WSET.com - ABC13Fall Frost Preview by Lyndsay Tapases

You've probably heard by now that we've got a very chilly couple of days coming up for us towards the end of this week. Overnight lows will likely be down into the upper 40s for some locations Thursday night through Sunday night (Friday through Monday mornings!) It is for sure going to be an early taste of Fall for us, which officially begins on Friday, September 23rd.

Although we certainly have a few weeks left before we have to worry about the temperatures affecting agriculture, I thought I would use this opportunity to remind everyone of the typical dates of frosts and freezes for our section of Virginia. Obviously, the temperature will impact the magnitude of the freeze. A light freeze occurs with temperatures between 29 and 32 degrees; this may hurt sensitive garden plants. A moderate freeze is caused by temperatures between 25 and 28 degrees and will be destructive to most vegetation. And a hard freeze, at 24 degrees or below, will damage or kill most plants.

The chart below shows the probability of seeing each type of freeze by the date listed. All values are for Lynchburg city. So, for example, there is only a 30% chance that we will see a frost by October 17th in Lynchburg, meaning we should have at least another month before having to worry about covering up plants or bringing them indoors at night! Additionally, there is a 90% chance that we will see a hard freeze by December 11th. Last year, the temperature did not fall below freezing until October 30th. Our first moderate freeze was November 7th, and a hard freeze occurred on November 28th.

One more side note.. sometimes, a frost may occur when the "official" temperature measurement is greater than 32 degrees. This is because official temperature measurements are usually taken at a distance of about five feet off of the ground. Just like the earth is heated from the ground up, it also loses heat from the ground up due to radiational cooling at night. This means that the actual air temperatures very close to or at the surface of the earth on a night with good radiational cooling (usually clear skies and calm winds) may be a few degrees cooler than five feet above the surface, where the observation was taken.