Darby climate

Darby Frost Dates and Growing Season

NOAA 1991-2020 normals for Philadelphia Intl AP, 6 km from Darby, elevation 10 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Philadelphia Intl 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 2 and the first fall freeze around November 10, a growing season of about 224 days.

Median at 36°F
last April 13, first October 31
Median at 32°F
last April 2, first November 10
Median at 28°F
last March 22, first November 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°FApr 28Apr 22Apr 18Apr 15Apr 13Apr 10Apr 7Apr 4Apr 1
32°FApr 13Apr 8Apr 6Apr 3Apr 2Mar 31Mar 29Mar 24Mar 19
28°FApr 3Mar 31Mar 28Mar 25Mar 22Mar 19Mar 16Mar 12Mar 7
24°FMar 26Mar 22Mar 18Mar 15Mar 12Mar 9Mar 6Mar 2Feb 24
20°FMar 18Mar 13Mar 8Mar 5Mar 1Feb 26Feb 21Feb 14Feb 5
16°FMar 7Mar 1Feb 24Feb 19Feb 13Feb 8Feb 3Jan 30Jan 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°FOct 18Oct 23Oct 26Oct 28Oct 31Nov 2Nov 4Nov 8Nov 12
32°FOct 30Nov 2Nov 5Nov 8Nov 10Nov 13Nov 16Nov 19Nov 24
28°FNov 7Nov 12Nov 16Nov 19Nov 23Nov 26Nov 29Dec 2Dec 8
24°FNov 17Nov 23Nov 28Dec 1Dec 4Dec 9Dec 13Dec 17Dec 23
20°FNov 30Dec 6Dec 12Dec 17Dec 21Dec 25Dec 31Jan 3Jan 11
16°FDec 14Dec 23Dec 29Jan 1Jan 4Jan 8Jan 14Jan 19Jan 29

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°F219211207204200197193189182
32°F243236232228224220217212207
28°F269260255250246242237232227
24°F293285279274269265259254245
20°F319309303299294289283277269
16°F347334330323318311305301294

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
96% 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
4,510
Cooling degree days, base 65°F
1,358
Growing degree days, base 40°F
6,595
Growing degree days, base 45°F
5,241
Growing degree days, base 50°F
4,047
Growing degree days, base 55°F
3,004
Growing degree days, base 60°F
2,107

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January97205
February81506
March666139
April33015179
May10482444
June10266706
July0425890
August1368832
September28175597
October23726270
November530166
December818013

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 6 km from Darby. Low ground, valleys and open fields freeze earlier and later than a town center or a hillside.

More

Sources

NOAA NCEI U.S. Climate Normals, 1991-2020, annual and seasonal normals for station USW00013739 (Philadelphia Intl AP). Public domain.