Istanbul has a temperate climate with hot and humid summers, and cold, wet winters. Humidity is generally rather high which can make temperatures feel much warmer or colder than they actually are. Snowfall is quite common, snowing for a week or two during the winter season, but it can be heavy once it snows. It is most likely to occur between the months of December and March. The summer months between June and September bring average daytime temperatures of 28 °C.
The warmest month is July with an average temperature of 23.3 °C. The coldest is January with 5.6 °C. The weather becomes slightly cooler as one moves toward eastern Istanbul. Summer is by far the driest season, although there is no real summer drought such as occurs further west, and so the climate cannot be considered truly Mediterranean.
Istanbul is huge as an area and it has so many topographic diversities that you can experience several different sub-climates in the same city. For example, Bosphorus Strait and surroundings are dominated by the Mediterranean climate (dry summers and mild and rainy winters) but to the north you can notice a Black Sea climate (mostly rainy). And meanwhile you can have a Balkan climate to the west (cold winters and snow) there is a Anatolian Continental climate to its east (hot summers and cold winters). You can even have completely different climates in the same day; while snowing in one area you can notice the sun shining (but cold of course) on another part of the city.
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Meteo Forecast Istanbul