Sweetwater County climate

Sweetwater County Frost Dates and Growing Season

NOAA 1991-2020 normals for Rock Springs AP, 16.2 km from Sweetwater County, elevation 6741 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Rock Springs AP. 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 22 and the first fall freeze around September 28, a growing season of about 126 days.

Median at 36°F
last June 5, first September 19
Median at 32°F
last May 22, first September 28
Median at 28°F
last May 7, first October 5

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

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 7Sep 11Sep 14Sep 16Sep 19Sep 21Sep 23Sep 26Sep 30
32°FSep 14Sep 20Sep 23Sep 25Sep 28Sep 30Oct 2Oct 4Oct 9
28°FSep 22Sep 27Oct 1Oct 3Oct 5Oct 10Oct 14Oct 18Oct 23
24°FSep 29Oct 4Oct 9Oct 14Oct 17Oct 21Oct 24Oct 28Nov 1
20°FOct 12Oct 17Oct 21Oct 24Oct 27Oct 30Nov 2Nov 5Nov 10
16°FOct 21Oct 26Oct 30Nov 1Nov 4Nov 7Nov 10Nov 13Nov 18

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°F123116112108104100969286
32°F148140135130126122117113106
28°F173165158154149146141136128
24°F193187181176171168163158150
20°F220212206200194190185179171
16°F246238231225220215210204194

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
8,064
Cooling degree days, base 65°F
319
Growing degree days, base 40°F
3,656
Growing degree days, base 45°F
2,713
Growing degree days, base 50°F
1,914
Growing degree days, base 55°F
1,250
Growing degree days, base 60°F
715

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,34700
February1,13900
March96102
April712017
May4422119
June15939341
July21159603
August38109537
September24211251
October637045
November1,01601
December1,35000

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 16.2 km from Sweetwater County. 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 USW00024027 (Rock Springs AP). Public domain.