Alpha climate

Alpha Frost Dates and Growing Season

NOAA 1991-2020 normals for Altona, 19.7 km from Alpha, elevation 765 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Altona. 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 28 and the first fall freeze around October 12, a growing season of about 165 days.

Median at 36°F
last May 7, first October 1
Median at 32°F
last April 28, first October 12
Median at 28°F
last April 18, first October 22

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 20May 16May 13May 10May 7May 4May 2Apr 30Apr 25
32°FMay 12May 7May 3Apr 30Apr 28Apr 25Apr 22Apr 19Apr 15
28°FMay 3Apr 28Apr 24Apr 21Apr 18Apr 15Apr 13Apr 10Apr 5
24°FApr 22Apr 17Apr 14Apr 11Apr 8Apr 5Apr 2Mar 31Mar 25
20°FApr 11Apr 6Apr 3Apr 1Mar 30Mar 27Mar 23Mar 18Mar 12
16°FApr 1Mar 29Mar 25Mar 21Mar 19Mar 16Mar 12Mar 9Mar 4

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 19Sep 24Sep 27Sep 29Oct 1Oct 5Oct 7Oct 10Oct 15
32°FSep 28Oct 2Oct 6Oct 9Oct 12Oct 14Oct 18Oct 21Oct 26
28°FOct 8Oct 13Oct 16Oct 19Oct 22Oct 24Oct 27Oct 30Nov 3
24°FOct 19Oct 24Oct 26Oct 29Nov 1Nov 2Nov 5Nov 8Nov 13
20°FOct 28Nov 1Nov 2Nov 6Nov 8Nov 11Nov 14Nov 19Nov 25
16°FNov 3Nov 8Nov 11Nov 14Nov 18Nov 22Nov 26Nov 30Dec 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°F165159155150147144140135130
32°F186178173169165160157152146
28°F203197193189185181177172165
24°F226217212207204200196191185
20°F246239234229224220215211203
16°F267260254248244240235230223

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
6,534
Cooling degree days, base 65°F
744
Growing degree days, base 40°F
5,136
Growing degree days, base 45°F
4,007
Growing degree days, base 50°F
3,005
Growing degree days, base 55°F
2,123
Growing degree days, base 60°F
1,362

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,35000
February1,10503
March860129
April4784107
May18852343
June23164591
July8243700
August12186638
September11482422
October41911151
November801021
December1,17601

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 19.7 km from Alpha. 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 USC00110140 (Altona). Public domain.