Heat-Safe Hours Planner

The planner answers one question (which hours of a normal day in a given month are workable for a given activity) and it answers it by combining two things that come from completely different places. Keeping those two apart is the whole design.

The measurements are official

Temperature and dew point for each of the 24 hours of each of the 12 months come from the NOAA National Centers for Environmental Information 1991–2020 hourly climate normals for Phoenix Sky Harbor International Airport, station USW00023183. A normal is an average: the 4 p.m. figure for July is the mean of every 4 p.m. in July across the thirty years from 1991 to 2020. It is not a typical day, because there is no such thing, but it is the most defensible single number available for "what is July like", and it is the same number the National Weather Service uses when it describes the climate of this station.

The heat index is computed rather than read. The equation is the Rothfusz regression, which is the official National Weather Service heat index equation, with the two standard adjustments the NWS applies at low humidity and at high heat. It is fitted only above 80 °F. Run below that it returns nonsense (a January hour at 49 °F comes back at 102 °F), so the computation returns the air temperature unchanged below the threshold, which is what the NWS itself does.

The categories are official; the ceilings are ours

The colour of each bar is the National Weather Service heat-index category for that hour: Caution from 80 °F, Extreme Caution from 90 °F, Danger from 103 °F, Extreme Danger from 125 °F. Those thresholds and those names are the NWS’s, and this site does not adjust them.

What the categories do not do is tell you whether to go outside. They describe risk from exposure, which is a different thing from risk during exertion. A person sitting in shade at a heat index of 95 °F is in a genuinely different position from a person running at the same figure, and one set of thresholds cannot speak to both. So the site sets its own ceiling for each of four activities, and says out loud that these are editorial:

• Sitting outside, in shade: 103 °F. At rest the body is close to balancing the heat, so this runs to the top of the Extreme Caution band.
• Walking and light errands: 96 °F. Moving about, but with buildings and cars to step into, so short hops are workable higher up the band than sustained work would be.
• Yard work or a dog walk: 90 °F. Sustained exertion, often in full sun, and hard to stop halfway. The ceiling sits at the floor of the Extreme Caution band so the work finishes before the risk starts.
• Running, cycling or hiking: 85 °F. Hard exertion in dry heat, where thirst arrives after the fluid deficit does. Below the first published category on purpose.

What the strip is doing

Each of the 24 bars is one hour, left to right from midnight. The fill is the NWS category. A green outline marks every hour below the chosen ceiling. The gap between the two (hours that are coloured but not outlined) is where the useful information is: those are hours the NWS would call Extreme Caution and the site would call too much for the activity you picked.

A safe window is the longest unbroken run of outlined hours, and it is allowed to cross midnight, because a run from 10 p.m. to 6 a.m. is one continuous stretch of usable time even though the calendar disagrees. Runs are capped at 24 hours, so a month with no dangerous hours reports "All 24 hours" rather than a window that starts and ends at the same place.

The table repeats every figure in text, including the categories, so nothing on this page is available only as a colour.