Atmospheric instability in urban area of Cluj-Napoca, Romania. An analysis of the distribution of precipitation amounts in the range of 10-50 mm/24h in 2006-2011 was made, having as result a distribution of precipitation of amounts larger than 20 mm mainly in the summer and a distribution of precipitation with values of less than 20 mm in all seasons. A total of 32 cases of heavy rain with intensity greater than 10 mm/h were identified, these being most common in July. From the analysis of the probability of precipitation not exceeding certain thresholds, it was found that values of 10 mm/h have a probability of non-exceedance 10% for a non-returning period of 1.1 years. In the second part of the paper, a case of pronounced instability - the afternoon of May 20, 2011, in Cluj-Napoca area - that prompted large amounts of water from rain associated with lightning and brief strong wind, which led to flooding on large portions of the city was studied.