(WGHP) — Just a month ago we were seeing days in the 80s and even near 90 degrees. Now we have already experienced nights in the 30s.
The period from mid-September to mid-October experiences wild temperature swings. The cold nights come on so fast that many think they are coming early.
I get that question every year: “Is this cold so early normal?” If you look at our history, we often get a frost in the Piedmont by mid- to late October. Our mountain counties often get their first frost the first week of October and the foothills by Oct. 12.
Frost is a little more difficult to track because it does not need a specific temperature. Since air temperatures are taken 5 feet above the ground and cold air is heavy, the air at ground level at night will usually be colder than where the temperature sensor is located.
Depending on wind speed and sky conditions, frost can form when official readings are in the mid- to upper 30s. The wind has to be near calm, and the skies need to be mostly clear. If it is cloudy or if there is a good breeze, the temperature at the ground is usually closer to air temperature due to the air being better mixed. Given those variables, 36° air temperature is accepted by most as the cutoff for frost. The air on surfaces (grass, car tops, rooftops, etc.) may be 4 or 5 degrees colder than 5 feet above, leading to frost if there is enough moisture in the air. If the air is very dry, no frost will form.
When it comes to a freeze, that by definition is easier to track. When the air temperature reading is 32 or colder, that is a freeze. When we have a low of 31 or 32, we have a freeze. The surface may actually dip all the way to 25 or 26 on those nights.
So with a freeze, many tender plants will die. The growing season will end. Here in the Piedmont Triad, our average first freeze happens near Halloween. In the cities, it is often into the first week of November and rural areas late October. Keep in mind that cities hold more heat.
Speaking of cities and impact. Below is a comparison of the average first freeze dates over the past 100 years. We will use the years that observations were taken at PTI which started in 1928.
1928-1960: Oct. 26
1961-1990: Oct. 26
1991-2023: Nov. 5
In the time period between 1991 and 2023, there have been more runways added at the airport, more highways nearby and the Triad cities have grown and expanded. Cities can be as much as 10° warmer on clear nights than rural areas. So, as the cities grow and expand, that warmth is measured at the airport sensor too. The airport used to be very rural and smaller. Now, the cities are growing closer to the airport, and the airport is also growing to a point that is acting much like a city. Not just here, but throughout the country, as cities grow and expand toward airports, night-time lows are warmed by that growth.
