Istanbul is located between two continents and city is divided by Bosphorus Strait which lets air streams from Balkans, Siberia and Marmara (south). Maybe that's why locals say; you can never trust Istanbul weather. Although dates differ from year to year, you can enjoy four seasons in Istanbul.

Spring:

Starting from end of March, temperatures increase significantly. During day time averages at 20' C but nights are colder around 15' C. Tulip blossom time is around mid April which is an indicator for spring. Rains are common and night temperatures are cooler than day time. Don't forget umbrellas and rain coats and get prepared for winds. You'll probably need a jackets or light coat.

Summer:

Generally starting from July, wheather is hot and humid. It's holiday season and most Istanbullers leave city for vacation. Light layers of clothing are perfect for this time of year; don't forget that Turks dress fairly conservatively, so save the shorts for the beach only!  In July and August day temperatures are over 30' C and you can feel it much more combined with humidity. Bosphorus bank is most popular area for summer offering breezes.

Autumn:

Temperatures do not decrease rapidly, in September and Cctober you can still enjoy good weather most of days. Cloudy days or light rains are common and maybe you can consider a jacket for nights. In early November, the temperatures start to fall significantly but its early for storms or snow.

Winter:

Starting from December till March  temperatures are around 5-10' C. Although its not common for Istanbul, get preapred for (-) degrees C in some days. Snow and strong winds negatively affect city transportation especially ferries. People prefer shopping malls or covered areas in this season except New Year's eve. Unluckily it didn't snow for plenty of years on that day but there are streets parties on that day forgetting cold weather.