Prince William County climate

Prince William County Frost Dates and Growing Season

NOAA 1991-2020 normals for Manassas, 3.7 km from Prince William County, elevation 245 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Manassas. 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 12 and the first fall freeze around October 27, a growing season of about 197 days.

Median at 36°F
last April 24, first October 18
Median at 32°F
last April 12, first October 27
Median at 28°F
last April 1, first November 5

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 10May 4Apr 30Apr 27Apr 24Apr 21Apr 18Apr 15Apr 11
32°FApr 24Apr 20Apr 17Apr 15Apr 12Apr 9Apr 7Apr 3Mar 30
28°FApr 14Apr 9Apr 6Apr 3Apr 1Mar 30Mar 27Mar 22Mar 17
24°FApr 4Mar 31Mar 28Mar 24Mar 20Mar 17Mar 14Mar 9Mar 2
20°FMar 25Mar 19Mar 15Mar 12Mar 9Mar 6Mar 2Feb 26Feb 20
16°FMar 15Mar 10Mar 5Mar 2Feb 28Feb 23Feb 19Feb 13Feb 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°FOct 6Oct 10Oct 12Oct 15Oct 18Oct 20Oct 23Oct 26Oct 30
32°FOct 13Oct 18Oct 22Oct 25Oct 27Oct 30Nov 1Nov 3Nov 7
28°FOct 24Oct 29Nov 1Nov 2Nov 5Nov 7Nov 10Nov 14Nov 19
24°FNov 2Nov 7Nov 10Nov 14Nov 16Nov 19Nov 23Nov 28Dec 4
20°FNov 13Nov 18Nov 22Nov 26Dec 1Dec 5Dec 10Dec 16Dec 24
16°FNov 27Dec 4Dec 9Dec 14Dec 20Dec 25Dec 30Jan 3Jan 12

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°F194188184179177172169164157
32°F216210205201197194190186180
28°F239232226222218214211206200
24°F267258251247242238233228221
20°F295285278274268263257252244
16°F320311305300294288283276267

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
4,707
Cooling degree days, base 65°F
1,270
Growing degree days, base 40°F
6,394
Growing degree days, base 45°F
5,076
Growing degree days, base 50°F
3,908
Growing degree days, base 55°F
2,886
Growing degree days, base 60°F
2,004

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January98208
February822014
March680148
April33519189
May11783433
June10257698
July0404869
August1340804
September46143548
October29121228
November575155
December849014

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 3.7 km from Prince William County. 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 USC00445204 (Manassas). Public domain.