In the Köppen–Geiger classification system, Istanbul has a borderline Mediterranean climate (Csa), humid subtropical climate (Cfa) and oceanic climate (Cfb), due to its location in a transitional climatic zone.
Since precipitation in summer months ranges from 20 to 65 mm (1 to 3 in), depending on location, the city cannot be classified as solely Mediterranean or humid subtropical.
Due to its size, diverse topography, maritime location and most importantly having a coastline to two different bodies of water to the north and south, Istanbul exhibits microclimates.
The northern half of the city, as well as the Bosporus coastline, express characteristics of oceanic and humid subtropical climates, because of humidity from the Black Sea and the relatively high concentration of vegetation.
The climate in the populated areas of the city to the south, on the Sea of Marmara, is warmer, drier and less affected by humidity.
The annual precipitation in the northern half can be twice as much (Bahçeköy, 1166.6 mm), than it is in the southern, Marmara coast (Florya 635.0 mm).
There is a significant difference between annual mean temperatures on the north and south coasts as well, Bahçeköy 12.8 °C (55.0 °F), Kartal 15.03 °C (59.05 °F).
Parts of the province that are away from both seas exhibit considerable continental influences, with much more pronounced night-day and summer-winter temperature differences.
In winter some parts of the province average freezing or below at night.

Istanbul's persistently high humidity reaches 80 percent most mornings.
Because of this, fog is very common, although more so in northern parts of the city and away from the city center.
Dense fog disrupts transportation in the region, including on the Bosphorus, and is common during the autumn and winter months when the humidity remains high into the afternoon.
The humid conditions and the fog tend to dissipate by midday during the summer months, but the lingering humidity exacerbates the moderately high summer temperatures.
During these summer months, high temperatures average around 29 °C (84 °F) and rainfall is uncommon; there are only about fifteen days with measurable precipitation between June and August.
The summer months also have the highest concentration of thunderstorms.

Winter is colder in Istanbul than in most other cities around the Mediterranean Basin, with low temperatures averaging 1–4 °C (34–39 °F).
Lake-effect snow from the Black Sea is common, although difficult to forecast, with the potential to be heavy and—as with the fog—disruptive to the city's infrastructure.
Spring and autumn are mild, but often wet and unpredictable; chilly winds from the northwest and warm gusts from the south—sometimes in the same day—tend to cause fluctuations in temperature.
Overall, Istanbul has an annual average of 130 days with significant precipitation, which amounts to 810 millimeters (31.9 in) per year.
The highest and lowest temperatures ever recorded in the city center on the Marmara coast are 40.5 °C (105 °F) and −16.1 °C (3 °F).
The greatest rainfall recorded in a day is 227 millimeters (8.9 in), and the highest recorded snow cover is 80 centimeters (31 in).
In the Köppen–Geiger classification system, Istanbul has a borderline Mediterranean climate (Csa), humid subtropical climate (Cfa) and oceanic climate (Cfb), due to its location in a transitional climatic zone. Since precipitation in summer months ranges from 20 to 65 mm (1 to 3 in), depending on location, the city cannot be classified as solely Mediterranean or humid subtropical. Due to its size, diverse topography, maritime location and most importantly having a coastline to two different bodies of water to the north and south, Istanbul exhibits microclimates. The northern half of the city, as well as the Bosporus coastline, express characteristics of oceanic and humid subtropical climates, because of humidity from the Black Sea and the relatively high concentration of vegetation. The climate in the populated areas of the city to the south, on the Sea of Marmara, is warmer, drier and less affected by humidity. The annual precipitation in the northern half can be twice as much (Bahçeköy, 1166.6 mm), than it is in the southern, Marmara coast (Florya 635.0 mm). There is a significant difference between annual mean temperatures on the north and south coasts as well, Bahçeköy 12.8 °C (55.0 °F), Kartal 15.03 °C (59.05 °F). Parts of the province that are away from both seas exhibit considerable continental influences, with much more pronounced night-day and summer-winter temperature differences. In winter some parts of the province average freezing or below at night.
Istanbul's persistently high humidity reaches 80 percent most mornings. Because of this, fog is very common, although more so in northern parts of the city and away from the city center. Dense fog disrupts transportation in the region, including on the Bosphorus, and is common during the autumn and winter months when the humidity remains high into the afternoon. The humid conditions and the fog tend to dissipate by midday during the summer months, but the lingering humidity exacerbates the moderately high summer temperatures. During these summer months, high temperatures average around 29 °C (84 °F) and rainfall is uncommon; there are only about fifteen days with measurable precipitation between June and August. The summer months also have the highest concentration of thunderstorms.
Winter is colder in Istanbul than in most other cities around the Mediterranean Basin, with low temperatures averaging 1–4 °C (34–39 °F). Lake-effect snow from the Black Sea is common, although difficult to forecast, with the potential to be heavy and—as with the fog—disruptive to the city's infrastructure. Spring and autumn are mild, but often wet and unpredictable; chilly winds from the northwest and warm gusts from the south—sometimes in the same day—tend to cause fluctuations in temperature. Overall, Istanbul has an annual average of 130 days with significant precipitation, which amounts to 810 millimeters (31.9 in) per year. The highest and lowest temperatures ever recorded in the city center on the Marmara coast are 40.5 °C (105 °F) and −16.1 °C (3 °F). The greatest rainfall recorded in a day is 227 millimeters (8.9 in), and the highest recorded snow cover is 80 centimeters (31 in).
Istanbul