Here is a bit of fun with simple map algebra. I took the average annual maximum and minimum temperatures for the lower 48 states from the PRISM Climate Group; these gridded data (rasters) are primarily based on the NCDC’s 1981-2010 U.S. Climate Normals. I then subtracted the minimum temperatures from the maximum temperatures to generate an average annual temperature range map.

The average annual temperature extrema are identified for a given location by recording the maximum and minimum temperature every day over the course of a year, then calculating the average of each set. Consequently, the difference between these averages is mathematically equivalent to the average of the daily temperature ranges at that location over the course of a year.

The interpolation technique applied by the PRISM group, converting weather station data into gridded data, results in the geographic extent of the map reaching slightly (about 6-10 km) out into the ocean/gulf/lakes. These data are likely real – not an artifact – and represent the smallest temperature ranges; the high thermal inertia of water as compared to land has a powerful moderating effect, which clearly minimizes coastal temperature swings. On the map, this effect appears as a dark blue fringe along the coasts.

In general, water, or the lack thereof, is responsible for a large part of the variation visible on the map. Deserts in the Basin and Range Province have large temperature swings because they can be hot during the day and then very cold at night. More humid climates in the eastern half of the country maintain warmer relative temperatures at night because of the moisture in the air.

The prevailing wind direction (the Westerlies), also play an important role here. I’ll skip the detailed meteorological explanation, but those interested should read up on the Coriolis Effect and its role in both oceanic and atmospheric circulation dynamics, which explain most of the observed non-orographic spatial variations in temperature range.

Data source: http://prism.oregonstate.edu (PRISM Climate Group, Oregon State University)