Abstract: To understanding the climatologic characteristics of hot days and weltering weather at Xi'an City, Shaanxi Province, China during recent decades, the paper analyzed the characteristics of monthly, seasonal, interannual and interdecadal changes of daily maximum temperature using the data of daily maximum temperature in Xi'an station from 1951 to 2013,, and the temperature-humidity index(THI)were calculated with the data of daily maximum temperature and daily minimum relative humid from 1980 to 2013, and an extremely hot day was defined as a day of THI more than 29.5 at the highest five level. With the comparison of hot day with extremely temperature-humidity, the results showed that there was a significant interdecadal variation of hot days, and there had about 24.7 d·a^-1 in 1950s and 26.5 d·a^-1 in 1960s, but in 1970s, the hot days was reduced to 21.8 d·a^-1 and 14.5 d·a^-1 in 1980s. In 1990s, the hot days were increased to 23.3 d·a^-1, and the number of hot days(27.5 d·a^-1) were at most from 2001 to 2013, and that the starting date of hot days was shifted forward and end date was delayed. The number of extremely hot days was significant less than that of hot days, and there were most extremely hot days in the July of a year, and that 87.5% of extremely hot days was related to hot days, which was with above 32.4℃ and high humid. Finally, the relationship between the extremely hot days and atmospheric circulation was studied. The result pointed out that the extremely hot days was significantly related to warm and humid current resulted from the westward and northward of subtropical high, and there was a sinking airflow in middle and high layer, and warm and humid current, hence there was a high humid and hot weather situation, a pure high temperature weather was related to continental high pressure and Tibet plateau high with warm downward flow, and which was dry and clear.

Key words: hot day, weltering weather, comparative analysis, Circulation situation