Ottawa climate

Ottawa Frost Dates and Growing Season

NOAA 1991-2020 normals for Ottawa 5SW, 6 km from Ottawa, elevation 525 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Ottawa 5SW. 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 20 and the first fall freeze around October 22, a growing season of about 183 days.

Median at 36°F
last May 1, first October 11
Median at 32°F
last April 20, first October 22
Median at 28°F
last April 8, first November 1

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 17May 12May 7May 4May 1Apr 29Apr 25Apr 22Apr 18
32°FMay 5Apr 30Apr 26Apr 23Apr 20Apr 18Apr 14Apr 11Apr 6
28°FApr 24Apr 19Apr 15Apr 11Apr 8Apr 5Apr 3Mar 31Mar 24
24°FApr 14Apr 7Apr 4Apr 1Mar 31Mar 27Mar 22Mar 18Mar 11
20°FApr 4Mar 31Mar 26Mar 22Mar 19Mar 15Mar 12Mar 8Mar 3
16°FMar 28Mar 21Mar 16Mar 13Mar 10Mar 7Mar 4Mar 1Feb 23

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 27Oct 2Oct 5Oct 9Oct 11Oct 14Oct 17Oct 20Oct 24
32°FOct 7Oct 13Oct 17Oct 20Oct 22Oct 24Oct 27Oct 30Nov 4
28°FOct 19Oct 24Oct 27Oct 30Nov 1Nov 3Nov 6Nov 10Nov 15
24°FOct 31Nov 2Nov 5Nov 8Nov 12Nov 15Nov 18Nov 22Nov 27
20°FNov 6Nov 11Nov 15Nov 19Nov 22Nov 25Nov 28Dec 1Dec 6
16°FNov 14Nov 20Nov 24Nov 27Nov 30Dec 2Dec 5Dec 10Dec 17

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°F181175170166162158154147140
32°F203196191187183179175170162
28°F227220214210206201198193184
24°F251243237232227223219214207
20°F271263257252248243238233225
16°F288279274269265260255249241

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
6,211
Cooling degree days, base 65°F
865
Growing degree days, base 40°F
5,382
Growing degree days, base 45°F
4,231
Growing degree days, base 50°F
3,202
Growing degree days, base 55°F
2,290
Growing degree days, base 60°F
1,508

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,30301
February1,06402
March829229
April4498122
May17763361
June28184606
July4272733
August9224680
September9099462
October37614175
November750030
December1,13102

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 km from Ottawa. 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 USC00116526 (Ottawa 5SW). Public domain.