The climate of Istanbul, classified variously as Mediterranean, oceanic, or a transitional climate between the two, is temperate, with cool, frequently rainy, somewhat snowy winters, and warm to hot, moderately dry summers. Strongly influenced by the Sea of Marmara to the south and the Black Sea to the north, it is thoroughly maritime; precipitation is moderately high, fog is frequent, and seasonal lag is noticeable both in winter and in summer. The city is sheltered from both extreme heat and frigid temperatures, with temperatures rarely reaching or dipping below , and it is quite windy year-round.
Precipitation is unevenly distributed, with winter typically experiencing very frequent precipitation, while summers are generally dry, punctuated by infrequent showers. Cloudiness also varies drastically by season; while the city has winters that receive around one–fifth of the possible sunshine duration, comparable to cities in northwestern Europe, in summer, the city gets considerably more sunshine than most of Western and Central Europe.
Istanbul's climate is rapidly changing due to the combined effects of climate change and the city's urban heat island. Recent data shows an immense increase in daily low temperatures and some increase in daily highs.
General climate Temperature Istanbul's average temperature is based on the latest normals covering 1991-2020, around . Its coldest month is usually February, with an average temperature of , while its warmest month is usually August, with an average temperature of . Unsurprisingly, Istanbul's temperature range is narrow for a non-oceanside locale; the annual mean maximum (hottest temperature in an average year), is about , while the annual mean minimum (coldest temperature in an average year) is about . Istanbul's climate is also well known for its noticeable seasonal lag; it is one of the few cities in the temperate Northern Hemisphere where March is, in many districts, colder than December.
Summers in the city are dominated by stable, northeasterly meltem-like air, with a narrow range of daytime temperatures, generally around to . A temperature of is only seen about once every ten years. Despite this, Istanbul's summer climate is generally considered as relatively uncomfortable, largely due to persistently high dew points; most thermal comfort indices rate July and August as less comfortable overall than June and September.
Winters are more variable, still with relatively stable daytime temperatures generally around to , but with abrupt, short-term extremes. The dominant northeasterly loses its dry quality during this time of year, giving way to a more moist wind, termed Poyraz (ultimately from ) from the same direction, causing persistent precipitation, sometimes in the form of snow; although it is still incapable of producing extreme cold due to its maritime nature. The Lodos (ultimately from ) is the southwesterly, warm, and dry wind "competing" with the northeasterly Poyraz and is responsible for the occasional day above in the city, as well as numerous severe windstorms. Therefore, while warmth records in winter are usually associated with and always colloquially blamed on the Lodos, cold records are the product of rare windless nights.
Spring and fall are mild, especially by the end of spring and the beginning of fall. Spring is generally colder than fall due to seasonal lag: March is quite chilly, much more akin to winter, in contrast to November's milder average temperatures. The last spring frost generally happens around late March in the most coastal and heavily urbanized areas, and late April well-inland; the first fall frost happens sometime in November in most of the city (except the far western suburbs, which may get their first frost in October), and early December in coastal locations.
It is somewhat well-established that summer mean temperatures have risen from about during pre-industrial times to about over the last two centuries, according to reconstructions done by Luterbacher et al. The same pattern is not found to the same extent in winter, where temperatures have seen no perceptible increase until very recently. This points to the combined effect of climate change and the urban heat island, which increases in intensity during the nights of the city's largely sunny summer days, and on hot summer days may create temperature differences of up to 8 °C at night, essentially creating a 4-5 °C difference between urban and rural areas.
Wind Istanbul is quite windy, with average yearly wind speeds of depending on location. It is quite anomalous for a city that is not on the immediate west-coast of a continent. These high averages are not caused by frequent windstorms; however, they are rather caused by the near-constant north-northeasterly wind over the city (specifically the etesians in summer, and the Poyraz in winter) that all but eliminates windless days and causes around 30 days with sustained wind speeds above per year.
Although rare, Istanbul can experience severe windstorms, mostly due to Lodos in winter, when the typical wind flow reverses and becomes southwesterly. These have, historically, created gusts of up to . Most years, however, see gusts up to , which rarely causes more than sporadic damage.
Precipitation Istanbul's precipitation varies considerably between years and districts. On average, however, it experiences around of precipitation annually. Most of this falls in the colder half of the year, with around to of precipitation in the wettest months. In the driest months, precipitation ranges from to depending on location, month and year, with prolonged dry periods punctuated by abrupt, heavy showers and thunderstorms, sometimes severe.
As Istanbul receives some amount of precipitation from both Atlantic and Mediterranean systems, precipitation, especially in the winter half of the year, is frequent and light, which is unusual for the Mediterranean basin: the average number of rainy days in the city is 131, and in some parts this may reach up to 152. Furthermore, during early and mid-winter, the city's frequency of precipitation is virtually unparalleled in the Mediterranean basin: January averages 20 days of precipitation when counting trace accumulations, 17 when using a 0.1 mm threshold, and 12 when using a 1.0 mm threshold, on par with cities such as Brussels, and rainier than famously humid locales in Western Europe, such as London.
