Mills climate

Mills Frost Dates and Growing Season

NOAA 1991-2020 normals for Casper Natrona CO AP, 10.1 km from Mills, elevation 5338 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Casper Natrona CO 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 26 and the first fall freeze around September 22, a growing season of about 117 days.

Median at 36°F
last June 9, first September 9
Median at 32°F
last May 26, first September 22
Median at 28°F
last May 11, first October 2

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 20Jun 16Jun 12Jun 9Jun 6Jun 3May 31May 25
32°FJun 10Jun 4Jun 1May 30May 26May 23May 20May 17May 12
28°FMay 27May 21May 17May 14May 11May 9May 6May 4Apr 30
24°FMay 14May 9May 5May 2Apr 30Apr 28Apr 25Apr 21Apr 16
20°FMay 1Apr 27Apr 24Apr 21Apr 17Apr 14Apr 12Apr 8Apr 3
16°FApr 23Apr 17Apr 13Apr 9Apr 5Apr 2Mar 31Mar 25Mar 17

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 26Sep 1Sep 3Sep 6Sep 9Sep 12Sep 15Sep 19Sep 24
32°FSep 5Sep 11Sep 15Sep 19Sep 22Sep 24Sep 28Oct 1Oct 4
28°FSep 17Sep 23Sep 27Sep 30Oct 2Oct 4Oct 7Oct 11Oct 15
24°FOct 1Oct 4Oct 6Oct 10Oct 13Oct 16Oct 19Oct 21Oct 25
20°FOct 8Oct 14Oct 18Oct 20Oct 22Oct 25Oct 27Oct 30Nov 3
16°FOct 16Oct 21Oct 25Oct 28Oct 31Nov 2Nov 5Nov 9Nov 14

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°F112105101969288847969
32°F13613012512111711310910396
28°F161155150146141137133128120
24°F183177172169165161157152146
20°F206199194189186182178174168
16°F229221215210206201196190185

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
7,486
Cooling degree days, base 65°F
443
Growing degree days, base 40°F
4,015
Growing degree days, base 45°F
3,006
Growing degree days, base 50°F
2,156
Growing degree days, base 55°F
1,450
Growing degree days, base 60°F
877

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,23700
February1,07500
March90707
April683025
May4094140
June12651380
July14200651
August31157591
September21431293
October611161
November93208
December1,24800

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 10.1 km from Mills. 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 USW00024089 (Casper Natrona CO AP). Public domain.