In order to recognize Tehran weather types, 22 atmospheric variables are studied for Tehran synoptic station from January 1, 1978 to December 31, 2004. Cluster analysis on the standardized matrix of data and linking days on the basis of ward method shows that Tehran has four Weather Types (WT). These are- 1) Cold, frosty, rainy WT, 2) Moderate WT, 3) Warm, dry WT and 4) Cold, windy WT. Based on this study results, warm and dry WT is the most dominant and durable and cold, frosty, rainy WT is the least frequent and short-lived weather types in Tehran. Only 25.4% days of a year, moderate WT can be seen. Warm and dry period involves almost 40% days of a year Because of the dominance of Azores subtropical high pressure, Tehran's weather is much stable around this time and only warm, dry WT occurs. Because of relative increase of the warm and dry weather type in this period, Tehran's weather has become warmer.
