A continental high-pressure system (albeit modified as it is spreading across Intermountain West and Plains) will move through the Ohio Valley tonight. This would create an ideal atmospheric situation known as radiative cooling; a phenomenon that occurs frequently and the temperature drops markedly during the colder months (Autumn – Spring). Since we’re in the spring and the temperatures have been unusual especially over the past few weeks (and overall warm winters to boot), this puts early vegetation at risk of frost and freezing. at the end of the season.
So how do we determine this using sound and what atmospheric conditions are fundamental for this to manifest?
First, we know that the Earth is continuously radiating geothermal heat from its core – that is a perpetual constant. However, we also know that we have an energy balance with the sun’s luminosity, so during the day the Earth also receives energy in the form of absorbed heat. At night, this energy can be lost to the atmosphere and space. This is where radiative cooling comes into play with the right atmospheric conditions, which is exacerbated. So what to look for are a few key things:
- cloudless sky
- High altitude subsidence through High pressure
- The wind is weak in the boundary layer
Clouds, like a blanket when you go to bed at night, trap heat in the troposphere thereby keeping the previous day’s heat from escaping.
Subsidence or subduction of air through high pressure (mass is converging overhead and towards the surface) yields relative humidity to very low values, thus also hindering any condensation from occurring. We know we need condensation to form clouds.
Finally, the wind needs to remain weak within 1-2 km below (basically the lowest ~3500 ft) so that no mix. The winds that are combined from the surface to the lower troposphere essentially help transport stronger winds that may be higher up to the surface. This causes heat to remain at the surface allowing the temperature to remain high. The exact opposite happens when the boundary layer separates from the troposphere, where mixing is not possible at present and heat loss is maximum. It even increases when we have snow cover, as it has an extremely high albedo (i.e. reflectance of solar radiation), allowing temperatures to drop even more sharply than without snow cover. The graphic below illustrates this very nicely!
Finally, we can use sonics (longitudinal thermodynamic profiles of the atmosphere) to identify key features of radiative cooling. We see more dry air in the troposphere, weak winds in the boundary layer, and a radiative cooling inversion due to subducting parcels of adiabatic heating combined with a sharp drop in surface temperatures.
This is especially noticeable during the winter months when we have snow cover, clear skies and weak winds – it’s certainly a way to bring temperatures down to the lowest, single digits and depending on the location. mind, even below 0 degrees. So now you get the hang of it whenever that term is used!
About the author
Hello! My name is Armando Salvadore and I am a Mississippi State graduate with a Bachelor of Professional Meteorology and an Activity Meteorologist working in the Private Sector. Stay tuned if you like technical, exotic, and general weather tweets! Also big on long-range forecasting as well! Twitter: @KaptMands