Climate quality map of Prague (based on the data of IMI Prague).
Climate
The territory of the Capital City of Prague is provided with a relatively dense network of stations regularly monitoring the climate. At present there are six principal climatological stations measuring all important meteorological characteristics three times a day. Moreover, the Ruzyně, Libuš and Kbely stations provide reports on the state of climate every hour which form part of an extensive international exchange of meteorological data serving mostly the weather forecast purposes. This network is supplemented with a network of seven precipitations stations measuring the sums of precipitations and the thickness of snow cover once a day.
Climate quality map of Prague (based on the data of IMI Prague).
The oldest meteorological station in Prague is situated in the Klementinum, where regular measurements started as early as 1752. The original records from the oldest period before 1775, however, have not been preserved and only some monthly extremes or mean temperatures and air pressure data are available. Precipitations measurements were introduced in 1804. The more than 200 years' relatively homogenous series of temperature measurements performed in Klementinum in Prague ranges among the most important in Europe, its length making it suitable for the investigation of long-term climatic changes. The development of mean annual temperatures from 1775 to 1996 is illustrated by the diagram on p. 37, bottom left.
Mean annual temperatures (oC) 1775 - 1996, Prague-Klementinum.
According to the "Climatic Atlas of the Czechoslovak Republic" Prague territory is situated at the boundary between the warm, dry area with mild winters, and the mildly warm, dry area with mostly mild winters. The climate of Prague is influenced markedly by the so-called big city thermal island. For the same elevation above sea level the mean temperature in the city centre is 1oC higher than in the open country. This is due to the high concentration of heat sources, but chiefly to minor heat losses during evaporation resulting from the urbanization of active surface with the marked prevalence of built-up or paved surfaces over natural surfaces with vegetation, where most precipitations flow immediately into the drainage network. Consequently the long-term annual mean air temperature (1951 - 1990) varies between 9.9oC in the centre of Prague (Klementinum) and 7.9o at the highest elevation on the outskirt of Prague (Ruzyně). Annual history of mean daily temperatures in the Klementinum and Ruzyně stations is shown in the diagram on p. 39 (lower right).
Annual progress of mean daily air temperature (oC) 1961 - 1990.
However, the city influences also the history of daily temperature. This is manifested particularly during clear and quiet situations, when the daily temperature amplitude in the city centre is reduced and the daily extremes are retarded by about one hour. The absolutely highest temperature was measured on July 27, 1983, at the Uhříněves station, viz. 40.2oC. Simultaneously this temperature was the highest temperature recorded in the Czech Republic since the beginning of measurements. The absolutely lowest temperature was recorded in the Research Institute of Plant Husbandry in Ruzyně on February 10, 1956, viz. - 31.1oC.
The mean annual precipitations in Prague (1951 - 1990) are 542 mm. The lowest precipitations are in Radotín (507 mm) and Podbaba (516 mm). Even lower precipitations have been observed in the stations of Karlov (442 mm) and Klementinum (484 mm) where, however, the precipitation meters are mounted in non-standard position - on the roofs of the buildings. The highest sums appear in the SE of Prague, in Uhříněves (571 mm) and Průhonice (597 mm). In Ruzyně the highest annual sum of precipitation, viz. 755 mm, was observed in 1985, the highest monthly sum of 227 mm in July, 1981, and the highest daily sum of 93.3 mm in July 19, 1981. The lowest annual sum of 345 mm was observed in 1953. The history of annual precipitations is shown in the diagram on p. 36 (below). The average of the annual maximum snow cover thickness is 20 cm in Ruzyně and 12 cm on Karlov. The highest measured value since 1961 was 57 cm in Ruzyně (March 6, 1970) and 34 cm on Karlov (December 20, 1969).
Average monthly precipitations sums (mm), Prague-Ruzyně 1951 1990.
The mean annual relative humidity is 77 % in Ruzyně and 71 % on Karlov. The mean annual sunshine is 1668 hours in Ruzyně and 1611 hours on Karlov. The mean annual wind velocity is 4.3 m/s in Ruzyně and 3.0 m/s on Karlov. The most frequent wind directions are SW and W. The absolute highest instantaneous wind velocity was 45 m/s (162 km/h) recorded in Ruzyně on January 28, 1994.
Phenology and Agrometeorology
Phenology records the history of significant periodically repeated manifestations of life of plants and animals (phenological phases) in dependence on the complex of external environmental conditions, particularly the changes of weather and climate. The result of phenological observations can be applied to the characterization of climatic condition of a site or a region.
On the outskirts of Prague and its nearest environs characterized by the cultivation of field produce and fruit the following dates characterizing the start of phenological phases have been recorded:
- start of spring work in the fields between March 18 and 24,
- start of hay harvest between June 1 and 15,
- winter rye: sowing start September 26 +/- 7 days, germination between October 4 and 10, blossoming between June 2 and 7, yellow maturity July 16 - 18, harvest start July 20 +/- 7 days
- winter wheat: sowing start September 29 - October 3, germination October 12 - 16, earing between May 31 and June 12, blossoming June 16 - 23, yellow maturity July 21 - 28, harvest July 26 - August 2
- spring barley: sowing start March 28 +/- 11 days, germination April 7 - 19, shooting between May 1 and 9, earing June 8 - 18, harvest start July 23 - 28
- sown oats: sowing start April 1 - 6, germination April 12 - 21, shooting May 7 - 10, earing June 23 - 26, harvest start between August 3 and 9
- late potatoes: planting start April 19 - May 2, germination May 17 - 30, blossoming July 4 - 15, harvest September 20 - 28
- sugar beet: sowing start April 14 - 19, germination April 29 - May 6, harvest start October 7 - 13
- cherry - first blossoms between April 22 and 25,
- blackthorn - blossoming start between April 18 and May 1,
- summer apple tree - first blossoms on May 4 +/- 9 days.
The above dates were derived from long term observations of phenological stations in Křeslice, Ruzyně, Troja, Velká Chuchle, Škvorec, Úhonice and Klokočná.
Mean soil temperature varies in individual decades between - 1.3oC and 19.0oC at a depth of 5 cm below surface, from - 1.0oC to 19.1oC at a depth of 10 cm, between - 0.8oC and 18.6oC at 20 cm, and between -0.8oC and 17.1oC at 50 cm. At a depth of 100 cm the soil temperature varies between 2.5oC and 15.4oC.
The greatest depth of occurrence of zero (0oC) temperature in clayey-loamy soil is 73 cm in the last decade of February (Uhříněves).
The frostfree period in the centre of Prague lasts 187 days on average (the shortest period 118 days, the longest 216 days). The first frost appears on October 20 on average (on September 24 at the earliest, on November 28 at the latest). The last frost appears on April 17 on average (March 30 at the earliest, on May 21 at the latest). The number of thaws in winter (from the beginning of the second November to the end of the first February decades is 1.48 day for 1 - 5 days duration, 0.62 day for 6 - 10 days duration and 0.52 day for over 10 days duration on average.
The moistening conditions characterized by the climatic moistening index (difference between evaporation and precipitations) vary between 6.8 mm (Uhříněves) and 31.7 mm (Karlov) on annual average.
From time to time a storm cleans the air also in the Botič Meanders (PP).