Phillips County climate

Phillips County Frost Dates and Growing Season

NOAA 1991-2020 normals for Helena, 25.8 km from Phillips County, elevation 195 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Helena. 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 March 15 and the first fall freeze around November 16, a growing season of about 246 days.

Median at 36°F
last March 30, first November 5
Median at 32°F
last March 15, first November 16
Median at 28°F
last March 3, first November 29

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

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 24Oct 29Nov 1Nov 3Nov 5Nov 8Nov 12Nov 16Nov 21
32°FNov 2Nov 6Nov 9Nov 13Nov 16Nov 19Nov 23Nov 28Dec 4
28°FNov 10Nov 16Nov 21Nov 25Nov 29Dec 3Dec 6Dec 12Dec 19
24°FNov 22Nov 29Dec 5Dec 9Dec 15Dec 20Dec 25Jan 1Jan 8
20°FDec 6Dec 15Dec 22Dec 27Dec 31Jan 5Jan 11Jan 18Feb 1
16°FDec 13Dec 25Dec 31Jan 3Jan 9Jan 15Jan 21Feb 1Feb 10

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°F245238232227223217213208200
32°F272263256250246240235230222
28°F299287280274270265259253242
24°F327317309303298290283276265
20°F363352343334328319311304293
16°F365365365365357348340330316

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
99% of years
20°F or colder
89% of years
16°F or colder
62% 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
3,072
Cooling degree days, base 65°F
2,097
Growing degree days, base 40°F
8,398
Growing degree days, base 45°F
6,840
Growing degree days, base 50°F
5,430
Growing degree days, base 55°F
4,166
Growing degree days, base 60°F
3,051

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January752029
February574144
March37910171
April14059378
May23215657
June0416865
July0513978
August0493958
September7304747
October12779424
November4048139
December667039

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 25.8 km from Phillips County. 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 USC00033242 (Helena). Public domain.