Whitman County climate

Whitman County Frost Dates and Growing Season

NOAA 1991-2020 normals for St John, 21.9 km from Whitman County, elevation 1952 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for St John. 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 29 and the first fall freeze around October 4, a growing season of about 161 days.

Median at 36°F
last May 16, first September 23
Median at 32°F
last April 29, first October 4
Median at 28°F
last April 5, first October 20

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°FJun 7May 30May 24May 20May 16May 13May 9May 4Apr 30
32°FMay 15May 9May 5May 1Apr 29Apr 26Apr 22Apr 17Apr 9
28°FApr 29Apr 22Apr 16Apr 10Apr 5Apr 1Mar 28Mar 21Mar 11
24°FApr 6Mar 30Mar 22Mar 16Mar 11Mar 6Mar 2Feb 24Feb 15
20°FMar 14Mar 7Mar 1Feb 26Feb 21Feb 15Feb 5Jan 29Jan 14
16°FMar 4Feb 26Feb 18Feb 10Feb 2Jan 26Jan 16Jan 7Dec 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°FSep 4Sep 11Sep 16Sep 19Sep 23Sep 26Sep 30Oct 3Oct 9
32°FSep 19Sep 26Sep 30Oct 2Oct 4Oct 8Oct 12Oct 18Oct 23
28°FOct 2Oct 7Oct 12Oct 16Oct 20Oct 24Oct 28Nov 1Nov 11
24°FOct 14Oct 20Oct 25Oct 30Nov 2Nov 8Nov 14Nov 21Nov 29
20°FOct 23Nov 1Nov 7Nov 14Nov 21Nov 27Dec 1Dec 6Dec 16
16°FNov 3Nov 15Nov 25Nov 30Dec 5Dec 9Dec 16Dec 23Jan 5

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°F15314413813212612111510898
32°F186177170165161156151145134
28°F230220210204198192185179168
24°F274260250244238231225217207
20°F319302292283275268260251240
16°F347334324314306298288276262

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
98% 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
6,085
Cooling degree days, base 65°F
375
Growing degree days, base 40°F
4,283
Growing degree days, base 45°F
3,109
Growing degree days, base 50°F
2,154
Growing degree days, base 55°F
1,381
Growing degree days, base 60°F
787

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,00300
February82001
March72108
April519046
May3008197
June14335343
July31155589
August36142571
September17534317
October503175
November79507
December1,03800

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 21.9 km from Whitman 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 USC00457267 (St John). Public domain.