This indicator examines U.S. and global surface temperature patterns over time.
U.S. surface measurements come from weather stations on land, while global surface measurements also incorporate observations from buoys and ships on the ocean, thereby providing data from sites spanning much of the surface of the Earth. This indicator starts at 1901 except for the detailed map of Alaska, where reliable statewide records are available back to 1925. For comparison, this indicator also displays satellite measurements that can be used to estimate the temperature of the Earth’s lower atmosphere since 1979.
This indicator shows annual anomalies, or differences, compared with the average temperature from 1901 to 2000. For example, an anomaly of +2.0 degrees means the average temperature was 2 degrees higher than the long-term average. Anomalies have been calculated for each weather station. Daily temperature measurements at each site were used to calculate monthly anomalies, which were then averaged to find an annual temperature anomaly for each year. Anomalies for the contiguous 48 states and Alaska have been determined by calculating average anomalies for areas within each state based on station density, interpolation, and topography. These regional anomalies are then averaged together in proportion to their area to develop national results. Similarly, global anomalies have been determined by dividing the world into a grid, averaging the data for each cell of the grid, and then averaging the grid cells together.