Boulder climate

Boulder Frost Dates and Growing Season

NOAA 1991-2020 normals for Boulder, 0.9 km from Boulder, elevation 4904 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 June 7 and the first fall freeze around September 8, a growing season of about 90 days.

Median at 32°F
last June 7, first September 8
Median at 28°F
last May 21, 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 25Jun 19Jun 14Jun 10Jun 7Jun 4Jun 2May 29May 24
28°FJun 5May 31May 27May 24May 21May 18May 14May 11May 7
24°FMay 22May 17May 13May 11May 8May 5May 2Apr 30Apr 24
20°FMay 12May 6May 2Apr 30Apr 27Apr 23Apr 20Apr 16Apr 9
16°FMay 2Apr 27Apr 22Apr 18Apr 14Apr 10Apr 5Apr 1Mar 24

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 25Aug 30Sep 2Sep 5Sep 8Sep 10Sep 14Sep 17Sep 21
28°FSep 6Sep 11Sep 15Sep 18Sep 21Sep 23Sep 26Sep 30Oct 3
24°FSep 15Sep 21Sep 25Sep 27Sep 30Oct 2Oct 4Oct 8Oct 12
20°FSep 26Sep 30Oct 3Oct 6Oct 9Oct 12Oct 15Oct 18Oct 23
16°FOct 3Oct 8Oct 13Oct 16Oct 19Oct 23Oct 27Oct 30Nov 3

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°F837772676359544739
32°F11010498949086827668
28°F14113312912412111611210699
24°F163157152148144140135131124
20°F187179173169164160156151144
16°F212204198192187182177170162

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
8,312
Cooling degree days, base 65°F
157
Growing degree days, base 40°F
3,143
Growing degree days, base 45°F
2,226
Growing degree days, base 50°F
1,477
Growing degree days, base 55°F
883
Growing degree days, base 60°F
439

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,28800
February1,09500
March96701
April728013
May483090
June24512232
July6880478
August8557440
September3077193
October696031
November1,03501
December1,31600

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 0.9 km from Boulder. 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 USC00241008 (Boulder). Public domain.