Natrona County climate

Natrona County Frost Dates and Growing Season

NOAA 1991-2020 normals for Powder Rvr (school), 17.6 km from Natrona County, elevation 5722 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Powder Rvr (school). 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 30 and the first fall freeze around September 22, a growing season of about 115 days.

Median at 36°F
last June 11, first September 13
Median at 32°F
last May 30, first September 22
Median at 28°F
last May 12, 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°FJun 25Jun 21Jun 18Jun 15Jun 11Jun 8Jun 5Jun 2May 26
32°FJun 13Jun 8Jun 4Jun 2May 30May 26May 22May 18May 12
28°FMay 29May 23May 19May 15May 12May 9May 6May 3Apr 29
24°FMay 11May 7May 4May 1Apr 30Apr 27Apr 23Apr 19Apr 12
20°FMay 2Apr 29Apr 25Apr 21Apr 16Apr 11Apr 6Apr 1Mar 22
16°FApr 22Apr 15Apr 10Apr 5Apr 1Mar 28Mar 20Mar 14Mar 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°FAug 31Sep 4Sep 7Sep 10Sep 13Sep 16Sep 18Sep 22Sep 26
32°FSep 8Sep 13Sep 16Sep 20Sep 22Sep 25Sep 28Sep 30Oct 4
28°FSep 16Sep 22Sep 26Sep 29Oct 1Oct 3Oct 6Oct 10Oct 15
24°FSep 26Oct 1Oct 5Oct 8Oct 11Oct 14Oct 16Oct 20Oct 25
20°FOct 6Oct 10Oct 14Oct 17Oct 20Oct 23Oct 26Oct 30Nov 7
16°FOct 14Oct 19Oct 23Oct 26Oct 30Nov 2Nov 7Nov 11Nov 19

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°F11210499959187837770
32°F13512912411911511110710195
28°F162154149145141136132126119
24°F187179173169165161156151144
20°F214205198192188183178173165
16°F242232223217211206200193186

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
8,059
Cooling degree days, base 65°F
330
Growing degree days, base 40°F
3,672
Growing degree days, base 45°F
2,725
Growing degree days, base 50°F
1,924
Growing degree days, base 55°F
1,252
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
January1,32700
February1,15000
March95204
April714019
May4503123
June16931320
July24155596
August43121543
September24320263
October670051
November98104
December1,33600

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 17.6 km from Natrona County. 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 USC00487375 (Powder Rvr (school)). Public domain.