Portal climate

Portal Frost Dates and Growing Season

NOAA 1991-2020 normals for Rocky Ford 4SE, 18.3 km from Portal, elevation 160 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Rocky Ford 4SE. 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 March 14 and the first fall freeze around November 15, a growing season of about 248 days.

Median at 36°F
last March 30, first November 5
Median at 32°F
last March 14, first November 15
Median at 28°F
last February 26, first November 29

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°FApr 16Apr 10Apr 6Apr 2Mar 30Mar 25Mar 20Mar 15Mar 7
32°FApr 5Mar 28Mar 22Mar 17Mar 14Mar 8Mar 3Feb 27Feb 19
28°FMar 20Mar 14Mar 8Mar 3Feb 26Feb 20Feb 14Feb 6Jan 29
24°FMar 7Feb 28Feb 20Feb 12Feb 6Feb 1Jan 28Jan 17Jan 6
20°FFeb 27Feb 12Feb 5Jan 31Jan 26Jan 18Jan 12Jan 5Dec 26
16°FFeb 11Feb 5Jan 31Jan 29Jan 21Jan 15Jan 11Jan 5Dec 31

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°FOct 22Oct 27Oct 31Nov 1Nov 5Nov 7Nov 10Nov 14Nov 18
32°FNov 1Nov 5Nov 8Nov 11Nov 15Nov 17Nov 21Nov 29Dec 5
28°FNov 9Nov 14Nov 19Nov 24Nov 29Dec 2Dec 8Dec 18Dec 31
24°FNov 23Dec 1Dec 6Dec 15Dec 24Jan 1Jan 6Jan 13Jan 28
20°FDec 4Dec 19Dec 27Jan 1Jan 7Jan 13Jan 19Jan 29Feb 8
16°FDec 26Jan 2Jan 7Jan 11Jan 16Jan 23Jan 28Feb 2Feb 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°F248239233228224219213208199
32°F278266258253248242236230222
28°F315303293286279271263256246
24°F354340331323314307301292278
20°F365365365359349339332321306
16°F365365365365365365365351333

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
94% of years
20°F or colder
68% of years
16°F or colder
25% 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
2,301
Cooling degree days, base 65°F
2,120
Growing degree days, base 40°F
9,022
Growing degree days, base 45°F
7,337
Growing degree days, base 50°F
5,789
Growing degree days, base 55°F
4,399
Growing degree days, base 60°F
3,167

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January569280
February4164105
March28318238
April10968412
May18223670
June0398847
July0505970
August0480945
September5305750
October9199476
November31215194
December4984102

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 18.3 km from Portal. Low ground, valleys and open fields freeze earlier and later than a town center or a hillside.

More

Sources

NOAA NCEI U.S. Climate Normals, 1991-2020, annual and seasonal normals for station USC00097528 (Rocky Ford 4SE). Public domain.