Hillsboro climate

Hillsboro Frost Dates and Growing Season

NOAA 1991-2020 normals for Marion Rsvr, 11.6 km from Hillsboro, elevation 1369 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Marion Rsvr. 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 8 and the first fall freeze around October 30, a growing season of about 202 days.

Median at 36°F
last April 19, first October 21
Median at 32°F
last April 8, first October 30
Median at 28°F
last March 30, first November 6

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 5Apr 30Apr 26Apr 23Apr 19Apr 16Apr 13Apr 9Apr 4
32°FApr 25Apr 20Apr 16Apr 12Apr 8Apr 5Apr 2Mar 30Mar 23
28°FApr 17Apr 9Apr 5Apr 2Mar 30Mar 26Mar 22Mar 17Mar 11
24°FApr 5Mar 31Mar 26Mar 22Mar 18Mar 14Mar 11Mar 6Mar 1
20°FMar 27Mar 19Mar 14Mar 11Mar 8Mar 5Mar 1Feb 26Feb 19
16°FMar 14Mar 10Mar 6Mar 2Feb 28Feb 24Feb 20Feb 14Feb 6

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 7Oct 13Oct 16Oct 19Oct 21Oct 23Oct 25Oct 29Nov 2
32°FOct 17Oct 21Oct 24Oct 27Oct 30Nov 1Nov 3Nov 7Nov 12
28°FOct 24Oct 29Nov 1Nov 4Nov 6Nov 10Nov 13Nov 16Nov 22
24°FOct 31Nov 5Nov 9Nov 12Nov 16Nov 20Nov 24Nov 28Dec 3
20°FNov 8Nov 15Nov 20Nov 24Nov 27Nov 30Dec 4Dec 7Dec 15
16°FNov 20Nov 25Nov 30Dec 3Dec 7Dec 10Dec 15Dec 20Dec 28

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°F202195190186183179174170163
32°F224216211207202199194189183
28°F244236231226222217213208200
24°F266258252246242237232226218
20°F288279272266262257251245237
16°F310299294288282277271266256

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
4,725
Cooling degree days, base 65°F
1,707
Growing degree days, base 40°F
6,993
Growing degree days, base 45°F
5,664
Growing degree days, base 50°F
4,479
Growing degree days, base 55°F
3,427
Growing degree days, base 60°F
2,504

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,04902
February834011
March600481
April30123225
May92123499
June6343787
July0501966
August1449913
September39220632
October24442298
November605261
December95504

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 11.6 km from Hillsboro. 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 USC00145039 (Marion Rsvr). Public domain.