Powell climate

Powell Frost Dates and Growing Season

NOAA 1991-2020 normals for Powell Fld Stn, 2.5 km from Powell, elevation 4372 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Powell Fld Stn. 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 19 and the first fall freeze around September 20, a growing season of about 123 days.

Median at 32°F
last May 19, first September 20
Median at 28°F
last May 5, first September 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°F–––––––––
32°FJun 4May 29May 26May 23May 19May 16May 13May 9May 5
28°FMay 20May 14May 10May 7May 5May 3Apr 30Apr 29Apr 25
24°FMay 7May 3Apr 30Apr 28Apr 25Apr 23Apr 20Apr 16Apr 11
20°FApr 29Apr 25Apr 22Apr 18Apr 16Apr 13Apr 9Apr 6Apr 1
16°FApr 22Apr 16Apr 13Apr 9Apr 6Apr 3Mar 31Mar 28Mar 20

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°FSep 7Sep 12Sep 15Sep 17Sep 20Sep 22Sep 24Sep 27Oct 1
28°FSep 16Sep 21Sep 24Sep 27Sep 29Oct 2Oct 4Oct 8Oct 14
24°FSep 26Oct 1Oct 3Oct 6Oct 10Oct 14Oct 17Oct 20Oct 25
20°FOct 5Oct 11Oct 14Oct 17Oct 20Oct 23Oct 26Oct 30Nov 3
16°FOct 13Oct 19Oct 22Oct 26Oct 29Nov 1Nov 3Nov 7Nov 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°F117109103989490837463
32°F141136131127123119115109102
28°F164159154150146142138133127
24°F186180176171167163159153146
20°F208202197192188183179174168
16°F227219214209205201196192184

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
8,227
Cooling degree days, base 65°F
329
Growing degree days, base 40°F
3,701
Growing degree days, base 45°F
2,743
Growing degree days, base 50°F
1,934
Growing degree days, base 55°F
1,260
Growing degree days, base 60°F
720

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,43200
February1,17600
March96304
April671030
May3866159
June14945354
July34152583
August55108519
September26017241
October670042
November1,04901
December1,38300

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 2.5 km from Powell. 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 USC00487388 (Powell Fld Stn). Public domain.