Climatological characteristics of summer precipitation in Helsinki during the period 1951-2000

Rainfall data, collected with a continuously-recording rain gauge in Helsinki, southern Finland, during the summers of the period 1951–2000 were used to study the climatology of the transient characteristics of summer precipitation. Despite some missing data and erroneous values, the quality of the data proved to be good. According to the observations, it typically rains 4.5% of the total time during summer, with rainfall lasting 60 minutes. Dry spells last on average 21 hours. The precipitation amount and the occurrence of moderate and heavy intensities have diurnal maxima both in the afternoon and in the morning, particularly in August. The main parts of the frequency distributions of rain event duration, dry spell duration and precipitation in a rain event could be reasonably well approximated by the sum of two exponential distributions. However, an extreme value distribution was more appropriate for the upper tails of the observed distributions.