Garfield County climate

Garfield County Frost Dates and Growing Season

NOAA 1991-2020 normals for Boulder, 5.9 km from Garfield County, elevation 6650 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Boulder. 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 May 4 and the first fall freeze around October 14, a growing season of about 160 days.

Median at 36°F
last May 18, first October 5
Median at 32°F
last May 4, first October 14
Median at 28°F
last April 23, first October 23

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°FJun 6May 31May 27May 22May 18May 14May 11May 6May 1
32°FMay 25May 17May 12May 7May 4Apr 30Apr 28Apr 25Apr 20
28°FMay 12May 3Apr 29Apr 26Apr 23Apr 19Apr 15Apr 11Apr 5
24°FApr 26Apr 20Apr 16Apr 12Apr 8Apr 4Mar 29Mar 22Mar 14
20°FApr 12Apr 4Mar 27Mar 21Mar 15Mar 11Mar 7Mar 2Feb 25
16°FMar 24Mar 12Mar 7Mar 3Feb 28Feb 23Feb 18Feb 13Feb 5

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°FSep 22Sep 27Sep 30Oct 2Oct 5Oct 7Oct 11Oct 14Oct 18
32°FSep 30Oct 4Oct 7Oct 11Oct 14Oct 17Oct 20Oct 23Oct 29
28°FOct 7Oct 13Oct 17Oct 20Oct 23Oct 27Oct 30Nov 4Nov 9
24°FOct 18Oct 24Oct 28Nov 1Nov 4Nov 8Nov 12Nov 16Nov 20
20°FOct 27Nov 2Nov 7Nov 12Nov 16Nov 19Nov 22Nov 25Nov 30
16°FNov 5Nov 13Nov 18Nov 22Nov 25Nov 29Dec 2Dec 6Dec 12

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°F160151147142137133128121113
32°F182175170165160156151145135
28°F206199192186182177173167160
24°F237227220214208203197190181
20°F267257252245238232225218206
16°F297288282276270265257250237

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
5,906
Cooling degree days, base 65°F
580
Growing degree days, base 40°F
4,808
Growing degree days, base 45°F
3,648
Growing degree days, base 50°F
2,651
Growing degree days, base 55°F
1,812
Growing degree days, base 60°F
1,122

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,07300
February86700
March721013
April518058
May28214229
June60108499
July5234694
August9177632
September10646395
October4222122
November76708
December1,07900

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 5.9 km from Garfield 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 USC00420849 (Boulder). Public domain.