Santa Clara County climate

Santa Clara County Frost Dates and Growing Season

NOAA 1991-2020 normals for Mt Hamilton, 13.2 km from Santa Clara County, elevation 4206 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Mt Hamilton. 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 May 3 and the first fall freeze around November 18, a growing season of about 197 days.

Median at 36°F
last May 27, first October 29
Median at 32°F
last May 3, first November 18
Median at 28°F
last April 10, first December 7

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°FJun 18Jun 9Jun 4May 31May 27May 21May 14May 7Apr 28
32°FJun 5May 26May 16May 9May 3Apr 29Apr 24Apr 17Apr 7
28°FMay 6Apr 29Apr 22Apr 15Apr 10Apr 3Mar 27Mar 14Feb 23
24°FApr 11Apr 1Mar 25Mar 13Mar 1Feb 21Feb 6Jan 18Dec 31
20°FMar 12Feb 27Feb 16Feb 4Jan 27Jan 14Jan 5Dec 31Dec 16
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 1Oct 10Oct 17Oct 22Oct 29Nov 3Nov 8Nov 15Nov 23
32°FOct 22Nov 1Nov 7Nov 13Nov 18Nov 25Dec 1Dec 4Dec 12
28°FNov 10Nov 20Nov 29Dec 3Dec 7Dec 12Dec 20Jan 1Jan 18
24°FNov 28Dec 7Dec 15Dec 26Jan 1Jan 10Jan 23Feb 8Mar 2
20°FDec 9Dec 20Jan 1Jan 4Jan 13Jan 22Jan 31Feb 14Feb 27
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°F192179170162155148141133121
32°F235220211204197188180171157
28°F294273261250241233224214202
24°F365350335319305289277266252
20°F365365365365365365351334309
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
100% of years
24°F or colder
86% of years
20°F or colder
38% of years
16°F or colder
7% 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
4,386
Cooling degree days, base 65°F
797
Growing degree days, base 40°F
5,803
Growing degree days, base 45°F
4,374
Growing degree days, base 50°F
3,173
Growing degree days, base 55°F
2,191
Growing degree days, base 60°F
1,403

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January640046
February609035
March619054
April514390
May34227214
June14297418
July22236679
August21224668
September83154526
October23951309
November4845102
December673033

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 13.2 km from Santa Clara County. 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 USC00045933 (Mt Hamilton). Public domain.