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Under the Köppen Climate Classification climate classification, "dry-summer subtropical" climates are often referred to as "Mediterranean". This climate zone has an an average temperature above 10°C (50°F) in their warmest months, and an average in the coldest between 18 to -3°C (64 to 27°F). Summers tend to be dry with less than one-third that of the wettest winter month, and with less than 30mm (1.18 in) of precipitation in a summer month. SMany of the regions with Mediterranean climates have relatively mild winters and very warm summers.
The Köppen Climate Classification subtype for this climate is "Csb". (Mediterran Climate).
The average temperature for the year in Mercer Island is 53.1°F (11.7°C). The warmest month, on average, is August with an average temperature of 67.0°F (19.4°C). The coolest month on average is December, with an average temperature of 40.9°F (4.9°C).
The highest recorded temperature in Mercer Island is 80.9°F (27.2°C), which was recorded in July. The lowest recorded temperature in Mercer Island is 28.5°F (-1.9°C), which was recorded in February.
The average amount of precipitation for the year in Mercer Island is 39.9" (1013.5 mm). The month with the most precipitation on average is November with 6.5" (165.1 mm) of precipitation. The month with the least precipitation on average is July with an average of 0.8" (20.3 mm). There are an average of 159.0 days of precipitation, with the most precipitation occurring in January with 20.0 days and the least precipitation occurring in July with 5.0 days.
In Mercer Island, there's an average of 18.0" of snow (45.7 cm). The month with the most snow is December, with 9.3" of snow (23.6 cm).