Bethlehem climate

Bethlehem Frost Dates and Growing Season

NOAA 1991-2020 normals for Bethlehem 2, 3.8 km from Bethlehem, elevation 1180 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Bethlehem 2. 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 24 and the first fall freeze around September 18, a growing season of about 116 days.

Median at 36°F
last June 5, first September 7
Median at 32°F
last May 24, first September 18
Median at 28°F
last May 7, first September 30

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 24Jun 15Jun 11Jun 8Jun 5Jun 3May 31May 27May 22
32°FJun 8Jun 3May 31May 28May 24May 21May 17May 12May 6
28°FMay 27May 21May 16May 11May 7May 4May 1Apr 28Apr 24
24°FMay 10May 5May 1Apr 28Apr 26Apr 23Apr 20Apr 16Apr 12
20°FApr 24Apr 21Apr 18Apr 15Apr 13Apr 10Apr 8Apr 5Apr 3
16°FApr 17Apr 12Apr 9Apr 6Apr 4Apr 3Apr 1Mar 31Mar 26

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°FAug 25Aug 31Sep 2Sep 5Sep 7Sep 10Sep 13Sep 16Sep 20
32°FSep 6Sep 10Sep 13Sep 15Sep 18Sep 21Sep 24Sep 27Oct 1
28°FSep 15Sep 19Sep 23Sep 26Sep 30Oct 1Oct 5Oct 8Oct 14
24°FSep 27Oct 1Oct 5Oct 9Oct 13Oct 17Oct 21Oct 25Nov 1
20°FOct 8Oct 15Oct 19Oct 25Oct 28Nov 1Nov 4Nov 9Nov 14
16°FOct 22Oct 30Nov 2Nov 7Nov 11Nov 14Nov 17Nov 21Nov 26

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°F112106101979389857972
32°F13713112512011611210810397
28°F163156151146142138133129122
24°F193185178174169164160154147
20°F218211206201197193188182173
16°F238231227222218213210202196

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
8,548
Cooling degree days, base 65°F
227
Growing degree days, base 40°F
3,536
Growing degree days, base 45°F
2,597
Growing degree days, base 50°F
1,780
Growing degree days, base 55°F
1,095
Growing degree days, base 60°F
569

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,53101
February1,33700
March1,17602
April740123
May3748156
June13842358
July4994510
August8064450
September26217229
October611146
November92706
December1,32200

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 3.8 km from Bethlehem. 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 USC00270706 (Bethlehem 2). Public domain.