Grosso, G.; Dell'Omo, G.; Carere, C.

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.