Crunching the numbers on the geography of winter's worst weather.

Attention, employees who wish to time their "sick days" with the most frigid, butt-chapping weather: Want to know what portion of your calendar to mark off with a big, red X?

It's impossible to identify the exact dates for the coldest time of year, as weather changes constantly, but we can make an educated guess looking back over decades of temperature records. And that's what the folks at NOAA's National Climatic Data Center have done with this illuminating map of bottom-barrel temps throughout the U.S:

What's immediately obvious is that America's West gets blasted with chill rather early—on average, much of the region sees its coldest days in December. Around Seattle it tends to fall somewhere between December 16 to 20, whereas in the Bay Area it could be between December 26 to 31. Winter then extends its icy hand to smack around the Midwest and East, pulling temperatures down to their annual lowest often in January and (to a lesser extent) February. Alaska and Puerto Rico are outliers in that they have small spots where the coldest weather is likely to arrive in March.

In an interesting side note, how much snow a region gets influences the timing of the coldest day. "In addition, areas with higher snowfall [averages], such as the Northeast and high-altitude regions in the West, tend to reach their climatological coldest day much later," the NCDC writes, "which is likely because of the increased reflection of solar radiation at the Earth’s surface due to the presence of snow cover."

For folks not in the lower 48, here are Alaska and Hawaii:

NCDC

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