Ask Brandon: Why is it sometimes colder even if the sun is out?

Brandon,

Our class has been recording the weather every day and noticed when it is sunny it is cold. When it is cloudy it is warmer. We know the sun is supposed to warm us up. Why is it colder if the sun is out?

-Mrs. Johnson’s Kindergarten Class
Lakewood Elementary School
Duluth, MN

Hi Mrs. Johnson’s Kindergarten Class!

Great Question! While generally it is warmer out when the sun is out, sometimes as you have noticed, it doesn’t always work that way! Most of the times it is because of snowfall and it’s albedo. In weather terms, albedo is a measure of how much light that hits a surface is reflected without being absorbed. Something that appears white reflects most of the light that hits it and has a high albedo, while something that looks dark absorbs most of the light that hits it, indicating a low albedo.

As you can see in the table above, fresh snowfall has the highest albedo out of any surface. Fresh snow generally has an albedo of 0.8 to 0.95 (about 80 to 95% reflective), which is why it always seems so bright out after a new snowfall! Even old snowfall has a relatively high albedo (near 0.4 to 0.5, which translate to around 40 to 50% reflective) compared to other surfaces like grass or soil! Because of this, the sunlight reflects from the surface and back into the upper atmosphere and doesn’t allow for any warming at the surface.

As for it being warmer on a cloudy day, that could be two different reasons. The first is that there is a new air mass moving in, likely from the south or west, and that is pulling in some warm air naturally with winds likely out of the south. The other reason is a little easier to understand. The clouds act like a lid on our atmosphere, just like a lid on your drink or food container. The clouds trap in the warmer air that was built up at the surface and keep things more mild. Generally, it is warmer at night when there is cloud cover because the clouds trap in the warmth that was built up by the sunshine during the day, meanwhile it is colder at night when it is clear out because the warm air from the day mixes into the upper atmosphere overnight and has nothing to block in at the surface. I hope that helps!

Do you have a question about the weather? Make sure to send it to Brandon at weather@kdlt.com or on Facebook or Twitter!