Climate

Рубрикатор / City Topography/Nature and Natural Phenomena
CLIMATE of St. Petersburg is defined by its geographic location as a transitional point from a marine to continental climate. It retains a relatively high air humidity throughout the year, with a mild, warm and humid summer (the mean temperature in July is 17.8 degrees Celsius), and a similarly mild, cold winter (the mean temperature in January and in February is -7.8 degrees Celsius) with well pronounced cyclonic activity and the frequent invasion of warm air masses causing thaws typical in marine climates. The high latitude of St. Petersburg causes a wide variation in the altitude of the sun throughout the year. The duration of the day changes from 5 hours 51 minutes on the winter solstice, 22 December, to 18 hours 50 minutes on 22 June. The White Nights can be observed from the end of May till the first ten days of July. The annual magnitude of direct solar radiation on the level during sunny conditions varies from 25 MJ/m2 in December to 686 МJ/m2 in June. Cloud cover reduces total solar radiation by 21% on average annually. Cloud cover diminishes solar radiation 60% on average annually. The mean annual total solar radiation is 3,156 МJ/m2. Atmospheric circulation mostly determines the climate during the winter when the region is mostly influenced by the Atlantic. A considerable quantity of heat comes with cyclones coming through from the Atlantic, as opposed to the Arctic, which leads to winter being milder than it would otherwise be, and autumn is usually milder than spring. Cyclonic activity subsides in spring and solar activity starts to play a more important role in determining the weather of St. Petersburg. Autumn is usually protracted; the transition of the daily temperature from 10 degrees Celsius to 0 degrees Celsius lasts for 55 days. On average the first frost comes on 10 October. The earliest frost was registered on 15 September 1944. Frosts come earlier on soil; the earliest frost on soil was registered on 29 August 1973. Extremely cold winters are connected with the prevalence of arctic air. The absolute minimum of temperature recorded in St. Petersburg is 36 degrees Celsius, registered in 1940. On average atmospheric temperature is below -25 degrees Celsius for three days during winter. Intrusions of warm air may lead to sudden rise of temperature up to six degrees Celsius. Spring is usually protracted. The mean daily temperature exceeds 0 degrees Celsius from 3 April. Mean daily temperature exceeds 10 degrees Celsius from 18 May. On average the last frost is registered on 5 May, but it is still possible until 28 May. The last frost on soil was registered on 6 June, in 1962. The mean duration of frost-free period is 157 days. Mean daily temperature is above 10 degrees Celsius during 127 days. High summer temperatures are connected with the settling of anti-cyclone activity over the region bringing with it dry warm air. On average the temperature is above 25 degrees Celsius for 16 days a year. The absolute maximum is 34 degrees Celsius (1972, 1985). Penetration of arctic air masses in rear parts of north-western cyclones may lead to sudden reduction of temperature in summer. The absolute minimum summer temperature was 0 degrees Celsius, registered in June 1930. Cyclonic activity has been active in the last ten years, especially in winter. The mean annual temperature has increased 0.9 degrees Celsius as compared with the norm (4.4 degrees Celsius) mainly due to the period from January to March. The temperature in this period has increased 2.5 to 3.1 degrees Celsius. The temperature of the warmest month July has remained within normal levels. Periods with anomalously cold weather in winter (December 2002) and with dry and warm weather in summer (July 2002) have been observed, as being a general tendency. Such periods happen when a large anti-cyclone sits over the European part of Russia. Westerly, south-westerly and north-westerly winds prevail, this is the case for 46% of the year. Most often the wind speed is about 2-3 m/s. Winds with stronger than 15 m/s occur in central districts of St. Petersburg up to two days a year on average, while in coastal districts they occur on 14-22 days. Overcast weather with cloud cover of eight to ten points is typical for St. Petersburg (up to 177 days a year). Stratocumulus clouds occur most frequently 48% of the time and are heaviest from September to January. Cumulus clouds are present 25-30% of the time during summer. On average relative air humidity does not fall below 80% for 145 days a year. Mists usually rise in the presence of relatively high air humidity, occurring particularly often in winter. On average 27 days a year are misty. The mean duration of mists is three - four hours but in exceptional cases can last 1.5 days. The number of days with low air humidity, dropping down to 30%, does not exceed more than ten days on average. Such days most often occur in May. St. Petersburg is located in the zone high precipitation. On average 620 mm of atmospheric precipitation falls annually. This quantity is 50-70 mm higher in northern districts of St. Petersburg. The highest quantity of atmospheric precipitates is 825 mm a year, the lowest quantity of atmospheric precipitates is 417 mm annually (1920). Rain precipitation makes up 65%, which also occurs in winter. Solid precipitation makes up 19%. Mixed precipitates make up 16%. A tendency has been noticed in recent years, in which the annual atmospheric precipitation has been reducing by 70-100 mm a year, mostly occurring during summer and autumn months. Mean maximum quantity of atmospheric precipitates is 30 mm, the absolute maximum is 76 mm, registered on 8 August 1947. There are 191 days a year with precipitation rising over 0.1 mm. The precipitation ratio (relation between atmospheric precipitation and evaporation) is 1.9. The depth of snow cover is not more than 33 centimetres on average; it rarely achieves 50-60 centimetres. The earliest snow fall has occurred on 3 October. Steady snow cover occurs from 7 December. The average date for the spring though is 16 April. The latest date is 9 May. Snowstorms usually occur 14 days a year. The maximum annual number of days with a snowstorm is 33. The number of days with ice-crusted ground may reach 26. Transportation suffers from hoarfrost in winter for up to 39 days a year. On average one to two days a year register hail. The mean annual number of days with thunderstorms is 18, the maximum number of days with thunderstorms is 32. The majority of thunderstorms are registered in summer (78%). Thunderstorms with a duration of approximately five hours have been registered. The city noticeably influences the formation of the climate. On average the integral atmospheric transparency in St. Petersburg is 3-5 % lower than in the Leningrad Region. As a result atmospheric radiation within the city is reduced and the number of hours of sunshine is consequently reduced. Yet the radiation balance in the city is higher than in the outskirts. St. Petersburg is 0.8-1.5 degrees Celsius warmer in the centre than in the outskirts in winter on account of industrial discharges and heating systems within the city. St. Petersburg is 0.5-0.7 degrees Celsius warmer in the centre than in the outskirts during summer because of the heating of stone buildings and pavements. Absolute minimum temperatures are 4-5 degrees Celsius higher in the city than in the outskirts. The frost-free period in the centre of St. Petersburg is 25 days longer than in Lesnoy District and 43 days longer than at Pulkovo Airport. The city construction considerably influences the wind conditions. Mean monthly wind speeds in central districts are 1-1.5 m/s lower than on the shore.

References: Климат Ленинграда. Л., 1982; Научно-прикладной справочник по климату СССР: Сер. 3: Многолетние данные. Вып. 3: Карельская АССР, Ленинградская, Новгородская, Псковская, Калининская и Смоленская области. Л., 1988; Ленинград: Ист.-геогр. атлас. 2-е изд. М., 1989.

I. G. Moskalenko. Ленинград: Ист.-геогр. атлас. 2-е изд. М., 1989
Климат Ленинграда. Л., 1982
Научно-прикладной справочник по климату СССР: Сер. 3: Многолетние данные. Вып. 3: Карельская АССР, Ленингр., Новгор., Псков. Калинин. и Смолен. обл. Л., 1988 Airports (entry)
1740
1940
1972