Ketchum Frost Dates and Growing Season

NOAA 1991-2020 normals for Ketchum RS, 0.5 km from Ketchum, elevation 5890 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Ketchum RS. A median date has an even chance of a later freeze in spring, or an earlier one in fall.

NOAA does not state freeze dates for this station.

Median at 28°F: last May 28, first September 19

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.

Threshold	10%	20%	30%	40%	50%	60%	70%	80%	90%
36°F	–	–	–	–	–	–	–	–	–
32°F	–	–	–	–	–	–	–	–	–
28°F	Jun 29	Jun 16	Jun 8	Jun 2	May 28	May 22	May 18	May 12	May 5
24°F	May 29	May 17	May 11	May 7	May 3	Apr 30	Apr 27	Apr 21	Apr 17
20°F	May 10	Apr 30	Apr 25	Apr 21	Apr 17	Apr 14	Apr 11	Apr 8	Apr 3
16°F	Apr 23	Apr 14	Apr 10	Apr 7	Apr 4	Apr 2	Mar 30	Mar 24	Mar 17

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.

Threshold	10%	20%	30%	40%	50%	60%	70%	80%	90%
36°F	–	–	–	–	–	–	–	–	–
32°F	–	–	–	–	–	–	–	–	–
28°F	Sep 3	Sep 7	Sep 11	Sep 15	Sep 19	Sep 22	Sep 25	Sep 30	Oct 5
24°F	Sep 12	Sep 20	Sep 25	Sep 29	Oct 2	Oct 6	Oct 11	Oct 15	Oct 20
20°F	Sep 24	Oct 3	Oct 8	Oct 12	Oct 17	Oct 21	Oct 24	Oct 28	Nov 2
16°F	Oct 10	Oct 18	Oct 23	Oct 26	Oct 29	Nov 1	Nov 4	Nov 8	Nov 12

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.

Threshold	10%	20%	30%	40%	50%	60%	70%	80%	90%
36°F	70	62	56	51	46	41	36	32	26
32°F	105	96	89	83	77	72	65	56	46
28°F	140	131	124	118	112	107	101	91	78
24°F	174	166	159	154	148	143	136	130	118
20°F	203	196	190	186	180	175	168	159	148
16°F	230	222	217	212	207	203	198	193	180

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: 9,068
Cooling degree days, base 65°F: 63
Growing degree days, base 40°F: 2,660
Growing degree days, base 45°F: 1,811
Growing degree days, base 50°F: 1,127
Growing degree days, base 55°F: 601
Growing degree days, base 60°F: 242

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Cooling DD	Growing DD 50
January	1,406	0	0
February	1,175	0	0
March	1,065	0	0
April	768	0	5
May	523	1	63
June	311	5	176
July	114	38	392
August	140	19	348
September	374	1	129
October	719	0	15
November	1,071	0	0
December	1,403	0	0

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 0.5 km from Ketchum. Low ground, valleys and open fields freeze earlier and later than a town center or a hillside.

Sources

NOAA NCEI U.S. Climate Normals, 1991-2020, annual and seasonal normals for station USC00104845 (Ketchum RS). Public domain.