Whenever I see clouds moving into the Tennessee Valley, I instantly think about how those clouds will impact the forecast, especially when it comes to temperatures.
It’s well known — and makes sense — that temperatures are cooler when the sun is obscured by the clouds.
But did you know that those same clouds make the nighttime temperature warmer than if they weren’t there?
Daytime Clouds Yield Cooler Temperatures
Consider the image at left. When medium to thick cloud cover is present, the afternoon daytime temperature is not as high as compared to a clear sky or a sky marked with high, thin clouds. In fact, much of the sun’s light is reflected off the clouds and back into space, which means that the Earth is not able to absorb the radiant heat from that reflected sunlight. The effect is similar to wearing a white t-shirt on a sunny day. The result is that the temperature on the surface of the Earth is cooler compared to a sunny day.
Nighttime Clouds Create Warmer Temperatures
Recall that incoming sunlight filters through clear sky or thin, high clouds and heats the surface of the earth. Fair sky afternoons can see temperatures warm as much as 30 degrees compared to their morning lows. However, at night, the heat absorbed by the Earth radiates out towards space. Without a thick layer of clouds to slow the progression of heat escaping the lower atmosphere, the overnight temperatures plummet as much as 30 degrees compared to the daytime highs.
However, when clouds are present at night, the cooling trend slows. The clouds act like a blanket that traps the outgoing heat, absorbs it, and re-radiates it back down to the Earth. In addition, the clouds reflect some of the heat back to the ground as well. So on a cloudy evening, temperatures can diminish as little as 10 degrees, effectively keeping nighttime lows higher than they would be on a clear night.
That’s a little peak behind the curtain when it comes to forecasting. Do you have a question about the science of weather? Feel free to contact me!
– Christina Edwards