It has long been recognized that with increase in altitude the density of air decreases and influences the voltage rating and temperature rise of electrical apparatus. However, different standards for apparatus do not give the same weight to these factors. Examination of Weather Bureau data shows that there is a decrease in average atmospheric temperature with altitude, which should compensate to some extent for the influence of the decrease in air density on the rate of cooling at higher altitudes. This led to a study of temperature records of the United States for the purpose of presenting a summary of the variation of mean temperature with altitude.