The human body can withstand high temperatures to some extent, but exposure to temperatures exceeding human heat tolerance leads to sickness and, at very high temperatures, to death. In this study, using the analyzed data on the maximum temperature at 2 m above the surface in a 73-year period from 1948 to 2020, the seasonal, annual, decadal and centennial frequency of lethal temperatures above 50 degrees C (TU50c) in the Northern Hemisphere (NH) was analyzed. The aim of this research was to extract the frequency and trend in TU50c in the NH. In this study, regression analysis, trend component analysis and wavelet analysis were used. Examination of the frequency of TU50c occurrence in the NH showed that the trend of annual changes in the frequency of TU50c in the NH is upward and significant. Inter-decadal changes indicate that in the fifth decade (1980-1989) and sixth decade (1990-1999), there was an unprecedented increase in the frequency of TU50c. Inter-century changes also show that the incidence of TU50c has increased gradually from the twentieth to the twenty-first century. The highest incidence of twentieth-century TU50c extreme temperatures occurred between 1986 and 1988. Spatially, the region with the highest frequency and strongest TU50c is in Africa, especially Sudan, West Asia (between Iraq, southwestern Iran, Kuwait and Saudi Arabia) and India in the Indian subcontinent. These temperatures are not uncommon in the United States, but TU50c has not been reported in Europe and East Asia. The results of statistical analysis show that the frequency of occurrence of TU50c in the NH is related to the annual frequency of sunspots and also, to a much lesser extent, to the concentration of carbon dioxide.