In general, the climate is the same as it remains in the rest of southern Europe during the summer months. It is hot and dry most of the time. From June to September the temperature is the highest and decreases gradually making January & February the coldest months. The best time to visit Istanbul, Turkey is in April when the weather is also littler moderate and is the season where in you can see the Tulips bloom. This gives a great view of the natural beauty of Istanbul.
Spring (April - June): The temperature starts rising up after the winters with the average high going upto 20°C where as in the night the temperatures are usually colder going to 15°C. Spring is considered to be the high season for Istanbul where in the entire hotel rooms are usually priced higher.
Autumn (September - October): The weather in Istanbul doesn’t vary much and it remains pleasant, though you can encounter slight rainfall.
Summer (June - August): Mid of June to August are quite hot. Comparatively nights are little colder. There is hardly any rainfall during the summers so it’s majorly a very dry season. At this time most of the local crowds are out for vacation so one looking out for a peaceful vacation can head over there in the summers.
Winter (November - February): Usually colder months with the temperature varying from 5-10°C. Rains, snow & storm are common in these months. Because of the winter the tourists footfalls are less and this is an off season so the hotel prices are relatively low which can be a good deal for a cheap accommodation in Istanbul.