January 8, 2015
As we welcome some of the coldest weather we have yet experienced this winter here in eastern Kansas, I wanted to share this interesting map that shows the typical date on which various locations in the US experience their coldest day. This map, created by the National Climatic Data Center (NCDC) from 30 years of historic records, shows some very interesting geographic trends.
According to the commentary provided by NCDC: “The map reveals several interesting regional differences across the country. Most prominently, the western half of the Lower 48 typically reaches its climatological coldest day in December, whereas most eastern stations reach their minimum in January. In addition, areas with higher snowfall Normals, such as the Northeast and high-altitude regions in the West, tend to reach their climatological coldest day much later, which is likely because of the increased reflection of solar radiation at the Earth’s surface due to the presence of snow cover.”
This results in interesting dichotomy between areas that may be very close to one another, such as southeast Wyoming where the Cheyenne proper may experience its coldest day in mid-December, whereas the mountains an hour west of town experience their coldest temperature a full 2-3 months later. To expand upon the commentary from NCDC, I would hypothesize that soil moisture content plays a significant part in the differences between east and west. Water has higher thermal inertia than dry dirt, meaning that it will take longer to change temperature. Thus the dry soils of the western US are more likely to cool off quickly, timing their coldest day very close to the Winter Solstice where the sun is at its lowest angle. Meanwhile the wetter soils of the eastern US carry heat from the summer and fall farther into the winter, and take a few extra months to cool down to their minimum, months later than the actual minimum in solar radiation. Just a guess.
Image and cited commentary from http://www.ncdc.noaa.gov/news/when-to-expect-coldest-day-of-year