Ontario climate

Ontario Frost Dates and Growing Season

NOAA 1991-2020 normals for Ontario Ksrv, 2 km from Ontario, elevation 2145 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Ontario Ksrv. 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 22 and the first fall freeze around October 9, a growing season of about 168 days.

Median at 36°F
last May 5, first September 30
Median at 32°F
last April 22, first October 9
Median at 28°F
last April 6, first October 21

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 21May 15May 11May 8May 5May 2Apr 29Apr 26Apr 21
32°FMay 7May 2Apr 29Apr 26Apr 22Apr 20Apr 17Apr 13Apr 7
28°FApr 23Apr 19Apr 14Apr 9Apr 6Apr 2Mar 30Mar 25Mar 18
24°FApr 6Apr 1Mar 27Mar 20Mar 16Mar 11Mar 7Mar 3Feb 27
20°FMar 16Mar 7Mar 3Mar 1Feb 26Feb 22Feb 18Feb 13Feb 6
16°FFeb 28Feb 24Feb 19Feb 15Feb 10Feb 6Feb 1Jan 25Jan 12

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 16Sep 20Sep 24Sep 27Sep 30Oct 2Oct 5Oct 8Oct 12
32°FSep 26Oct 1Oct 4Oct 7Oct 9Oct 12Oct 15Oct 18Oct 23
28°FOct 6Oct 11Oct 15Oct 18Oct 21Oct 24Oct 27Oct 31Nov 4
24°FOct 19Oct 23Oct 27Oct 30Nov 1Nov 5Nov 9Nov 14Nov 20
20°FOct 27Nov 1Nov 6Nov 11Nov 14Nov 19Nov 22Nov 28Dec 5
16°FNov 4Nov 12Nov 18Nov 22Nov 26Nov 30Dec 5Dec 11Dec 21

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°F163157153149145141137132122
32°F187181176172168164160155149
28°F219210204200196192187181175
24°F255246241236230224218211201
20°F291281274267262257250244234
16°F325312304297290283276269260

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
5,788
Cooling degree days, base 65°F
921
Growing degree days, base 40°F
5,280
Growing degree days, base 45°F
4,091
Growing degree days, base 50°F
3,072
Growing degree days, base 55°F
2,215
Growing degree days, base 60°F
1,499

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,10800
February84700
March656015
April436187
May19040323
June48144546
July4368829
August11296750
September9870425
October457293
November82603
December1,10700

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 2 km from Ontario. 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 USC00356294 (Ontario Ksrv). Public domain.