First Frost

According to the National Weather Service, the average first frost in western Pennsylvania happens on October 15. This year, the records indicate it occurred on October 18.

Some say that they can smell frost in the air and they probably can, in a way. The sensation of cooling air, becoming denser with moisture is certainly discernable, but what you really need to know is how cold is it at ground level - everywhere.

Frost forms as air, saturated with water near the freezing point, comes in contact with a surface at or below freezing. Above freezing, dew forms; below, water vapor changes directly from gas to solid producing the ice crystals that mark the ending of our growing season. Cold, clear nights create the perfect conditions for frost by allowing air to stratify, the coldest air dropping down to ground level and staying there where it can cool below freezing. Each point on the ground can experience a unique temperature depending upon its elevation, the amount of heat it can retain and how it affected by air currents. So, frost does not form uniformly.

Some of us fight to keep that first frost off of the plants we hope to keep going another few weeks. But it's actually not the frost itself that kills plants; it's the temperature the plant experiences as the "host" for frost formation. In other words, a leaf or flower must be at the freezing point for frost to form. What kills the plant is the disruption of cells as water crystals form, expand and eventually rupture cells. This happens on the inside, not on the outside where we see frost.

This picture is of a staghorn sumac leaf whitened with ice crystals early was taken by WPC's County Natural Heritage Coordinator Jeff Wagner on the morning of the region's first heavy frost.

Special thanks to Meteorologist Anthony Sadar for securing frost date information.

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