Royal City climate

Royal City Frost Dates and Growing Season

NOAA 1991-2020 normals for Smyrna, 7.6 km from Royal City, elevation 560 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Smyrna. 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 23 and the first fall freeze around October 12, a growing season of about 172 days.

Median at 36°F
last May 6, first October 2
Median at 32°F
last April 23, first October 12
Median at 28°F
last April 3, first October 24

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 18May 14May 11May 8May 6May 3Apr 30Apr 28Apr 23
32°FMay 7May 2Apr 29Apr 26Apr 23Apr 20Apr 16Apr 11Apr 3
28°FApr 24Apr 17Apr 11Apr 7Apr 3Mar 31Mar 27Mar 21Mar 13
24°FApr 5Mar 31Mar 26Mar 20Mar 15Mar 11Mar 7Mar 3Feb 24
20°FMar 18Mar 10Mar 5Mar 1Feb 27Feb 23Feb 18Feb 11Jan 31
16°FMar 5Feb 28Feb 22Feb 16Feb 8Feb 2Jan 26Jan 16Dec 31

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 21Sep 25Sep 28Oct 1Oct 2Oct 4Oct 6Oct 10Oct 14
32°FOct 1Oct 3Oct 5Oct 8Oct 12Oct 15Oct 18Oct 21Oct 25
28°FOct 6Oct 14Oct 17Oct 20Oct 24Oct 26Oct 29Nov 1Nov 6
24°FOct 20Oct 25Oct 28Oct 31Nov 2Nov 5Nov 11Nov 17Nov 25
20°FOct 29Nov 3Nov 7Nov 12Nov 17Nov 22Nov 27Dec 3Dec 11
16°FNov 4Nov 12Nov 18Nov 25Nov 29Dec 4Dec 9Dec 18Dec 30

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°F167160155152148145142138131
32°F192185179175172167163158152
28°F227217212207202196192185176
24°F261250243236231226220214205
20°F301287277271266259253245237
16°F336324314305298290283274262

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
98% 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
5,115
Cooling degree days, base 65°F
923
Growing degree days, base 40°F
5,656
Growing degree days, base 45°F
4,369
Growing degree days, base 50°F
3,262
Growing degree days, base 55°F
2,324
Growing degree days, base 60°F
1,547

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January99301
February76202
March605027
April3702116
May14541363
June23147575
July1343807
August5302763
September7584459
October3893136
November745010
December1,00302

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 7.6 km from Royal City. 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 USC00457727 (Smyrna). Public domain.