Mill City climate

Mill City Frost Dates and Growing Season

NOAA 1991-2020 normals for Detroit Dam, 17.9 km from Mill City, elevation 1220 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Detroit Dam. A median date has an even chance of a later freeze in spring, or an earlier one in fall.

The last spring freeze typically comes around March 9 and the first fall freeze around December 3, a growing season of about 270 days.

Median at 36°F
last April 17, first November 11
Median at 32°F
last March 9, first December 3
Median at 28°F
last February 5, first December 26

Last freeze in spring

The chance of a later freeze than the date shown. At 10 percent, one spring in ten still freezes after the date; planting after the 10 percent date for 32°F is the cautious choice.

Threshold10%20%30%40%50%60%70%80%90%
36°FMay 12May 4Apr 29Apr 22Apr 17Apr 12Apr 8Apr 2Mar 26
32°FApr 8Mar 31Mar 23Mar 15Mar 9Mar 3Feb 24Feb 16Feb 2
28°FMar 8Feb 28Feb 20Feb 14Feb 5Jan 29Jan 18Jan 7Dec 23
24°FFeb 24Feb 16Feb 7Jan 29Jan 16Jan 7Jan 1Dec 23Dec 10
20°FFeb 18Feb 7Jan 25Jan 11Jan 3Dec 27Dec 21Dec 16Dec 9
16°F–––––––––

First freeze in fall

The chance of an earlier freeze than the date shown. At 10 percent, one fall in ten freezes before the date.

Threshold10%20%30%40%50%60%70%80%90%
36°FOct 25Oct 30Nov 4Nov 7Nov 11Nov 15Nov 19Nov 25Dec 2
32°FNov 8Nov 15Nov 22Nov 28Dec 3Dec 8Dec 15Dec 24Jan 4
28°FNov 22Dec 1Dec 8Dec 18Dec 26Jan 2Jan 9Jan 20Feb 5
24°FDec 1Dec 9Dec 19Dec 27Jan 1Jan 8Jan 18Feb 1Feb 16
20°FDec 7Dec 12Dec 17Dec 22Dec 29Jan 4Jan 12Jan 27Feb 15
16°F–––––––––

Growing season length

Days between the last spring and first fall freeze at each threshold, with the chance of a season at least that long.

Threshold10%20%30%40%50%60%70%80%90%
36°F237227219213207199193185175
32°F311296286278270260251241227
28°F365358345335327318309299284
24°F365365365365362354340329312
20°F365365365365365365365359341
16°F365365365365365365365365365

How often it gets that cold

The share of years with at least one night at or below each temperature.

36°F or colder
100% of years
32°F or colder
100% of years
28°F or colder
84% of years
24°F or colder
54% of years
20°F or colder
27% of years
16°F or colder
6% of years

Degree days

A degree day is one degree of the daily mean temperature beyond a base, for one day. Heating degree days count cold (the fuel a house burns), cooling degree days heat, growing degree days the warmth a crop needs; each crop has its own base.

Heating degree days, base 65°F
4,896
Cooling degree days, base 65°F
373
Growing degree days, base 40°F
4,788
Growing degree days, base 45°F
3,391
Growing degree days, base 50°F
2,304
Growing degree days, base 55°F
1,462
Growing degree days, base 60°F
819

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January78000
February65202
March626015
April475166
May29811202
June15735331
July36128556
August30139573
September10357404
October3563140
November603013
December78001

What this means

NOAA computes these from the daily lows at the station over 1991-2020, fitting the dates of each year's last and first freeze to a distribution, so a date can be stated at any level of risk. 36°F is a frost risk for tender plants on a clear, calm night, since the ground cools below the air above it; 32°F is a freeze; 28°F is a hard freeze that kills most annuals.

The station is 17.9 km from Mill City. Low ground, valleys and open fields freeze earlier and later than a town center or a hillside.

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Sources

NOAA NCEI U.S. Climate Normals, 1991-2020, annual and seasonal normals for station USC00352292 (Detroit Dam). Public domain.