According to the updated Köppen–Geiger classification system, Istanbul has a borderline Mediterranean climate (Csa) and humid subtropical climate (Cfa). Since it has only two summer months with less than 40 millimeters (1.6 in) of rainfall, the city cannot be classified as solely Mediterranean or humid subtropical. Due to its vast size, diverse topography, and maritime location, Istanbul exhibits microclimates. Northern parts of the city express characteristics of an oceanic climate because of humidity from the Black Sea and the relatively high concentration of vegetation. The climate in the populated areas of the city in the south is warmer and less affected by humidity. Indeed, one of the most salient characteristics of the climate in parts of Istanbul is its persistently high humidity, which reaches 80 percent most mornings. Because of these conditions, fog is very common, although more so in northern parts of the city and away from the city center. Notably dense fog events that disrupt transportation in the region, including on the Bosphorus, are perennial occurrences 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 still has the effect of exacerbating 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. Nevertheless, despite the low precipitation, 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 4–5 °C (39–41 °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 115 days with significant precipitation, which amounts to 852 millimeters (33.5 in) per year. The highest and lowest temperatures ever recorded in the city are 40.5 °C (105 °F) and −16.1 °C (3 °F), respectively. The highest amount of rainfall recorded in a single day is 227 millimeters (8.9 in), whereas the highest recorded snow cover is 80 centimeters (31 in).