Athens is known as one of the sunniest cities in Europe, with a semi-arid climate and low average annual rainfall. The rain that does occur falls during the winter months, between mid-October and mid-April, usually as short, heavy showers. Summers are very hot, exacerbated by smoggy conditions, and heat waves are common during July and August when the mercury soars to over 104°F (40C). Winters are mild, although frost can occur and nights can be cold.