Non-standard temperatures affect the accuracy of pressure-altimeter readings, which can pose a threat for general aviation especially near elevated terrain during colder-than-standard temperatures. During colder temperatures, these aircraft are flying at lower altitudes than indicated by their altimeters. To quantify this risk, we are using archived model reanalysis data from the National Center for Atmospheric Research to create a 30-year climatology of Corrected D-values for the winter months of December, January, and February. Air transport pilots currently use a parameter called D-value, which provides the estimated error between the true altitude and the indicated altitude due to non-standard temperatures (and humidity). The traditional D-value, however, assumes the altimeter base setting is constant at the standard mean sea-level pressure (29.92” Hg). Therefore, D-values are only appropriate for use in Class A airspace. Below Class A airspace, pilots are required to set their altimeters to the of the actual mean sea-level pressure (i.e., altimeter setting). Since general aviation pilots typically operate below Class A airspace, the D-values should be “corrected” for the non-standard surface pressure values to isolate the impact of non-standard temperature. A climatology of Corrected D-values will be analyzed for the 30-year period from January 1981 to December 2010. The goal is to help quantify the degree of altimeter error in an effort to improve pilot education and awareness. Comparisons will also be made with simpler rules of thumb that are often used to correct altimeters for temperature using only the temperature at flight altitude.