How fast a parked car gets hot in Phoenix
The measured rate a closed car heats, minute by minute, and what a normal Phoenix July afternoon does to it. Nearly all of the rise is done in the first half hour.
Short answer
A closed car heats fastest in its first half hour. The 2005 Pediatrics study measured a rise of 3.2 °F every five minutes, with 80% of the hour’s total climb finished inside thirty minutes and the interior still rising until about the hour mark. In Phoenix the outside air is already 104.8 °F at four in the afternoon in a normal July, so the car starts its climb from a number most cities never reach.
A closed car is not a slow oven. It is a fast one, and the reason is worth understanding before the numbers: sunlight passes through glass, is absorbed by the seats, dashboard and trim, and leaves again as heat that the glass will not let out. The air inside is not being heated from the outside in. It is being heated from the inside out, which is why a car at 100 °F outside can pass 140 °F inside while the air around it stays where it was.
Everything below comes from two documents. One is a measurement. The other is a calculation built on that measurement, and the two are kept separate on purpose.
Sixty minutes in a closed car at 100 °F outside
The interior temperature at five-minute steps, drawn from the published rise table. The dashed line is the outside air, which does not move.
Rise table published by Jan Null, CCM (San José State University) and labelled by its author as calculated from the 2005 Pediatrics study. The ambient line is the outside air at 100 °F and is the same in every column of the published table, which is a property of the calculation rather than a finding about shade or windows. The tool draws finer five-minute steps and lets you set the outside temperature.
The timeline, in the study’s own numbers
Read the climb as a sequence rather than a curve, because that is how it happens to somebody running an errand.
Minute 0. The doors close on air at whatever the outside temperature is. This is the coolest moment the interior will have for the rest of the day: the outside air is not a floor the car will return to, it is a starting point the car leaves behind immediately.
Minute 5. The published table puts the interior 10 °F higher. That is the fastest five minutes of the hour, and on the 2005 study’s own measured rate it works out at 3.2 °F per five minutes.
Minute 10. 19 °F above the outside air. Nearly a fifth of the hour’s total climb is done before a person has finished a short queue.
Minute 20. 29 °F above. Two-thirds of the hour is behind it.
Minute 30. 34 °F above, which is 80% of everything the hour will do. This is the figure worth carrying around: the car is four-fifths of the way to its maximum before half an hour has passed. An errand that feels quick is not.
Minute 45. 40 °F above. The curve has flattened noticeably, five more degrees across fifteen minutes where the first fifteen minutes produced twenty-four.
Minute 60. 43 °F above, and the published study found the interior stopped climbing here. It does not keep going up in a straight line, and it does not come back down while the sun is still on it.
| Outside air | After 10 minutes | After 30 minutes | After 60 minutes |
|---|---|---|---|
| 100 °F | 119 °F | 134 °F | 143 °F |
| 105 °F | 124 °F | 139 °F | 148 °F |
| 110 °F | 129 °F | 144 °F | 153 °F |
The rise is the same in every row. That is what the published table does: it holds the climb constant and moves the starting point, so the only difference between the rows is the air the car was parked in.
What the study actually measured
The measurement behind all of this is narrow, and knowing how narrow is the difference between trusting it and over-reading it.
McLaren, Null and Quinn, writing in Pediatrics in 2005, placed a sensor 15 inches above the rear seat of a single dark-blue car and recorded it across sixteen cloud-free days. The outside air over those days ran from 72 °F to 96 °F. The interior rose by an average of 41 °F over the hour, with individual days ranging from 28 °F to 49 °F of rise, a spread wide enough that any single number from the study should be treated as a typical figure rather than a promise.
Three results from it are worth keeping:
- The rate. 3.2 °F every five minutes.
- The shape. 80% of the hour’s rise happened in the first thirty minutes.
- The coolest day. The outside air was 72 °F, a genuinely pleasant afternoon, and the interior still reached 117 °F.
That third one is the sentence that does more work than any Phoenix figure, because it removes the excuse. This is not a hazard that requires a hundred-degree day to exist.
