Air temperature, hot days, tropical nights and icing days - Cities in Mercer

About this data

Indicator

Air temperature, hot days, tropical nights and icing days - Cities and FUAs — Change in air temperature at 2 meters from 1981–2010 period

Unit

Degrees celsius

Source

Organisation for Economic Co-operation and Development

Licence

OECD Terms and Conditions (attribution required)

Coverage

1,325 places, 59,625 data points, 1981–2025

Last refreshed

17 August 2026

This dataset provides indicators of air temperature, hot days, tropical nights and icing days. Data sources and methodology Indicators are estimated using ERA5-Land data. Air temperature indicators correspond to the average annual air temperature at a height of 2 meters. Hot days correspond to the number of days when the maximum temperature exceeds 35°C. Tropical nights correspond to the days when the minimum temperature is above 20°C, and icing days to the number of days when the maximum temperature is below 0°C. ERA5-Land is used as it provides harmonised, globally consistent coverage at fine spatial resolution (0.1 degree), enabling the production of comparable subnational indicators also where national meteorological data are not available at the required scale. These estimates may differ from official subnational climate statistics due to differences in methodological approaches, such as the use of reanalysis based top down modelling versus in situ observations, along with variations in input data sources, spatial resolution, and the models and algorithms used to generate temperature estimates. Defining FUAs and cities The OECD, in cooperation with the EU, has developed a harmonised definition of functional urban areas (FUAs) to capture the economic and functional reach of cities based on daily commuting patterns (OECD, 2012). FUAs consist of: A city – defined by urban centres in the degree of urbanisation, adapted to the closest local administrative units to define a city. A commuting zone – including all local areas where at least 15% of employed residents work in the city. The delineation process includes: Assigning municipalities surrounded by a single FUA to that FUA. Excluding non-contiguous municipalities. The correspondence table between SAUs and FUAs/cities is available in parquet and csv format. The definition identifies 1 272 FUAs and 1 269 cities in all OECD member countries except Costa Rica and three accession countries. Cite this dataset OECD Regions, cities and local areas database (Air temperature, hot days, tropical nights and icing days - FUAs), http://oe.cd/geostats Further information OECD Local Data Portal OECD Regions and Cities at a Glance For questions and/or comments, please email CitiesStat@oecd.org