Rain remains the dominant form of precipitation throughout the year, with snow representing only one fourth of precipitation even in the coldest months of January and February. Despite this, an annual average of more than of snow falls on the area of the airport, making Istanbul the snowiest major city in the Mediterranean basin. Snowfall varies widely between years and different areas of the city, with districts facing north more prone to receive snow than southerly ones. This effect is largely caused by lake-effect snow, which forms when cold air, originating from the North Pole or Siberia, develops into moist and unstable air that ascends to form snow squalls along the lee shores of the Black Sea, upon contact with the relatively warm water. These snow squalls are heavy snow bands and occasionally thundersnows, with accumulation rates approaching per hour. Almost half of snowy periods do not leave accumulated snow for more than a day, reducing the 'number of days with snow cover' statistic, published daily by the TSMS, by half.
Particularly severe winter storms have included January 1942, March 1987, and most recently January 2017 and 2022. Some of these broke daily snow depth records, with on 4 January 1942, and in the northern suburbs on 11 January 2017; unofficial measurements in hilly and northern regions have included in March 2022, and a putative measurement of in some snow drifts in March 1987.
Normally uncommon precipitation types, especially graupel (, or ) are a staple of winter due to insufficient cooling of the southern Black Sea, somewhat well known internationally for delaying a Champions League match between Galatasaray and Juventus.
Sunshine and cloudiness Istanbul's sunshine is profoundly seasonal, with cloudy winters and mostly sunny summers. Yearly, it averages around 2000 hours of sunshine, cloudier than the rest of the Mediterranean and generally similar to southern oceanic and sub-Mediterranean locales, like the Pacific Northwest in the US or areas around the Bay of Biscay in France.
In winter, the city is known for its persistently cloudy weather, with many regions getting less than 20 percent of their potential sunlight, and winter months of certain years getting less than 30 hours – equivalent to less than one tenth of possible sunshine. This brings Istanbul's sunshine hours to levels closer to Western Europe, while its generally sunny summers create a seasonal sunshine pattern perhaps most closely mirrored by Cascadia, specifically Seattle.
Fog is known to be a major component limiting sunshine in Istanbul. Data before 1980 count annual foggy and misty days at above 200, and in some summer months, records show more than 25 days of fog and mist.
However, fog has been steadily declining since the 1970s. Data from the 1980s show around 100 days of fog and mist, and this has declined to about 60 in the last 10 years. This is likely due to the city's urban heat island, which has been raising overnight temperatures since the city's initial rapid expansion in the 1950s and 60s. While forested regions outside the city (especially in nearby Kocaeli Province) have lagged behind in the decrease of fog – the station in Gölcük had consistently been seeing over 200 days of fog until 1993, and now also averages around 60 – the decrease of fog poses a significant threat to the flora of the region, which feed on morning humidity in the relatively dry summer months.
Severe weather Istanbul's climate is generally not conducive to severe weather, due to its maritime heating and cooling. However, instances of severe weather do occur in Istanbul with some regularity, mostly in the form of heavy snow, severe thunderstorms and heatwaves.
Thunderstorms
Istanbul experiences around 10–15 days of thunderstorms, with a primary peak in June, and another one in September. Unlike many central European cities, July and August are too dry to support a high amount of thunderstorms, and yet unlike many cities in the Mediterranean basin, winters are too cold and cloudy to support thunderstorms, causing two warm-season stormy periods with a summerly nadir in between.
Despite the relatively uncommon nature of thunderstorms in the city, a notable anomaly is the amount of strong to severe thunderstorms that occur yearly; about half of the thunderstorms that occur in the city (about 6–7) can be qualified as such. These thunderstorms peak around early-fall, likely due to warmer waters, while their anomalously high number is likely due to high wind shear and other conditions conducive to severe storm development.
Very damaging storms have happened on a somewhat consistent basis, a significant one in recent memory is the July 2017 storm, where hail causing damage to buildings, straight line winds of and widespread flooding was recorded. Tornadoes, even weak ones, are very uncommon, nevertheless not impossible, with recent storms in 2020 and 2021 dropping weak, non-damaging tornadoes.
Heat waves Due to its maritime position, Istanbul is not susceptible to heat waves by southern European standards. Not only do cities with cooler summers often have higher summer temperature records than Istanbul, Istanbul escapes heat-waves even when they happen over northwestern Turkey. Most recently in September 2022, when southern Marmara was experiencing daily highs of about , Istanbul's high temperature was , with a strong northeasterly wind.