Shirley climate

Shirley Frost Dates and Growing Season

NOAA 1991-2020 normals for Fitchburg Muni AP, 8.8 km from Shirley, elevation 348 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Fitchburg 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 April 29 and the first fall freeze around October 11, a growing season of about 164 days.

Median at 36°F
last May 9, first October 1
Median at 32°F
last April 29, first October 11
Median at 28°F
last April 16, 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 24May 19May 16May 12May 9May 7May 5May 2Apr 29
32°FMay 13May 8May 4May 1Apr 29Apr 27Apr 25Apr 22Apr 19
28°FApr 29Apr 24Apr 21Apr 19Apr 16Apr 13Apr 10Apr 7Apr 4
24°FApr 16Apr 12Apr 8Apr 5Apr 3Apr 2Mar 31Mar 29Mar 23
20°FApr 5Apr 2Mar 31Mar 29Mar 26Mar 22Mar 19Mar 15Mar 11
16°FMar 30Mar 26Mar 22Mar 19Mar 16Mar 13Mar 11Mar 8Mar 2

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 20Sep 24Sep 27Sep 30Oct 1Oct 3Oct 5Oct 8Oct 13
32°FOct 1Oct 3Oct 5Oct 8Oct 11Oct 14Oct 18Oct 22Oct 27
28°FOct 9Oct 14Oct 17Oct 21Oct 23Oct 26Oct 30Nov 2Nov 5
24°FOct 21Oct 26Oct 30Nov 1Nov 4Nov 7Nov 10Nov 14Nov 19
20°FNov 1Nov 6Nov 10Nov 13Nov 17Nov 20Nov 23Nov 27Dec 2
16°FNov 15Nov 20Nov 24Nov 26Nov 29Dec 2Dec 5Dec 10Dec 15

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°F160155151147144140137133126
32°F183176171167164161157153147
28°F206201197192188184180175170
24°F231225220216212209205201195
20°F258250245240236232227222214
16°F278271265261257253249244237

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
6,483
Cooling degree days, base 65°F
641
Growing degree days, base 40°F
4,771
Growing degree days, base 45°F
3,664
Growing degree days, base 50°F
2,701
Growing degree days, base 55°F
1,876
Growing degree days, base 60°F
1,188

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,24300
February1,06001
March921111
April548567
May25128259
June58117509
July6237696
August14189640
September12761387
October4494111
November746018
December1,06002

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 8.8 km from Shirley. 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 USW00004780 (Fitchburg Muni AP). Public domain.