Fairfax climate

Fairfax Frost Dates and Growing Season

NOAA 1991-2020 normals for Ralston, 8.5 km from Fairfax, elevation 825 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Ralston. 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 11 and the first fall freeze around October 24, a growing season of about 196 days.

Median at 36°F
last April 23, first October 14
Median at 32°F
last April 11, first October 24
Median at 28°F
last March 31, first November 1

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 6May 2Apr 29Apr 26Apr 23Apr 20Apr 16Apr 13Apr 8
32°FApr 27Apr 22Apr 17Apr 14Apr 11Apr 8Apr 4Mar 31Mar 26
28°FApr 17Apr 10Apr 6Apr 3Mar 31Mar 28Mar 25Mar 20Mar 14
24°FApr 5Apr 1Mar 27Mar 22Mar 18Mar 15Mar 12Mar 8Mar 2
20°FMar 23Mar 17Mar 13Mar 10Mar 7Mar 4Feb 28Feb 25Feb 18
16°FMar 14Mar 8Mar 4Mar 1Feb 26Feb 22Feb 17Feb 10Feb 1

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 29Oct 4Oct 8Oct 11Oct 14Oct 18Oct 21Oct 24Oct 28
32°FOct 7Oct 14Oct 18Oct 21Oct 24Oct 27Oct 30Nov 1Nov 6
28°FOct 17Oct 23Oct 27Oct 30Nov 1Nov 4Nov 7Nov 11Nov 17
24°FOct 25Nov 1Nov 4Nov 7Nov 11Nov 14Nov 18Nov 23Nov 29
20°FNov 3Nov 8Nov 13Nov 17Nov 21Nov 25Nov 29Dec 3Dec 9
16°FNov 12Nov 19Nov 24Nov 28Dec 2Dec 5Dec 10Dec 16Dec 23

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°F197189184180175171167161154
32°F216209205200196192188181174
28°F238231225220216212207202195
24°F261253246241236231226220214
20°F282275268263258253248241233
16°F311301294287281276270262253

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
3,742
Cooling degree days, base 65°F
1,981
Growing degree days, base 40°F
7,840
Growing degree days, base 45°F
6,384
Growing degree days, base 50°F
5,075
Growing degree days, base 55°F
3,903
Growing degree days, base 60°F
2,873

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January882012
February662236
March45512142
April19848320
May51169583
June1382831
July05411,006
August0494959
September21260690
October17666371
November4876105
December808019

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 8.5 km from Fairfax. 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 USC00347390 (Ralston). Public domain.