We are just a couple of days from the start of October, and nights are starting to get cooler. As nights get cooler, frost will soon become quite common here in NE Ohio and NW Pennsylvania. The National Weather Service Office in Pittsburgh has started issuing frost advisories for locations in NW Pennsylvania. What is frost? What conditions are needed for it to form? Lastly, why can there be frost when air temperatures are above freezing?
What is frost and how does it form?
The National Weather Service describes frost as the formation of thin ice crystals on the ground and other surfaces that form when water vapor freezes.
Frost usually forms overnight and occurs on nights when skies are clear, winds are calm and either air temperatures or surface temperatures are below freezing. Why are these conditions needed for frost to form?
First, let’s talk about temperature. For frost formation, the temperature at either the surface or above the surface needs to be 32˚ or below. That is because water turns to ice at 32˚. In addition to freezing temperatures, moisture is also needed at the surface to promote ice crystal development.
Another condition is calm winds. We need calm winds under 5 mph. Calm to light winds prevent stirring of the atmosphere, which allows a thin layer of supercooled temperatures to develop at the surface. Stronger winds will prevent frost from forming.
We also need clear skies. Clear skies lead to radiational cooling, allowing the greatest amount of heat to exit the atmosphere. On clear nights, temperatures are typically colder, especially at the surface.
Why can frost form when temperatures are above 32°?
Frost can form when the air temperature is above 32˚. Now, why is that? Well, the temperature is typically measured about 5-6 feet above the surface. Since cold air sinks, surface temperatures can be 5 to 10 degrees cooler than the air just above the surface. For example, if the air temperature is at 36˚, the surface temperature will likely be below 32˚. That is why frost can occur when the air temperature is in the mid to upper 30s.
When is typically the first frost in NE Ohio and NW Pennsylvania?
On average, the first frost in the Youngstown area is October 1, and the first freeze usually occurs by mid-October. The average first freeze in the Youngstown area is October 14.