YOUNGSTOWN, Ohio (WKBN) – Another late spring round of cool air is on the way Wednesday night into the day on Thursday.

Since the growing season has started, the National Weather Service offices in Cleveland and Pittsburgh have issued numerous frost and freeze warnings across the Valley during the month of May.

Typically, these cold weather alerts will include information about the development of frost, especially in low-lying areas like a valley.

What is it about valleys that attract cold temperatures? The answer to that question is directly related to the density of air.

You probably learned about density in your high school science classes. Density, technically speaking, is defined as the object’s mass divided by unit volume. Now, that definition probably does not mean much to you.

Here is an alternative explanation: Water has a density of 1 gram/mL, and a chunk of lead has a density of 11 grams/mL. If you throw the chunk of lead into water, then it will sink because it is more dense than the water.

On the other hand, ice has a density of 0.92 grams/mL. Therefore, if you throw ice into water, then it will float because it is less dense than water.

The atmosphere is full of air with different densities. These differences in density often drive the weather patterns that affect our area. Warm air is less dense than cold air, thus warm air rises and cold air sinks.

During the nighttime hours, the Earth releases stored heat to the atmosphere, which allows the surface of the Earth to cool. In a valley, the cool air will slide down the sides of the valley and collect at the valley floor. This is a process known as “cold pooling.”

If the weather is calm and clear, then this can lead to temperatures that are significantly cooler at the valley floor relative to the surrounding areas of elevation.

Graphic showing how cold air moves into valleys during the nighttime.

One extreme example of this is in Peter Sinks, Utah, which is known for extremely cold temperatures. Peter Sinks is a natural limestone sinkhole that is 8,164 feet in elevation with no outlets at the bottom of the hole.

This past year, Peter Sinks recorded a low temperature of -62°F at the bottom of the sinkhole on January 30. The temperature at the rim of the sinkhole, which is only 400-500 feet higher in elevation, only recorded a temperature of -6°F the same night.

The lowest temperature in the state of Ohio of -39°F was recorded in Milligan, Ohio on February 10, 1899, when a massive blizzard hit the state. Milligan, Ohio is located in a valley alongside the Moxahala Creek, where cold air filters in from the surrounding hills.

Therefore, if you live in a valley, make sure you cover up those plants when the cold air arrives this week!