Derby climate

Derby Frost Dates and Growing Season

NOAA 1991-2020 normals for Ansonia 1 Ne, 3.1 km from Derby, elevation 136 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Ansonia 1 Ne. 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 17, a growing season of about 175 days.

Median at 36°F
last May 8, first October 7
Median at 32°F
last April 25, first October 17
Median at 28°F
last April 10, first October 30

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 22May 17May 14May 10May 8May 5May 2Apr 29Apr 24
32°FMay 10May 6May 2Apr 29Apr 25Apr 22Apr 18Apr 14Apr 11
28°FApr 27Apr 20Apr 16Apr 13Apr 10Apr 7Apr 5Apr 2Mar 30
24°FApr 10Apr 5Apr 3Apr 1Mar 30Mar 28Mar 24Mar 20Mar 15
20°FApr 1Mar 30Mar 26Mar 23Mar 20Mar 17Mar 14Mar 10Mar 5
16°FMar 25Mar 21Mar 17Mar 14Mar 11Mar 8Mar 4Feb 28Feb 22

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 27Oct 1Oct 3Oct 5Oct 7Oct 9Oct 13Oct 16Oct 21
32°FOct 4Oct 8Oct 11Oct 14Oct 17Oct 20Oct 23Oct 27Nov 1
28°FOct 14Oct 19Oct 23Oct 27Oct 30Nov 1Nov 4Nov 8Nov 13
24°FOct 26Nov 1Nov 4Nov 8Nov 12Nov 14Nov 18Nov 23Nov 30
20°FNov 9Nov 14Nov 17Nov 21Nov 25Nov 28Dec 1Dec 4Dec 10
16°FNov 20Nov 26Nov 30Dec 2Dec 5Dec 8Dec 11Dec 16Dec 24

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°F172164159156151147143138132
32°F197189182179175170165159153
28°F220215210206202196192186179
24°F247240234229225222217211205
20°F270262258253249244240236228
16°F292284278274269265260255247

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
6,070
Cooling degree days, base 65°F
686
Growing degree days, base 40°F
4,974
Growing degree days, base 45°F
3,827
Growing degree days, base 50°F
2,823
Growing degree days, base 55°F
1,968
Growing degree days, base 60°F
1,259

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,17200
February1,02201
March86506
April516159
May23218265
June39116528
July3237699
August6224683
September10786430
October3983131
November696020
December1,01501

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 3.1 km from Derby. 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 USC00060128 (Ansonia 1 Ne). Public domain.