From Glossary of Meteorology
Evaporative cooling effect due to heat advection when a source of water exists in an otherwise arid area.
In addition to true desert oases, the oasis effect is also characteristic of natural bodies of water in arid surroundings, melting snow patches, irrigated fields in arid areas, or irrigated urban lawns and parks. Latent heat flux from such an oasis can exceed the locally available radiative flux twofold; advection of sensible heat from surrounding warmer surfaces and airmass subsidence over the cooler area provides the remainder. Evaporation also exceeds the local precipitation, the extra water coming from wells, river flow, and irrigation.