Automobile traverses revealed an intense urban heat island in Dublin city during winter nights with light winds and clear skies. Urban rural contrasts of over 6.5°C may be anticipated, and a close correspondence between building density and temperature exists. The city's location close to warm sea and cold uplands appears to be responsible for the generation of active mesoscale air movement during such nights. These circulations bring both cold mountain air into the southern suburbs and warmer oceanic air into the eastern fringes and are responsible for creating substantial variations in temperature within the urban area itself. Cold air drainage along surprisingly gentle slopes occurs, channelled along the valleys of small streams particularly in the south of the city. Considerable intra-urban differences in aggregate energy demand may be anticipated as a consequence.