First half of the month resembles July in many ways. Rain events are typically short lived and rather heavy. After mid-August over 25 ºC temperatures are rare, these are reached only twice a month on the average. During the north wind the lowest night time temperatures may decline close to 0 ºC. According to normal period 1971-2000, the average temperature of the month in Helsinki Kaisaniemi was 15,8 ºC. The average of the highest daytime temperatures was 19,3 ºC, and the average of the lowest night time temperatures was 12,6 ºC, respectively. The highest measured temperature was 31,2 ºC while the lowest was 3,4 ºC. The warmest August since year 1900 occurred in 1939 (20,0 ºC), and the coolest in 1916 (12,9 ºC).
In Helsinki Kaisaniemi, diurnal variation of temperature may be about 4 ºC in totally cloudy, and about 12 ºC in totally clear situation. The following figure illustrates daily average temperature (green color) in Kaisaniemi in August 2004. Smoothed curves represent 2,5%, 50% and 97,5% probabilities. The average accumulated precipitation was 78 mm during normal period 1971-2000. The greatest measured accumulated precipitation has been 174 mm (1972) and the smallest 1,2 mm (1996).
In capitol area, local temperature conditions are influenced by distance from the sea, topography, heat island effect and solar radiation. Heat island effect results from replacement of natural surfaces with construction materials such as stone and asphalt, sewage systems and draining, anthropogenic heat production and various emissions to the atmosphere. As a consequence, urban city areas are warmer than their surroundings. This can be observed in calm and clear nights in late summer, for instance. In these conditions, large parks in city are about 2 ºC cooler than nearby constructed area. Close regions right outside the city are still 2-3 ºC cooler than central parks.
The most dangerous weather events in August are thunderstorms with heavy rain, wind gusts and lightning. For example, on 5th of August 2003, 11 soldiers had to be hospitalized when lightning stroke to a close spruce. Roughly two weeks later again (21.8.2003) lightning and thunder were observed widely in southern Finland. The next figure shows lightnings detected on this day. Color scale on right represents occurrence times of lightnings in UTC-time. Thunder caused alarms to rescue departments and at least in Helsinki and Porvoo happened water damages. Heavy showers passed Helsinki approximately at 19.30-20.30. Also hail was observed, the biggest ones being about 15 mm in diameter. Accumulated precipitation was 12,5 mm in Helsinki Kaisaniemi during this day.
