Clackamas climate

Clackamas Frost Dates and Growing Season

NOAA 1991-2020 normals for Oregon City, 6.4 km from Clackamas, elevation 167 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Oregon City. 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 18 and the first fall freeze around November 18, a growing season of about 244 days.

Median at 36°F
last April 15, first October 31
Median at 32°F
last March 18, first November 18
Median at 28°F
last February 12, first December 8

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 4Apr 30Apr 26Apr 19Apr 15Apr 11Apr 5Apr 1Mar 26
32°FApr 11Apr 1Mar 27Mar 23Mar 18Mar 12Mar 5Feb 27Feb 19
28°FMar 7Feb 28Feb 23Feb 18Feb 12Feb 4Jan 27Jan 14Jan 1
24°FFeb 21Feb 12Feb 2Jan 26Jan 16Jan 8Jan 2Dec 23Dec 12
20°FFeb 16Feb 1Jan 19Jan 9Jan 3Dec 30Dec 23Dec 15Dec 3
16°F–––––––––

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 16Oct 21Oct 25Oct 28Oct 31Nov 3Nov 7Nov 13Nov 20
32°FOct 30Nov 4Nov 9Nov 13Nov 18Nov 22Nov 28Dec 3Dec 12
28°FNov 16Nov 24Nov 30Dec 3Dec 8Dec 13Dec 18Dec 26Jan 5
24°FNov 29Dec 6Dec 13Dec 19Dec 26Jan 1Jan 6Jan 17Feb 4
20°FDec 1Dec 6Dec 13Dec 21Dec 28Jan 3Jan 11Jan 26Feb 8
16°F–––––––––

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°F226216209204198193187181173
32°F280268259252244238230223214
28°F342329317309302294287278264
24°F365365365357347338332321309
20°F365365365365365365362345332
16°F365365365365365365365365365

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
97% of years
24°F or colder
69% of years
20°F or colder
33% of years
16°F or colder
8% 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
4,015
Cooling degree days, base 65°F
543
Growing degree days, base 40°F
5,772
Growing degree days, base 45°F
4,202
Growing degree days, base 50°F
2,908
Growing degree days, base 55°F
1,893
Growing degree days, base 60°F
1,110

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January69807
February560014
March499050
April3524127
May19125302
June7663437
July12181634
August9189645
September6577462
October2945193
November542032
December71905

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 6.4 km from Clackamas. 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 USC00356334 (Oregon City). Public domain.