Keeping a rain log worth keeping

The rain gauge is the only weather instrument most households will ever own, and it is also the only one that produces a record nobody else has. The National Weather Service can tell you what a storm did to a polygon; Maricopa County can tell you what its own network saw. Neither can tell you what fell in your street, and monsoon rain here is local enough that the difference is routinely an inch.

Everything a useful log needs is small: one gauge, one reading a day, a written record, and something to compare it against. What makes the difference between a log worth keeping and a page of numbers nobody reads is doing the same thing every time.

Step 1: decide what the log is for

A log answers one of two different questions, and picking one changes what you write down.

If the question is “how does my place compare with Phoenix”, the log needs a monthly total, because that is the resolution the normals are published at. If the question is “what did this storm do”, the log needs the date and the amount, because a monthly total erases the only interesting thing about monsoon rain: that it arrives in a handful of storms rather than across the month.

The figures make the case. In the normal year, August produces 0.93 inches of rain across 4.6 days, of which 1.2 days reach a quarter of an inch and 0.1 days reach a full inch. July looks the same from the other side: 0.91 inches, 3.9 days with rain, 0.2 of them at an inch or more. A monthly total of 0.93 inches tells you almost nothing about whether that month was one storm or six, and in a desert city that is the difference between a street that drained and a wash that ran.

Step 2: accept that one gauge describes one place

Monsoon rain here does not fall evenly across the Valley. The evidence for that is the county’s own monitoring: the Flood Control District runs 355 automated precipitation gauges and publishes them as a real-time rainfall map, which is a number of instruments that only makes sense if a single one is not enough.

That is the honest limit of a backyard log, and it is worth writing on the first page. Your gauge measures your address. It is not a sample of Phoenix, and a dry month at your house is not evidence that the airport had a dry month.

Where exactly to stand the gauge is a question this article does not answer. The volunteer networks that run backyard observation programmes publish their own training on siting and mounting, and this site did not open that material; what can be said here is only the thing that follows from the county’s network: put it somewhere it can catch what falls on your plot, and then leave it there, because a gauge that moves is two short records rather than one long one.

Step 3: read it at the same hour every day

The discipline is the whole method. A gauge read at seven in the morning one day and five in the afternoon the next is measuring two different storms, and neither reading is comparable with the other.

The volunteer network’s own figure for the routine is five minutes a day, which is the right order of magnitude. The hour matters less than the consistency: pick one you will actually keep in July, when the mornings are already 85 °F at sunrise and the evening is when you would rather not be outside. Whatever hour it is, the number you record covers the twenty-four hours ending at that moment, so a storm that arrives at five in the afternoon appears in the next morning’s reading, not the same day’s.

Two things are easy to get wrong and both are worth a note on the log’s first page. Record the zeros. A day with nothing in the gauge is data, and a month of zeros is the most interesting thing a Phoenix rain log has to show. And do not round up. The normals on this page are quoted to the hundredth of an inch because that is the precision the instrument and the record both carry.

Step 4: write the date down, not just the number

A page of monthly totals loses the shape of the season. A page of dated entries keeps it.

The dates are what let you answer the questions you will actually ask later: which storm filled the wash, how long the ground stayed wet, whether the rain your neighbour got on the same afternoon reached you at all. In a season where the wettest months carry under an inch and the second half of June carries almost nothing, the gaps between entries are as informative as the entries.

If you want the wider frame for a given day, the Monsoon Season Clock places any date in the 108-day season and in the phase the rainfall normals put it in: build-up, wet core or wind-down. Knowing that your biggest storm of the year landed in the wind-down, after the wet core had passed, is the kind of thing only a dated log can show.

Step 5: compare it with something that exists

The comparison is where a log becomes useful, and there are two honest ways to make it.

The first is against the normals:

Those are Phoenix Sky Harbor’s figures, and the bottom row is this site’s sum of the four months above it. Read the rain-day columns rather than the totals: they say that a normal monsoon season delivers its two and a half inches in about eleven days, of which fewer than three days reach a quarter of an inch and fewer than half a day reaches an inch.

The second comparison is against the district’s own published rainfall data, which is the closest thing to an official record for a specific part of the county. It is a published product rather than something you have to ask for, and it is the right thing to check a suspicious reading against.

What a log cannot tell you

Three limits, and they are the same ones every rain record has.

A gauge is a point measurement. Monsoon cells are small enough that a storm can drop an inch on one side of a street and nothing on the other, and the rainfall normals by month describe a station, not a neighbourhood.

The normals are thirty-year averages, so a season that runs twice the average is not an error. Phoenix’s own record has both kinds of year, and the dew point is the reason for most of the variation, which is what the article on when the season really starts is about.

And no log forecasts anything. It is a record, and its use is retrospective: it tells you what your own drainage did, how your street behaves, and how far your address sits from the airport’s number. For the current picture, the district’s weather outlook and the county’s real-time map are the places to look, and the Rain Log keeps the entries in your browser rather than sending them anywhere.