Riley climate

Riley Frost Dates and Growing Season

NOAA 1991-2020 normals for Manhattan, 24.3 km from Riley, elevation 1060 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Manhattan. 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 16 and the first fall freeze around October 19, a growing season of about 185 days.

Median at 36°F
last April 27, first October 9
Median at 32°F
last April 16, first October 19
Median at 28°F
last April 6, first October 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°FMay 11May 5May 2Apr 30Apr 27Apr 23Apr 21Apr 17Apr 13
32°FMay 1Apr 26Apr 22Apr 19Apr 16Apr 13Apr 10Apr 7Apr 1
28°FApr 24Apr 18Apr 14Apr 10Apr 6Apr 3Mar 31Mar 29Mar 22
24°FApr 15Apr 8Apr 4Apr 1Mar 29Mar 25Mar 21Mar 17Mar 11
20°FApr 4Mar 30Mar 26Mar 22Mar 18Mar 14Mar 11Mar 7Mar 2
16°FMar 26Mar 19Mar 15Mar 12Mar 8Mar 5Mar 2Feb 27Feb 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°FSep 25Sep 29Oct 2Oct 5Oct 9Oct 12Oct 15Oct 18Oct 22
32°FOct 1Oct 6Oct 12Oct 16Oct 19Oct 21Oct 24Oct 28Nov 1
28°FOct 11Oct 18Oct 22Oct 25Oct 29Nov 1Nov 3Nov 6Nov 12
24°FOct 21Oct 28Nov 1Nov 4Nov 6Nov 9Nov 13Nov 18Nov 25
20°FOct 30Nov 4Nov 8Nov 12Nov 17Nov 21Nov 25Nov 30Dec 5
16°FNov 5Nov 13Nov 19Nov 23Nov 28Dec 1Dec 5Dec 10Dec 16

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°F183178173169165162157152145
32°F205198192189185180175170163
28°F226218213208204199195189181
24°F247239233228224219214209200
20°F268260253247242238232226216
16°F289279272267262256251245234

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
5,112
Cooling degree days, base 65°F
1,520
Growing degree days, base 40°F
6,590
Growing degree days, base 45°F
5,313
Growing degree days, base 50°F
4,172
Growing degree days, base 55°F
3,158
Growing degree days, base 60°F
2,275

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,11101
February87909
March658467
April34120199
May111111469
June8309751
July1467931
August2395859
September54180577
October29332254
November656249
December1,00004

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 24.3 km from Riley. 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 USC00144972 (Manhattan). Public domain.