Most of Turkey has a Mediterranean climate and enjoys four seasons that can be more and less harsh depending on the region. Areas of Turkey nearer to the Black Sea experience the cooler Oceanic climate. Areas of Turkey near the Sea of Marmara (this area includes Istanbul) have a transitional climate that straddles both Mediterranean and Oceanic climates. Here, the seasons can be harsher than on the Mediterranean.
The Turkish coast that borders the Mediterrean Sea enjoys a mild climate—hot, dry summers and cool, wet winters. It rarely snows in these areas. However, the coast that borders the Black Sea experiences harsher seasonal changes and receives more precipitation (and more frequently) than the rest of the country. And the areas bordering the Sea of Marmara (like Istanbul) can experience the best and the worst of their neighboring climates. It usually snows in winter in these areas, as well as in mountainous regions, but not heavily.
The interior of the country experiences the harshest climates and seasons, as most of the interior is isolated from climate influence due to mountain ranges. Most of the interior has a Continental climate. The central Anatolian plateau, for example, suffers from severe climate changes from season to season, with temperatures reaching -40°F at their lowest and 90°F at their highest.
The temperature in Istanbul (and the western regions of Turkey) typically ranges between 35°F and 85°F throughout the year. Average annual humidity is around 80%. Istanbul receives less than 25 inches of rain per year.
As in any country, weather depends on your region, but generally Turkey enjoys a warm, mild climate, with a noticeable temperature fluctuation from season to season.
Turkish Winter: December to February
Turkish Summer: May to September