Alta climate

Alta Frost Dates and Growing Season

NOAA 1991-2020 normals for Storm Lake, 8.5 km from Alta, elevation 1442 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Storm Lake. 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 April 25 and the first fall freeze around October 13, a growing season of about 170 days.

Median at 36°F
last May 4, first October 3
Median at 32°F
last April 25, first October 13
Median at 28°F
last April 15, first October 23

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°FMay 16May 11May 8May 6May 4May 2Apr 30Apr 27Apr 22
32°FMay 7May 3May 1Apr 29Apr 25Apr 22Apr 19Apr 16Apr 11
28°FApr 29Apr 24Apr 21Apr 18Apr 15Apr 12Apr 9Apr 6Apr 2
24°FApr 20Apr 15Apr 11Apr 9Apr 6Apr 3Apr 1Mar 28Mar 22
20°FApr 13Apr 8Apr 4Apr 1Mar 31Mar 26Mar 22Mar 18Mar 13
16°FApr 6Apr 1Mar 28Mar 24Mar 20Mar 17Mar 14Mar 10Mar 6

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 21Sep 25Sep 28Oct 1Oct 3Oct 5Oct 8Oct 11Oct 15
32°FOct 1Oct 4Oct 7Oct 10Oct 13Oct 16Oct 18Oct 21Oct 25
28°FOct 11Oct 15Oct 18Oct 21Oct 23Oct 26Oct 28Nov 1Nov 5
24°FOct 20Oct 24Oct 27Oct 30Nov 1Nov 3Nov 6Nov 10Nov 16
20°FOct 27Nov 1Nov 3Nov 6Nov 8Nov 11Nov 15Nov 19Nov 27
16°FNov 2Nov 6Nov 10Nov 13Nov 16Nov 20Nov 24Nov 29Dec 5

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°F169162158154151148144140134
32°F189182178174170166162158151
28°F211203198194190187183178171
24°F230222216212207204199195188
20°F248240234229224219215209202
16°F261254249245240235230224217

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
7,230
Cooling degree days, base 65°F
754
Growing degree days, base 40°F
4,934
Growing degree days, base 45°F
3,865
Growing degree days, base 50°F
2,908
Growing degree days, base 55°F
2,066
Growing degree days, base 60°F
1,345

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,47900
February1,21100
March954116
April530590
May22547308
June30171591
July6254713
August15190640
September12979408
October4708127
November887014
December1,29600

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 8.5 km from Alta. 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 USC00137979 (Storm Lake). Public domain.