How the County Freeze Numbers Are Made

From stations to counties

NOAA publishes 1991-2020 climate normals for about 7,000 US weather stations, including the average number of nights each month below 32, 20, 10 and 0°F. Counties do not have normals; stations do. For each county we take its Census internal point, look up its elevation, and blend the best three of the eight nearest stations within 120 km.

Why elevation is in the blend

Temperature falls with height, so a station 1,000 m above the town is a worse match than one 30 km away at the same height. Stations are ranked by distance plus a tenth of the height gap, and weighted by the inverse square of that. 71 counties still have their closest station 300 m or more above or below them, and their pages carry a lower grade. It is why Summit County, CO reads 177 nights below 20°F while Denver County, CO, about 70 miles east, reads 61 nights.

Match grades

Each page names its closest station and grades the match: close 2,544, fair 526, distant 70. A distant match means no station sits near the county at a similar height - mostly big western and Alaskan counties. Examples: Anchorage Municipality, AK, Bethel Census Area, AK, Copper River Census Area, AK. Treat those figures as a rough guide and check a nearby station that matches your own elevation.

Known weak spots

One point stands for the whole county, and in a county that spans a valley and a mountain range it can land on either. Anchorage's internal point sits high in the Chugach foothills, so its page reads colder than the city. Puerto Rico and four remote Alaska boroughs are left out because no station falls within 120 km. And every figure is a normal year: the 1991-2020 average, not this winter and not tonight.

Where to go next

See the range: Hennepin County, MN, Cook County, IL, Harris County, TX, Miami-Dade County, FL.

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