We are now back in the time of year where temperatures are dipping below 32 degrees Fahrenheit. If you’re like me and must go outside and start your vehicle in the morning, you have to allow a few extra minutes to scrape off that layer of frozen moisture on the windows.
During early fall and late spring, when there is a chance of frost, the National Weather Service will issue a frost warning for plants that may be susceptible. Sometimes, instead they will issue a freeze warning. So what is the difference between the two warnings? First we should explain what frost is.
Frost is made up of ice crystals that have formed directly from water vapor, a process called deposition. In this case, water does not change to liquid at any stage. For this to occur, the air must cool down to or below 32 degrees before reaching 100% relative humidity. Whatever temperature that happens to occur at is called the frost point. It will not always be one specific temperature, because the amount of water vapor in the air is highly variable. Next, we’ll explain how frost is formed.
Frost occurs during the night when there is calm air and skies are clear. This allows the ground to cool very quickly, which then cools the air in direct contact with the ground. With no wind to move air around, this cooling layer stays very close to the ground. While temperatures may still be several degrees above 32 just a few feet above ground level, temperatures may be at 32 degrees or below right on the ground and other surfaces, such as cars and other objects. This allows frost to form on those surfaces.
When the temperature goes to or below 32 degrees at the surface, but the air is so dry that frost cannot form, this is a freeze. A freeze can be caused by the same processes as a frost, but another way a freeze can occur is when a large mass of very cold air moves into an area. Many times, the air temperature during a freeze event is at or below 32 degrees, not just at the surface, but several feet above ground as well.
When conditions are favorable for frost or freeze, and when there is live vegetation present, the National Weather Service will issue a frost or freeze warning. While there are general time frames that are used for guidance based on location, the NWS also relies on information from others, such as crop producers, specialty growers and landscape entities that can provide detailed updates on what type of live vegetation is present. This is especially important early in the growing season.
The Midwest has seen the growing season expand over the last few decades. On average, the length of time from the last frost in the spring to the first frost in the fall has increased by seven to 15 days. This trend for a longer growing season is expected to continue as our climate continues to change.
Folks should still look at the average dates frost in their location as guidance. An early spring does not mean frost will not occur.
There are several great maps for Illinois supplied by State Climatologist Trent Ford. These maps not only show the average frost dates, but also dates for chances those that are less risky and more risky, along with earliest and latest spring and fall frost dates: https://tinyurl.com/mtakprf7.
Another map from the Midwest Regional Climate Center is information on frost/freeze guidance: https://tinyurl.com/3bufkzup.
Duane Friend is Illinois Master Naturalist Coordinator and Climate Specialist, University of Illinois Extension.