Land-based thermometers are frequently compromised by the urban heat island effect. The microclimate around a land-based thermometer can change due to urban encroachment, such as nearby asphalt, concrete, buildings, air conditioners, cars, electrical appliances, or changes in vegetation. The annual mean air temperature of a city with 1 million people can be 13C warmer than its surroundings . In the evening, the difference can be as high as 12C.