Statistically, the coldest days of the year should be a pretty simple thing to map. So why does this map look so splotchy?
The NOAA recently resurfaced this map first published last year, explaining a bit more about its idiosyncrasies. It represents the average date of the coldest day of the year based on 29 years of climate data — the greenish areas have their coldest days later in the year, while the purple and blue regions see their temperatures plummet in December.
But what about those odd sprinkles of green in the western US? Well, it's obvious that those are mountainous areas. The splotches fit the outlines of the Sierra Nevadas in California, and the Rockies cutting straight north-south through Colorado, Montana, and Wyoming:
The NOAA explains that these outliers exist "likely" because these regions see a lot of snow — which reflects sun back into the atmosphere when it hits the white cover. "Other conditions being equal, the more sunlight the ground reflects, the less solar heating the location experiences," the NOAA writes.
It makes sense: In lower-lying regions of the Western US, the coldest days occur in December, clustered around the winter solstice — the day of the year when we get the least sunlight. But in regions where lots of snow falls, the white cover continues to reflect sunlight well into the new year, keeping temperatures low.
That still doesn't explain why the southeastern US, which doesn't see much snow, conforms to this pattern. Kevin Walter, who is Director of Meteorology at the sustainable energy startup Tradewind Energy, has a blog post theorizing another reason the mountains are so different. He explains his hypothesis:
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.
By that reasoning, the ultra-dry west gets cold more quickly, while the damp east coast takes a longer time to get cooler.