What Phoenix adds
The normals make the derived table’s columns the normal case rather than the extreme one.
A normal July day at Phoenix Sky Harbor peaks at 104.8 °F in the four o’clock hour, and August at 103.3 °F. The normal July high is 106.5 °F. The 105 °F row in the table above is therefore not a hypothetical; it is a Tuesday. And eight hours of a normal July day are at or above 100 °F, so the exposure window is not the twenty minutes around noon: it runs from mid-morning to well into the evening.
Put the two documents together and the arithmetic for a normal July afternoon is straightforward: air at 104.8 °F, interior at roughly 139 °F after half an hour, roughly 148 °F after an hour. Nothing alive should be inside it, and the decision is not about how long the stop is, because the number that matters is reached in the first thirty minutes.
The two things these sources do not measure
It is one car. One dark-blue vehicle, one sensor position, sixteen days, all cloud-free. Interior colour, glass area, sun angle and how much of the cabin is in shade all move the figure, and the study was not designed to isolate any of them.
The table is a calculation, not a measurement at each temperature. The five-minute rise grid published from the study is labelled by its author as calculated from published research. It is a defensible way to extend a narrow measurement across a temperature range, and it is still an extension, which is why this page separates the measured figures from the derived ones rather than blending them into a single number.
Neither source tests a cracked window, a sunshade or a shaded parking spot, so this page makes no claim about any of them.
The part that is about more than a car
Everything on this page applies to an animal in the same way. The reason the site pairs this article with the one on ground temperature and paws is that the two are the same afternoon seen twice: a surface that is 50 °F hotter than the air under the dog, and an enclosed space that is 40 °F hotter than the air around it.
For a person, the end point of that interior temperature is the condition the CDC describes as heat stroke, in which the body can no longer control its own temperature. The Heat-Illness Triage Guide is where this site keeps the CDC’s symptom list and first-aid steps, and the comparison article is where it explains why sweating is a poor test for it.
Where this page stops
The numbers here are published figures about a closed car in sun on cloud-free days. They are not a measurement of your car on your street, and the tool that draws them lets you set the outside temperature precisely because your number will differ from these.
What the page can say without hedging is smaller and more useful than a temperature: the rise is nearly complete within half an hour, so a closed car in a Phoenix summer is not a place to leave anyone, however brief the stop is meant to be, and the Parked-Car Heat Timeline exists so that the figure in front of you is a real one rather than a remembered one. If a person or an animal has already been in a hot car and is unwell, the guidance on the triage tool and on the CDC’s own page is to treat it as an emergency and call 911.
- McLaren C, Null J, Quinn J. “Heat Stress From Enclosed Vehicles.” Pediatrics, July 2005; 116(1):e109–e112
The measured rate of rise (3.2 °F every five minutes), the 80% share completed in the first thirty minutes, the hour mark at which the interior stopped climbing, the 72–96 °F ambient range of the study, the mean rise of 41 °F, the 28–49 °F spread, and the 117 °F the interior reached on the coolest day of the study.
- Jan Null, CCM, San José State University: “Vehicle Interior Temperature Rise vs. Elapsed Time”, calculated from the 2005 Pediatrics study
The five-minute rise figures the chart and the table are drawn from, published from 70 °F to 110 °F ambient and labelled by its author as calculated from the study rather than measured.
- NOAA National Centers for Environmental Information: U.S. Climate Normals 1991–2020, hourly summaries, Phoenix Sky Harbor (USW00023183)
The normal hourly temperature of a July and August day: the 104.8 °F peak hour, and the eight hours of a normal July day at or above 100 °F.
- NOAA National Centers for Environmental Information: U.S. Climate Normals 1991–2020, monthly summaries, Phoenix Sky Harbor (USW00023183)
The normal July high of 106.5 °F, quoted in the section on what Phoenix adds to the measured figures.
- CDC National Institute for Occupational Safety and Health: “Heat-related Illnesses”
The definition of heat stroke used in the closing section: that the body can no longer control its temperature.