Dillon climate

Dillon Frost Dates and Growing Season

NOAA 1991-2020 normals for Dillon U Of Montana Western, 0.8 km from Dillon, elevation 5129 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Dillon U Of Montana Western. 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 June 5 and the first fall freeze around September 13, a growing season of about 98 days.

Median at 32°F
last June 5, first September 13
Median at 28°F
last May 16, first September 25

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 23Jun 18Jun 13Jun 9Jun 5Jun 2May 29May 24May 18
28°FJun 3May 26May 22May 18May 16May 13May 10May 7May 3
24°FMay 18May 14May 10May 7May 5May 2Apr 30Apr 26Apr 20
20°FMay 7May 2Apr 29Apr 25Apr 21Apr 18Apr 14Apr 11Apr 6
16°FApr 23Apr 16Apr 11Apr 8Apr 5Apr 2Mar 29Mar 24Mar 16

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°FAug 29Sep 3Sep 7Sep 10Sep 13Sep 16Sep 19Sep 22Sep 27
28°FSep 9Sep 14Sep 18Sep 22Sep 25Sep 28Oct 1Oct 4Oct 8
24°FSep 19Sep 26Sep 29Oct 2Oct 4Oct 7Oct 11Oct 15Oct 20
20°FOct 1Oct 4Oct 8Oct 12Oct 15Oct 19Oct 23Oct 27Nov 2
16°FOct 8Oct 15Oct 19Oct 23Oct 27Oct 30Nov 2Nov 6Nov 13

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°F867974696561554839
32°F1211121071029893888274
28°F150144138133130126121116108
24°F173166161157153149145141133
20°F200191185181176172168162155
16°F230220214208203198192186179

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
8,176
Cooling degree days, base 65°F
116
Growing degree days, base 40°F
3,172
Growing degree days, base 45°F
2,231
Growing degree days, base 50°F
1,461
Growing degree days, base 55°F
844
Growing degree days, base 60°F
392

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,27400
February1,08500
March95501
April710013
May458196
June2338237
July5866473
August9337412
September3024190
October671038
November1,01601
December1,32100

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 0.8 km from Dillon. 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 USC00242409 (Dillon U Of Montana Western). Public domain.