Is it time for a frost or freeze?

Feel the chill yet? That chill is the first push of winter air over southeast Ohio this fall, and a foreword of the winter season to come. A strong cold front, bringing in a much colder air mass from Canada, will give us our first taste of winter.

Temperatures for the last three days have been in the upper 70s and even lower 80s, roughly 10-15 degrees higher than average. This summer-like weather was caused by a strong southerly flow fueled by a strong high pressure off the east coast; but a northerly wind flow is now bringing in cold air from Canada. This is in response to a strong surface trough. The trough is stationed over the area, and will be reinforced by an upper level low sitting over Quebec. That will bring troughs rotating counter clockwise around its axis.

Should the temperature drop below 32° F this weekend, it would mark the first vegetation-killing temperatures and the end of the growing season.

Here is the forecasted minimum temperature for Sunday:



Frost is predicted for the weekend, and is expected to occur during the early hours of the morning on the 20th. If that occurs, it would be a full 3 days prior to the first freezing temperature average of October 22nd. The earliest the first freezing temperature occurred in Athens was 1988, when the temperature went down to 31° F during a frost on the 5th of October. The latest the first freezing temperature has occurred in a year was 1985 on the 20th of November.

With the temperature dropping below 32° F, this means that either a frost or freeze will occur. These can be farmer’s worse nightmare if it comes too early in the year. Both can burn or even kill all live vegetation that is exposed outdoors. They can mean the complete death of tropical plants that were planted outdoors earlier in the warm seasons of late spring and summer and the ending of new growth on annuals. Nearly all woody plants that have not already begun going into dormancy will get the switch flipped with this frigid temperature.

Frost:


Although freezes and frosts both occur with the same temperatures, and have the same final result on plants, there is a big difference between the two. Frosts only occur within certain parameters. For a frost to occur, there must be proper atmospherically conditions that allow it. First, dew must settle on the ground/surfaces and be allowed to freeze into ice crystals. There cannot be any wind that consistently blows during the night as it mixes the air. That prohibits the dew from settling on the ground before it gets a chance to freeze. Another thing that keeps frost from forming is cloud cover. Clouds keep the air warmer because daytime heating cannot escape into the atmosphere due to raditional cooling. Radiational cooling is the warm air lifting because nothing is preventing the cooler air to sink to take its place. This also prevents dew from forming, which in turn prevents the formation of any frost.

There are several types of frost that can occur, some of which don’t normally occur during this time of year. An advection frost can also occur when the temperature may not be below 32° F, however, there is a strong wind that blows through the night that causes wind chills to drop below the freezing mark and form frost on moist surfaces it crosses. Another type of frost, hoar frost, occurs in the middle of winter when the temperatures drop greatly below freezing.

A freeze differs from a frost significantly. A freeze can form in nearly any weather condition as long as the temperature is below the freezing mark for an extended period of time, usually more than three hours. Freezes can happen when there are clouds, wind, and even precipitation. A “hard” freeze occurs when the temperature is below freezing for a very long time such as 5 or 6 hours.

There are a few exceptions of when the first below freezing temperature can occur for certain locales. One is at the bottom of a valley, (or frost hollow) such as Perry Run in Vinton County, Ohio. Below freezing temperatures can occur even when the surrounding hillsides are warmer. This is due to nocturnal drainage, also called a flood frost. This occurs when the colder air that develops on the hillside forces its way down the hillside due to its smaller density than the surrounding warmer air. This can cause an early frost/freeze for an area. A farmer must keep all of this in mind when planting fields, as it can lead to an earlier harvest.

By taking the proper steps you can prevent damage to yourself and your plants. You can prepare for this frost/freeze by dressing very warm. The temperatures outside would be cold enough for snow if there was moisture in the atmosphere, so dress as if it were the middle of winter. You can easily get frostbite, even though winter may seem far off. Take all plants that may be vulnerable to winter temperatures inside for the night, and do not bring them back out until the temperature warms above the 30s. If the temperature warms in the daytime plants will be ok, just take them back in if temps are going to drop below freezing the next night.



Christopher Redmond
cr898005@ohio.edu