When is it cool enough to walk the dog? Hot hours counted from ten years of hourly weather in eight cities: our calculation, 2026-10-30

The answer first (our calculation [1] from hourly ERA5 weather 2016-2025, daylight hours, June-August, air temperature)

Lines used: 20 C, from which Vets Now says dogs can be at risk of heatstroke, and 25 C, from which The Kennel Club says not to walk a dog, both as reported by GoodTo [2]. 28 C is an extra example line (labelled), not a guideline.

• Phoenix: 97.8% of summer daylight hours are at or above 25 C; even the coolest daylight hour (6:00) averages 28.1 C.
• Miami: 97.0% at or above 25 C; its coolest summer daylight hour (7:00) averages 26.0 C.
• London: 9.0% at or above 25 C, about 21 days a year reaching 25 C; Seattle 12.9%, about 37 days.
• In every city the coolest summer daylight hour is the first one or two after sunrise (4:00-7:00 local), and the hottest is mid-afternoon (13:00-16:00).
• So for an owner who is out all day, a dog walker booked for early morning beats lunchtime in all eight cities; in Phoenix and Miami no summer daylight hour is below 25 C on average.

The table (our calculation [1]; June-August daylight hours, 2016-2025)

| City | Hours at or above 20 C | at or above 25 C | at or above 28 C (example line) | Days a year reaching 25 C | Coolest daylight hour (mean) | Hottest hour (mean) | | Phoenix | 100.0% | 97.8% | 91.8% | 246 | 6:00 (28.1 C) | 16:00 (40.2 C) | | Miami | 100.0% | 97.0% | 67.3% | 311 | 7:00 (26.0 C) | 13:00 (30.0 C) | | Denver | 77.8% | 56.0% | 38.6% | 122 | 6:00 (14.8 C) | 15:00 (29.6 C) | | Los Angeles | 80.4% | 54.0% | 33.4% | 188 | 6:00 (16.8 C) | 14:00 (30.2 C) | | New York | 88.4% | 52.4% | 24.9% | 102 | 6:00 (19.6 C) | 15:00 (27.5 C) | | Chicago | 81.3% | 36.7% | 15.4% | 78 | 6:00 (19.1 C) | 15:00 (25.6 C) | | Seattle | 45.0% | 12.9% | 3.6% | 37 | 6:00 (14.0 C) | 16:00 (23.1 C) | | London | 40.0% | 9.0% | 3.3% | 21 | 4:00 (12.9 C) | 15:00 (21.8 C) |

How we did it

Hourly ERA5 reanalysis from the Open-Meteo historical weather API (https://open-meteo.com/en/docs/historical-weather-api), 2016-01-01 to 2025-12-31, one point at each city centre, local time; temperature_2m (air temperature at 2 m) and is_day. "Days a year reaching 25 C" counts days with at least one daylight hour at or above 25 C, averaged over the ten years. The same data and method as walkitoffdaily.com's walkable-hours count.

What this cannot tell you

• Air temperature, not pavement: asphalt in sun is much hotter than the air. GoodTo [2] quotes a trainer's test: if you cannot stand on the surface in bare feet for 10 seconds, it is too hot for paws.
• Humidity, sun and the dog matter: flat-faced breeds, old, overweight or thick-coated dogs are at risk sooner. A vet's advice for a given dog comes first.
• One grid point per city centre; suburbs, parks and shade differ.
• The 20 C and 25 C lines are reported secondhand (GoodTo, 2022); we could not open the original Vets Now and Kennel Club pages from our tools.

Sources

[2] GoodTo, Ellie Hutchings, "When is it too hot to walk your dog?", 19 July 2022, https://www.goodto.com/family-news/when-too-hot-walk-your-dog