Calhan climate

Calhan Frost Dates and Growing Season

NOAA 1991-2020 normals for Rush 1N, 26.2 km from Calhan, elevation 6054 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Rush 1N. 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 9 and the first fall freeze around October 4, a growing season of about 146 days.

Median at 36°F
last May 19, first September 25
Median at 32°F
last May 9, first October 4
Median at 28°F
last April 30, first October 12

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 1May 28May 25May 22May 19May 17May 14May 11May 7
32°FMay 22May 17May 14May 11May 9May 6May 4May 2Apr 29
28°FMay 11May 8May 5May 2Apr 30Apr 28Apr 25Apr 22Apr 18
24°FMay 3Apr 29Apr 25Apr 22Apr 20Apr 18Apr 14Apr 10Apr 5
20°FApr 23Apr 19Apr 16Apr 12Apr 8Apr 5Apr 2Mar 30Mar 23
16°FApr 16Apr 9Apr 5Apr 2Mar 30Mar 24Mar 19Mar 13Mar 6

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 14Sep 17Sep 21Sep 23Sep 25Sep 27Sep 30Oct 3Oct 7
32°FSep 21Sep 25Sep 29Oct 1Oct 4Oct 6Oct 8Oct 12Oct 16
28°FSep 30Oct 4Oct 7Oct 10Oct 12Oct 15Oct 18Oct 21Oct 25
24°FOct 8Oct 12Oct 16Oct 18Oct 21Oct 24Oct 27Oct 31Nov 4
20°FOct 15Oct 20Oct 25Oct 28Nov 1Nov 3Nov 6Nov 9Nov 14
16°FOct 23Oct 29Nov 2Nov 5Nov 7Nov 10Nov 14Nov 17Nov 22

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°F145139134130127123119115109
32°F163158153149146143139134127
28°F182176171167164161157153148
24°F202195191187183179175170164
20°F225217212207203199194189181
16°F250240233228223218213206199

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
6,488
Cooling degree days, base 65°F
511
Growing degree days, base 40°F
4,526
Growing degree days, base 45°F
3,413
Growing degree days, base 50°F
2,462
Growing degree days, base 55°F
1,661
Growing degree days, base 60°F
1,010

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,10200
February94901
March820012
April590156
May31212209
June7997470
July14203654
August24152594
September15446355
October503197
November837015
December1,10301

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 26.2 km from Calhan. 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 USC00057287 (Rush 1N). Public domain.