Planning an outdoor event in a Phoenix summer

An outdoor event in Phoenix is a scheduling problem wearing a party hat. The venue, the guest list and the catering are the easy parts; the hard part is a date and an hour that the desert has agreed to, and the only way to find one is to ask the station record rather than the monthly average.

The distinction matters more here than in most cities. A monthly normal tells you what a typical July afternoon looks like. It does not tell you what your date has actually done, and an event gets one attempt at its date. That question (how often has this exact day been this hot) is what the Outdoor Event Heat Check answers, and it is the question this article is built around.

Everything below is counted from the observation record at Phoenix Sky Harbor International Airport, station USW00023183, which runs back to June 1933. Each date in the chart has between 85 and 90 years with a usable reading behind it.

Twelve months out: pick the date from the record

The chart has one message. The 15th of January, February, March, November and December has never once reached 100 °F in the record. October and April are a coin toss weighted heavily against. June, July and August are not a coin toss at all.

The percentages hide how different the risk is at the top of the season:

Two things in that table are worth the space. The first is 1 July at 110 °F: a date that is not merely likely to be hot but, in more than a quarter of the years on record, hot enough to be in the National Weather Service’s Danger band. The second is the cliff between 15 May and 1 June. Two weeks of calendar move the odds of a century day from roughly one year in four to better than one year in two.

If the event can move, that is where to move it. If it cannot, everything after this paragraph is about the hour.

Six months out: pick the hour

Moving the start time is the standard answer, and in Phoenix it works less well than the advice suggests.

On a normal July day the dry-bulb temperature peaks at 105 °F at 4 p.m. and is still 93 °F at midnight; the coolest hour of the day is 86 °F, shortly before sunrise. Counting humidity, every one of the day’s 24 hours sits at or above the first National Weather Service category, Caution, at a heat index of 80 °F; 16 hours sit at or above Extreme Caution. An 8 p.m. start is not a cool start. It is a start in the mid-nineties with the sun off the table.

Sunset does help, and it is the part worth calculating. Sunset at Phoenix Sky Harbor runs from 7:42 p.m. on 1 July to 6:55 p.m. on 1 September and 5:58 p.m. by 13 October. West-facing tables have a hard cut-off in the hour before that; an east-facing patio loses its shade at midday instead.

The Heat-Safe Hours Planner prints the whole day hour by hour for a month and an activity. One caveat belongs here rather than in a footnote: the hourly “feels like” figure is computed by this site from the Rothfusz heat-index equation, using the normal temperature and the normal dew point for the same hour. A normal high and a normal dew point do not necessarily occur together, so it is a modelled hour rather than an observed one, and the tool page says so under its limits.

Three months out: the shade question

Shade is the cheapest thing an outdoor event can add, and Phoenix is unusually well suited to measuring it. In the Arizona State Climate Office’s urban heat island survey in Mesa, asphalt in full sun measured 155 °F and concrete in shade 93 °F, a 62-degree gap between two surfaces in the same city.

That is a bigger difference than any plausible start time can buy. A shaded 3 p.m. is a different venue from an unshaded 3 p.m., and the Sun Path & Shade Finder will say where the shadow falls at a given hour on a given date, including which wall takes the afternoon sun.

Two weeks out: start reading the forecast

The National Weather Service now issues its heat alerts against HeatRisk, a five-level index that runs from little or no risk to extreme. HeatRisk is a forecast rather than a normal, so it is the first product in this article that describes the actual week rather than the climate of the date.

Two weeks is early enough to move a vendor and late enough to be looking at something with a real signal. The value at this stage is not precision; it is knowing which of the five levels is being discussed, and what the venue’s thresholds are for each one.

The day before

Confirm the shade, the water and the ice. The substances that matter are the boring ones. If there is any work to be done at the site, do it in the morning: the normal high crosses 100 °F on 31 May and does not fall back below it until 18 September, a run of 111 days, and July is the middle of it.

Check the alert level for the day and know what the plan does if it rises. And write down the fallback in a form somebody else can execute, because the person deciding at 4 p.m. will be the one standing in it.

The day itself

Two decisions, kept simple. Start earlier than the plan says, and end earlier than the plan says. The first hour of a Phoenix July evening is the best of it; the last is the worst.

If anybody at the event shows confusion, slurred speech, a seizure, fainting, or skin that is very hot or has stopped sweating, that is a medical emergency and the answer is 911. Heat stroke can also present with heavy sweating, which is the mistake most people make. The Heat-Illness Triage Guide sets out the symptoms and the first-aid steps in the CDC’s own wording.

After

Log what the day actually did. The record this article leans on is built from 93 years of exactly that, and one more observation at one more station is how it keeps growing.