Modoc County climate

Modoc County Frost Dates and Growing Season

NOAA 1991-2020 normals for Alturas Muni AP, 17.3 km from Modoc County, elevation 4375 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Alturas Muni 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 June 3 and the first fall freeze around September 12, a growing season of about 97 days.

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

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°F877871676359534739
32°F1241141071029792878173
28°F157151145140136131126120110
24°F188181176171167163158152144
20°F224215208202198194189183174
16°F266256247240233227220213204

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
6,558
Cooling degree days, base 65°F
235
Growing degree days, base 40°F
3,804
Growing degree days, base 45°F
2,714
Growing degree days, base 50°F
1,839
Growing degree days, base 55°F
1,143
Growing degree days, base 60°F
603

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,02900
February84600
March78802
April622020
May3871133
June16925313
July25123562
August5073489
September21013266
October543050
November82103
December1,06900

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 17.3 km from Modoc 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 USW00094299 (Alturas Muni AP). Public domain.