Athens has a subtropical Mediterranean climate (Köppen Csa) and receives just enough annual precipitation to avoid Köppen's BSh (semi-arid climate) classification. The dominant feature of Athens's climate is alternation between prolonged hot and dry summers and mild, wet winters. With an average of 414.1 millimetres (16.30 in) of yearly precipitation, rainfall occurs largely between the months of October and April. July and August are the driest months, where thunderstorms occur sparsely once or twice a month. Winters are cool and rainy, with a January average of 8.9 °C (48.0 °F); in Nea Filadelfeia and 10.3 °C (50.5 °F) in Hellinikon; Snowstorms are infrequent but can cause disruption when they occur. Snowfalls are more frequent in the northern suburbs of the city.
Mount Parnitha creates a rain shadow for the city, as a result of which precipitation is typically lower than in other parts of the Balkans; for a typical comparison, Tirana receives over three times more rainfall and Shkodra about five times as much. Daily average highs for July (1955–2004) have been measured at 33.7 °C (92.7 °F) at Nea Filadelfeia weather station, but other parts of the city may be even warmer, in particular its western areas in part due to industrialization or in the main several natural reasons, knowledge of which has been available from the mid-19th century. Temperatures often surpass 38 °C (100 °F) during the city's notorious heatwaves.
The city of Athens is affected by the urban heat island effect in some areas which is caused by human activity, altering its temperatures compared to the surrounding rural areas, and bearing detrimental effects on energy usage, expenditure for cooling, and health. The urban heat island of the city has also been found to be partially responsible for alterations of the climatological temperature time-series of specific Athens meteorological stations, due to its impact on the temperatures and the temperatures trends recorded
