It is easy to understand the weather in Istanbul by dividing it to 4 climates, which also makes it easier for you to choose when to visit Istanbul.
The weather is still cold and rainy in March, so if you have chance, it is better to postpone your trip by one month. It depends on the period, in April, it may be rainy, or the days may be lovely and sunny, it is up to chance. There is still risk of spring rain. By April, wild flowers start to bloom, good news for nature lovers. Istanbul Tulip Festival, which colors the city, is held in April as well. May is less rainy, much warmer than April, and days are longer. High season hasn’t started yet, so you can still get discounted prices.
Sun shines in the whole country in summer months, and the possibility of raining is so low. Despite the hot weather, Istanbul becomes more crowded. If you are also planning to visit other cities, especially the Southern coasts, it will be better to rent your room in advance. June is perfect time to visit Turkey. It won’t rain so much and is still not as hot as summer months. If you want to go to south of Turkey, it is better to visit there before July. It gets hotter and because of high season, prices are doubled. July and August are the hottest months
The weather is growing cooler and cities become less crowded in September. It is considered as one of the best months to visit Turkey. The weather may be fine until early November. There is a high chance of rain, and the days are getting shorter, but still, if you want to travel without tourist crowds, you can visit Turkey until middle November.
It is cold and rainy during winter months, and crowds are mostly packed in shopping malls. You can visit ski resorts such as Uludag and Kartepe. Ferry services may be cancelled because of wind, and traffic becomes unbearable during rainy days, so bear that in mind if you want to get somewhere on time. Don’t forget to bring your umbrella and a thick coat with you.