The climate in Istanbul is generally similar to Marmara region; summer months are warm and humid with very little rain especially between July-August, winter months can be cold and wet with some snow but not that much extreme as some other areas of Turkey. Spring and autumn are mild and this would be the best time for visitors.
Istanbul is so 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.
During the summer months, air temperature can go up to 30 degrees Celsius (86 degrees Fahrenheit), highest recorded temperature in Istanbul was 45 degrees (113 Fahrenheit). During winter months, the temperature is around 10 degrees (50 Fahrenheit) with much colder nights, lowest recorded temperature was -15 degrees (5 Fahrenheit). We occasionally get some snow but it's not more than 10-12 days in a total winter period. In the springtime, especially between April-May and September-October, there is a very comfortable climate with around 15-25 degrees (59-77
Fahrenheit). Average temperature in Istanbul, in a year, is around 14 degrees (57 Fahrenheit) with average relative humidity of 76%.
Average rainfall is 787 millimeters a year (31 inches). 35% of the rainfall occurs in winter, 23% in spring, 14% in summer, and 28% in autumn. Generally, northeastern winds
prevail in Istanbul (Poyraz in Turkish), with a long term average of 3,2 meters/second (11 feet/sec). Strongest wind speed recorded until today is 42,4 m/sec (139 feet/sec). Summers are dominated by northeastern winds (Poyraz), winters by northwestern (Karayel) or northern (Yildiz) or southwestern (Lodos) winds. Most foggy period for Istanbul is from March to May.