By Tom Ross
Last month, we discussed how September 10th was traditionally the peak of hurricane season — wow! That certainly was the case this year with Harvey and Irma and Maria and José. Cleanup and rebuilding will continue well into 2018. If you take another quick peak at last month’s graphic, you can see how the frequency of hurricanes and tropical storms now drops through October, which is good news. If we contrast the last two summers, this year was more of a “cool green Asheville” summer, with the exception of two or three hot weeks in July. In contrast, last summer we had 18 days with a high of 90 or better; this year it was only five days, all in July.
The amount of solar radiation received drives our climate. The length of daylight in October drops from about 11 hours 48 minutes to 10 hours 44 minutes, with corresponding highs and lows of 72 and 50 on the Oct. 1 and 64 and 40 on Oct. 31.
We should expect some frost by month’s end. Frost forms on solid objects when the water vapor in the atmosphere changes from its vapor phase to small ice crystals. If you see frost, the temperature of the object reached 32 or lower. However, the air temperature, measured at 5 feet above ground in the vicinity of this object, is likely several degrees higher. Conversely, not every air temperature recorded at or below 32 means frost formed on solid objects in the area. The average date of the last spring frost is April 10 for downtown Asheville, April 27 for Black Mountain, May 5 for Bent Creek and April 26 for Hendersonville.
The average date of the first autumn frost is October 23 for Asheville, October 17 for Black Mountain, October 9 for Bent Creek, and October 12 for Hendersonville. To get the earliest date of the last frost, add 14 days to the average or subtract for the latest frost. In autumn, add or subtract about 12 days to get the earliest or latest dates. In Fairview, our dates run pretty close to Bent Creek; deeper valleys will tend to have frost/freezing conditions later in the spring and earlier in the fall.
On a final note, we’ll enjoy fall colors in the mountains over the next 4-6 weeks. In some years, our mountains look like a patchwork quilt of glorious color and other years not so much. Take a look at the map at smokymountains.com/fall-foliage-map to see estimated dates of peak fall colors across WNC.
October Trivia Question
When were the wettest, driest and snowiest Novembers?
September Trivia Answer
What is the mechanism of why low clouds so often form in the valleys between our mountain ridges?
Valley fog is most common when it is clear and calm during late and early morning hours. Rivers and streams that flow in the base of valleys can also enhance the fog potential because of the relatively warm water. If the air temperature around a river, stream or in a sheltered valley cools down to the dewpoint, an instant source of condensation and water vapor becomes available for the production of fog. Visibility as a result of valley fog can drop from unrestricted across higher terrain down to near zero in just a few hundred feet! Valley fog can also make for some picturesque scenery during the morning hours as the sun is rising.
Meteorologist Tom Ross managed the Climate Database Modernization Program at the National Climatic Data Center.