Jefferson County climate

Jefferson County Frost Dates and Growing Season

NOAA 1991-2020 normals for Hamer 4 Nw, 16.7 km from Jefferson County, elevation 4790 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Hamer 4 Nw. 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 June 3 and the first fall freeze around September 11, a growing season of about 98 days.

Median at 32°F
last June 3, first September 11
Median at 28°F
last May 14, first September 21

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°F–––––––––
32°FJun 21Jun 17Jun 12Jun 7Jun 3May 31May 26May 21May 15
28°FJun 2May 26May 21May 17May 14May 11May 9May 6May 2
24°FMay 14May 10May 7May 4May 1Apr 30Apr 27Apr 23Apr 17
20°FMay 4Apr 30Apr 26Apr 21Apr 18Apr 15Apr 12Apr 8Apr 4
16°FApr 21Apr 16Apr 12Apr 8Apr 5Apr 2Mar 30Mar 25Mar 17

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°F–––––––––
32°FAug 30Sep 2Sep 6Sep 9Sep 11Sep 14Sep 17Sep 20Sep 24
28°FSep 5Sep 11Sep 15Sep 18Sep 21Sep 24Sep 27Oct 1Oct 6
24°FSep 16Sep 22Sep 27Sep 30Oct 3Oct 5Oct 9Oct 13Oct 17
20°FSep 28Oct 4Oct 8Oct 11Oct 15Oct 18Oct 20Oct 23Oct 28
16°FOct 11Oct 16Oct 19Oct 23Oct 26Oct 28Oct 31Nov 3Nov 8

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°F928479757065625648
32°F1221141081039894898376
28°F149141136132127123118113104
24°F175168163159154149145139131
20°F199192187183179175171165156
16°F224216210206202198193187179

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
100% of years
24°F or colder
100% of years
20°F or colder
100% of years
16°F or colder
100% 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
7,992
Cooling degree days, base 65°F
308
Growing degree days, base 40°F
3,791
Growing degree days, base 45°F
2,805
Growing degree days, base 50°F
1,964
Growing degree days, base 55°F
1,261
Growing degree days, base 60°F
707

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,45500
February1,16500
March92102
April611028
May3573156
June14436347
July26148587
August36106535
September21814264
October620044
November1,03401
December1,40600

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 16.7 km from Jefferson County. 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 USC00103964 (Hamer 4 Nw). Public domain.