As September draws to a close and we enter October, signs that the weather is changing is all around. Soon enough, the colors on the leaves will begin to change. One other indicator of the changing seasons is the beginning of frost and freezes. What is the difference between frost and freeze? Why do some surfaces like grass and windows get frosty, but others not? How susceptible are my plants to frost/freeze? Read below for answers to all of these questions!
FROST
Frost occurs during the nighttime, as temperatures cool to near freezing, typically around 36 degrees. Despite the air temperatures being above freezing, many surfaces such as grass and windows fall below freezing. So when the water droplets, already cooled by the air, freezes when it touches the ground below freezing. This explains why fog is so selective, it only forms on surfaces below freezing. This typically includes glass and grass, hence why car windows get frosty but the rest of the car is fine.
Weather conditions to create frost include: clear skies, to allow for the surface to cool; cool temperatures, generally near or slightly above freezing; and calm winds, which allow for the air to settle to form frost.
On average, eastern Nebraska and western Iowa sees our first frost during the last week of September and the first week of October. Locations that saw frost develop last night is right on average, while Omaha and points south will have to wait a few days.
FREEZE
While frost is a physical process, where frost develops on surfaces. A freeze occurs anytime when the temperature falls below freezing, meaning any surfaces or plants with water in them could be damaged. A freeze can damage sensitive plants, such as: melons, tomatoes, cucumbers, eggplants, peppers, squash, and sweet corn.
The colder the temperature gets, the harder the freeze. A "hard freeze" occurs when the temperatures drops below 28 for an extended period of time, or goes below 24. A hard freeze can kill most plants, with vegetables such as broccoli, turnips, spinach, and cabbage able to survive.
If there is moisture in the air, the freeze can also create frost which is visible. However, as the temperature gets colder it will hold less moisture, so sometimes when the temperature gets really cold there is no water droplets in the air to freeze on.
The first freeze typically happens in our area one week after our first frost. In Omaha, the second week of October typically brings the first freeze. While the first hard freeze happens in the mid-later part of October.
