Is that a chill in the air!? Residents in Blanco County may have gotten their first hints of Jack Frost nipping at their noses during the last two weeks. The WeatherBug station at Blanco Middle School got down to 36 degrees overnight Thursday, October 28. A mere 24 hours later, the thermometer was flirting with freezing temperatures as the mercury hovered at 33 degrees. Marilyn Mikes, the NWS official COOP observer for the city of Blanco, said “We did have our first below freezing temperature on Saturday, October 30th with a cool 31 degrees.” Mikes’ weather station is located within city limits. Temperatures can and do vary that much even within a close area like that because of instrumentation differences and terrain, etc. The overnight temperature in Blanco dropped below freezing again on Friday night, November 5, into Saturday.
Following unseasonal warm afternoon highs earlier in the week, the afternoon high temperatures on Thursday and Friday, October 28-29, were near normal for late October as a cold front passed through the area. Clear skies, light winds, and very dry air combined to bring the area the coldest overnight low temperatures of the season so far. Friday was sunny and mild with afternoon highs in the 70s, followed by another cold night as temperatures fell to the low 30s before dawn.
Three meteorological elements that contribute to low overnight temperatures are winds, cloud cover, and humidity. If the sky is filled with clouds, those clouds trap the heat of the day close to the ground, making it a bit warmer. When the sky is clear, there are no clouds to trap the heat, so all of the warmth gained during the day dissapates overnight.
Dry air is colder than humid air. Water vapor holds on to heat, so the more humidity in the overnight air, the warmer the temperatures. Water vapor acts as a greenhouse gas, so as the humidity increases, the amount of longwave radiation held in the lower atmosphere also increases. Conversely, when the air is dry, that same longwave radiation emitted from Earth escapes into space. Also, drier nights tend to be associated with fewer clouds, further lowering overnight temperatures.
Finally, winds affect temperature, as the higher the wind, the greater the wind chill effect. The light winds during the nights at the end of last month effectively lowered the temperatures just a little bit.
For Blanco, the average first freeze (1950-2009) is November 10. For Johnson City, the average first freeze is November 13, and the earliest freeze on record was October 19 in 1989. The earliest freeze on record occured on October 8 in 1952. So, according to averages, we were a week or two early!
The forecast for the rest of the week is:
Wednesday: Cloudy, then gradually becoming mostly sunny, with a high near 78. South wind between 5 and 10 mph.
Wednesday Night: Mostly cloudy, with a low around 59. South southeast wind between 5 and 10 mph.
Veterans Day: A 10 percent chance of showers. Mostly cloudy, with a high near 80. South southeast wind between 10 and 15 mph, with gusts as high as 20 mph.
Thursday Night: A 20 percent chance of showers. Cloudy, with a low around 64. South southeast wind between 5 and 10 mph.
Friday: A 30 percent chance of showers and thunderstorms. Mostly cloudy, with a high near 73. South southeast wind 5 to 10 mph becoming north northwest between 15 and 20 mph. Winds could gust as high as 25 mph.
Friday Night: A 20 percent chance of showers and thunderstorms. Partly cloudy, with a low around 45. North northwest wind between 10 and 20 mph, with gusts as high as 25 mph.
Saturday: Sunny, with a high near 67.
Saturday Night: Partly cloudy, with a low around 34.
Sunday: Mostly sunny, with a high near 67.
Sunday Night: Mostly cloudy, with a low around 42.