No one likes it hot, but hotter cities adjust by staying active later

TL;DR

This study examines how extreme heat affects urban activity patterns across different climates, revealing that cities adapt by shifting activity to cooler evening hours rather than reducing overall activity.

Contribution

It introduces a Bactrian index to quantify bimodality in daily activity profiles and shows how heat influences activity timing differently across climates.

Findings

Hot days reduce overall activity but increase evening activity.

Hotter cities exhibit smaller activity losses and more evening substitution.

The Bactrian index measures the shift from unimodal to bimodal activity patterns.

Abstract

Extreme heat suppresses urban activity, but its effects need not be uniform across climates or across the day. Using data on activity at points of interest in 20 cities spanning temperate, tropical, and arid environments, we show that hot days reduce activity overall while shifting it away from midday and toward later hours. This rescheduling is substantially stronger in historically hotter cities, which exhibit smaller losses and larger evening substitution. To understand these changes, we introduce a Bactrian index of bimodality, which measures the degree to which a city's daily activity profile has one hump or two - one during the day and another during the evening. Arid desert cities like Doha, Amman, and Kuwait City are more Bactrian in level, but cities like Milan become Bactrian on hot days. Together, our results suggest that adaptation to heat in cities operates less through avoiding activity altogether than through moving it to cooler hours. This provides channels for adaptation in cooler cities, but it also suggests limits to adaptation in warmer ones: as evenings become warmer, these too may become intolerable.