The climate of St. Petersburg is moderate and humid, transitional from continental to marine. This region is characterized by frequent changes in air masses, due largely to cyclonic activity. In summer, westerly and North-westerly winds prevail, in winter, westerly and South-westerly.
Petersburg weather stations have data since 1722. The highest temperature recorded in St. Petersburg for the entire period of observations is +37.1 °C, and the lowest is -35.9 °C. In the new Millennium, the lowest temperature was recorded on January 11, 2003 — -28.8 °C.
Because of the small amount of solar heat, moisture evaporates slowly. The total inflow of solar radiation here is 1.5 times less than in the South of Ukraine, and half as much as in Central Asia. During the year, St. Petersburg has an average of 75 Sunny days. Therefore, during most of the year, days with cloudy, cloudy weather and scattered lighting prevail. The length of the day in St. Petersburg varies from 5 hours 51 minutes in the winter solstice to 18 hours 50 minutes in the summer solstice.
In the city there are so-called "white nights", coming on may 25-26, when the sun falls below the horizon no more than 9°, and the evening twilight almost merges with the morning. The white nights of July 16-17 end. In total, the duration of white nights is more than 50 days. The annual amplitude of the amounts of direct solar radiation on a horizontal surface in a clear sky from 25 MJ / m2 in December to 686 MJ/m2 in June. Cloud cover reduces the average annual arrival of total solar radiation by 21 %, and direct solar radiation by 60 %. The average annual total radiation is 3156 MJ / m2. The number of hours of sunshine — 1628 per year.
From the West and South-West, from the Atlantic ocean, where the Azores anticyclone and the Icelandic minimum pressure are located, moist sea air enters the territory of the region, which has a softening effect on the climate. The city is characterized by warm, rainy summers and relatively mild winters.
From the North and North-East, from the polar region of high pressure, the region receives cold and dry Arctic air masses formed over the ice of the
Arctic ocean. It has an impact on the climate of the region and the area of high pressure in Central Asia. From these regions —
from the East and South-East — continental air comes to the region, bringing dry and clear weather.
The greatest influence on the climate of the region has air masses coming from the Atlantic. In an average year winds West, North-Western and South-Western areas comprise almost 46 % (in autumn, about 50 %) of all winds, the winds of the North and East areas — 28 %, and South and South East — 26 %. The consequence of the change and interaction of air masses in different directions is typical for the city long-term variability of the weather and its instability during the year.
A noticeable influence on the climate of St. Petersburg (as well as any major city) have urban conditions that create a special microclimate. Dust, smoke, soot and other impurities in the air during the day reduce solar radiation, and at night delay the earth's radiation, slowing down the cooling of the earth's surface. In summer, stone buildings and road surfaces are very hot and accumulate heat, and at night give it to the atmosphere; in winter, the air receives additional heat from the heating of buildings. In summer, in the center of St. Petersburg, the daytime temperature is higher by 2-3 degrees than in the suburbs, and the relative humidity is 15-20 % lower; in winter, the temperature difference can reach 10-12 degrees, and humidity - up to 40 %. The warmest part of the city is Nevsky Prospekt. The wind in the city is weaker than in the surrounding open areas.
Winter comes in St. Petersburg usually in early December; its beginning coincides with the establishment of snow cover (but there are years when the snow cover is established only in mid-January) and the ice age in the upper reaches of the Neva. According to the average long-term data, this happens on December 5.
In the first half of winter, the weather is usually unstable, with frequent thaws. The sun is low, the day is short, the snow cover is small.
The air brought from the West by cyclones quickly cools, and the water vapor contained in it condenses, which causes high clouds and fogs.
During December, on average, there are only 2 clear days. The second half of winter is noticeably colder than the first. Coming from the West, the air becomes colder, but less humid. Therefore, cloud cover decreases, fogs become less frequent, less precipitation falls, more often the Arctic air invades, lowering the temperature.
Spring in St. Petersburg usually comes at the end of March, when daytime temperatures become steadily positive and snow begins to descend. Spring "wakes up" more slowly than in neighboring, more continental areas: the influence of large reservoirs cooled during the winter affects. The average daily temperature above 0 °C is set in early April, shortly after the snow melts; it reaches 5 °C by the end of April and 10 °C in may. Atmospheric pressure is highest in the spring, and cyclones are rare, so the weather is relatively stable.
At the beginning of summer, frosts stop. The average daily temperature in June reaches +14.8 °C, and in July +17.8 °C. St. Petersburg is in an area of low pressure, and it receives Atlantic air from the West. Cyclones are increasing, and the nature of the summer weather largely depends on the ways of their movement.
When passing the center of the cyclone South of the city, St. Petersburg falls into the rear part of the cyclone and North winds
prevail, bringing cold rainy weather. When the center of the cyclone passes over the Barents and White seas, St. Petersburg falls into the warm sector
of the cyclone and southern winds prevail and there is good warm weather.
Above +25 °C the air temperature is on average 16 days a year, the absolute maximum of +37.1 °C was observed on August 7, 2010. The absolute minimum in the summer of 0 °C was observed in June 1930.In the second half of summer, cyclones are more frequent and stronger. Such weather prevails in years with strong cyclones. In August, it gets colder, the average daily temperature drops to +16 °C.
Autumn in St. Petersburg begins at the end of August, when the average daily temperature becomes below 15 degrees, and the heat in the afternoon becomes very rare. The first frost occurs on average on October 10, the earliest recorded on September 15, 1944.
On the ground, frosts occur earlier, the earliest-August 29, 1973. During the first half of September, the weather is warm and dry; the average daily temperature usually exceeds +10 °C, although at night there are frosts. From the second half of October, cyclonic activity increases, gradually cloudy, wet and windy weather with drizzling rains becomes predominant; cloud cover and relative humidity increases (81-87 %), wind speed increases. The average monthly temperature drops from +10.8 °C in September to +4.8 °C in October and to + 0.5 °C in November.