Aspen climate

Aspen Frost Dates and Growing Season

NOAA 1991-2020 normals for Aspen 1SW, 1.9 km from Aspen, elevation 8175 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Aspen 1SW. 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 2 and the first fall freeze around September 23, a growing season of about 113 days.

Median at 36°F
last June 16, first September 14
Median at 32°F
last June 2, first September 23
Median at 28°F
last May 18, first October 1

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°FJul 1Jun 25Jun 22Jun 19Jun 16Jun 13Jun 10Jun 7Jun 3
32°FJun 17Jun 12Jun 8Jun 5Jun 2May 31May 27May 22May 17
28°FJun 5May 31May 26May 22May 18May 15May 11May 6May 3
24°FMay 23May 16May 12May 8May 4May 1Apr 28Apr 25Apr 20
20°FMay 9May 3Apr 30Apr 26Apr 22Apr 20Apr 16Apr 13Apr 8
16°FApr 27Apr 21Apr 17Apr 13Apr 9Apr 7Apr 4Mar 31Mar 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°FSep 1Sep 5Sep 9Sep 12Sep 14Sep 16Sep 19Sep 21Sep 25
32°FSep 12Sep 16Sep 19Sep 21Sep 23Sep 26Sep 28Oct 1Oct 4
28°FSep 18Sep 23Sep 27Sep 29Oct 1Oct 4Oct 7Oct 10Oct 16
24°FSep 27Oct 1Oct 4Oct 7Oct 10Oct 13Oct 17Oct 20Oct 25
20°FOct 4Oct 9Oct 13Oct 17Oct 21Oct 23Oct 27Oct 31Nov 4
16°FOct 13Oct 18Oct 22Oct 26Oct 29Nov 1Nov 4Nov 9Nov 15

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°F10610096928985817568
32°F13112412011611310910410092
28°F157149144139135130126121112
24°F178172167162158154150144135
20°F200192188184179175170166158
16°F221214209204201195191185179

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
8,880
Cooling degree days, base 65°F
20
Growing degree days, base 40°F
2,713
Growing degree days, base 45°F
1,834
Growing degree days, base 50°F
1,109
Growing degree days, base 55°F
543
Growing degree days, base 60°F
173

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,36600
February1,15100
March1,04600
April80603
May543054
June2602209
July11213367
August1575315
September3470143
October698018
November1,03500
December1,36100

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 1.9 km from Aspen. 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 USC00050372 (Aspen 1SW). Public domain.