by Tom Ross
I remember looking out the window on the first day of 2018 and being greeted by an inch of snow on top of a glaze of freezing drizzle that played havoc and cancelled many New Year’s eve activities. Actually, only a tiny bit of precipitation fell across the area – only a hundredth or two of an inch – but the form in which it fell is one of the most dangerous. What actually happened on New Year’s Eve was that we had a moist, low-level flow from the east, which causes the air to flow uphill across the Carolinas and get lifted and moistened. This results in widespread low cloudiness and precipitation. In this case, surface temperatures were well below freezing and we had four to eight hours of light, freezing drizzle and freezing mist. The upslope flow then turned more to a west-northwesterly flow and we went into a more typical light snow event that brought a dusting across the region. In retrospect, these local or “mesoscale” events, especially the ones that bring freezing drizzle or freezing rain, can be the most dangerous. In the New Year’s Eve event we were also “bone cold” with temperatures in the mid 20s, which made for excellent conditions for icy roads. In this case, it only took a trace of precipitation to cause many car accidents across the region.
I think we will see a fair amount of changeable weather for February 2018. We are starting to see more daylight, as there is an increase in the amount of solar radiation received each day in February. The sunrise and sunset times for February 1 are 7:30 am and 5:57 pm, but by month’s end they will be 7:01 am and 6:24 pm. That’s almost an hour of additional sunshine. However, the average high and low temperatures for the beginning of the month are 49 and 28, respectively, and they will rise about four to five degrees by month’s end. We also have to deal with the “seasonal lag of temperature,” which will be discussed more later in 2018. Basically, though, bodies of water and the ground have a certain heat capacity, which is the amount of energy it takes to change the temperature of an object by a given amount. In fact, water takes much longer to heat up than land. Since our earth is around 71 percent water, our temperatures on land are highly dependent on the temperatures of the bodies of water near it. Water also tends to absorb and release heat at a slower rate than the land does, and causes the seasonal lag we see in our temperature record. This is the main reason why the coldest and warmest days do not occur on the winter and summer equinoxes in mid-December and mid-June but lag or occur later in the season.
In July, terrestrial radiation (outgoing radiation from Earth) is still reaching its peak based on the large heat capacity of the oceans and, to a lesser extent, land masses. This added terrestrial radiation allows air temperature to continue to rise even though energy from the sun is decreasing.
Finishing up on a lighter note, when I worked at the National Climatic Data Center, now the National Center for Environment Information (NCEI), one of the most popular webpages I did was checking out our furry friend, the groundhog, and its relatively poor weather-predictive skills. You can read all about Groundhog Day at ncei.noaa.gov/news/groundhog-day-forecasts-and-climate-history.
Meteorologist Tom Ross managed the Climate Database Modernization Program at the National Climatic Data Center.