What is the single most important factor in determining the temperature of a region?

Geographical location is the single most important factor in determining the temperature of a region.

Explanation:

The location of an area and its surrounding environment is most important in its temperature determination. As the area located near to the ocean has a low temperature. And areas which are far away from the waters and there is low precipitation which means low rain. It indicates the higher temperature areas.

Elevation of region, height from the sea level also affects the temperature of region.