually fell where a person lives, and it is honest about the fact that the airport’s total and one’s own will differ in most months.

Counting the season’s share

One figure on this page is this site’s own arithmetic rather than a published one, and it should be labelled as such. The monsoon window runs from 15 June, so it contains sixteen days of June, all of July and August, and all of September. Counting those sixteen days of June at June’s daily rate gives about 0.01 inches for them, and the window total is about 2.42 inches, a little over a third of the 7.22-inch annual normal.

NOAA publishes rainfall normals by month and not by day, so the sixteenth-of-June figure is an approximation of this site’s rather than a measurement. The monthly numbers behind it are the agency’s own, and they are the ones to quote. The month-by-month guide to when the season starts covers the other half of the question: why the calendar window and the wet weather do not line up.