Beyond the urban surface: bird-borne sensing reveals the atmospheric structure of urban heat

Abstract

Urban heat is commonly characterized near ground level, while its vertical atmospheric structure remains difficult to observe across cities. We used bird-borne temperature sensors on homing pigeons (Columba livia) and yellow-legged gulls (Larus michahellis) to investigate the horizontal, vertical and daytime structure of the lower urban atmosphere over Rome. The dataset comprised 61,917 air-temperature observations from 47 individuals of gulls and pigeons, spanning 17 km and altitudes up to 400 m above ground level. Generalized additive models related air temperature to distance from the city centre, altitude, time of day, land cover, tree-patch area and topography, and reconstructed three-dimensional temperature fields. Air temperature showed a centre-to-periphery gradient that weakened with altitude and through the daytime period. At 20 m above ground level, its magnitude declined from -0.40 {degrees}C km--1 at 09:00 to -0.30 {degrees}C km--1 at 13:00 and -0.19 {degrees}C km--1 at 17:00, with weaker gradients at higher altitudes. Reconstructed isothermal boundaries showed a complementary vertical pattern. At 13:00, median heights decreased from 135 m for the 34 {degrees}C boundary to 76 m for 35 {degrees}C and 37 m for 36 {degrees}C. The 34 {degrees}C boundary reached its greatest extent around midday, rising from 57 m at 09:00 to 135 m at 13:00 before declining to 14 m at 17:00. Horizontal thermal contrast and the vertical development of high-temperature air represent complementary dimensions of urban heat. Bird-borne sensing offers a promising approach for resolving three-dimensional atmospheric temperature patterns that conventional monitoring networks cannot capture.