There is something about an autumn morning in Missouri that I absolutely love. The crisp air, the colorful leaves, and that first morning when you step outside and realize summer has officially packed its bags and headed south. Of course, that is also about the time meteorologists begin talking about frost advisories, freeze watches, and freeze warnings. You might hear those terms on television, see them on social media, or receive an alert on your phone. But what exactly do they mean, and more importantly, what should you do when one is issued?
As we move deeper into October and eventually toward winter, these weather alerts become increasingly common across Missouri and Illinois. While they may sound similar, there are important differences between them. Understanding those differences can help protect your plants, gardens, and landscaping from an unexpected encounter with Old Man Winter.
Let’s begin with something many people find surprising. Frost and freezing temperatures are not necessarily the same thing. Frost can form even when the official air temperature is above 32 degrees. That is because temperatures are generally measured by about five feet above the ground, while the temperature of grass, rooftops, vehicles, and other exposed surfaces can be several degrees colder. On clear, calm nights, heat escapes from the ground through a process called radiational cooling. If the surface becomes cold enough and moisture is available, frost can develop even when your thermometer reads to 35 or 36 degrees.
That brings us to the first alert, Frost Advisory. The National Weather Service generally issues a Frost Advisory when temperatures are expected to fall into the middle 30s, usually between 33 and 36 degrees, and conditions favor the development of widespread frost. Clear skies, light winds, and sufficient moisture are often the perfect ingredients. While a little frost on the grass might make for a beautiful autumn photograph, it can damage sensitive vegetation, including flowers, tomatoes, peppers, and other plants that have not yet been exposed to colder temperatures.
Next comes the Freeze Watch. Remember, in meteorology, a watch means conditions are favorable for something to happen, but there is still some uncertainty about the timing, location, or severity. A Freeze Watch is issued when widespread temperatures of 32 degrees or colder are possible, generally within the next day or two. Think of it as your early heads-up that significantly colder weather may be on the way. It is the perfect time to start making plans to protect vulnerable plants rather than waiting until the last minute.
A Freeze Warning, on the other hand, means freezing temperatures are expected or already occurring. When temperatures reach 32 degrees or colder over a widespread area during the growing season, the National Weather Service may issue a Freeze Warning. This is when action becomes especially important. Sensitive plants can suffer significant damage or even die, particularly when freezing temperatures persist for several hours. The longer temperatures remain below freezing, the greater the potential for damage.
You may also hear the term “hard freeze.” Generally, this refers to temperatures dropping to around 28 degrees or colder, especially for an extended period. A hard freeze can damage or kill many types of vegetation, including plants that might survive a brief dip to 32 degrees. Although the National Weather Service has simplified some of its frost and freeze alert terminology, the distinction between a light freeze and a hard freeze remains important when considering potential impacts.
Another interesting fact is that frost and freeze alerts are generally issued during the growing season. In the spring, they warn gardeners and farmers about potentially damaging cold after plants have begun growing. In the fall, they provide notice of the first significant cold temperatures that may bring the growing season to an end. Once a widespread freeze has effectively ended the growing season, the National Weather Service typically stops issuing these alerts until the following spring. After all, a temperature of 25 degrees in January is not particularly unusual in Missouri, but the same temperature in early October could be devastating to a garden.
So, what should you do when one of these alerts is issued? If you have potted plants, consider bringing them indoors or moving them into a protected location. For plants that cannot be moved, lightweight frost cloths or breathable coverings can help retain warmth near the ground. Make sure coverings extend all the way to the ground and avoid placing plastic directly against delicate leaves. Watering the soil ahead of a cold night can sometimes help because moist soil retains heat better than dry soil. However, no covering can guarantee protection during a prolonged or particularly hard freeze.
It is also important to remember that temperatures can vary considerably over relatively short distances. Low-lying valleys and rural areas often become colder than nearby hillsides or urban neighborhoods. This is because cold, dense air tends to settle into lower elevations, especially on calm nights. That is why one neighborhood might wake up to a heavy coating of frost while another location just a few miles away escapes without any noticeable frost at all.
As a meteorologist, I often remind people that weather alerts are not issued simply to get your attention. They are designed to help you make informed decisions before potentially damaging conditions arrive. A Frost Advisory tells you to be aware. A Freeze Watch tells you to be prepared. A Freeze Warning tells you it is time to act. Three different messages, each serving an important purpose.
The next time you wake up to a sparkling layer of frost covering your lawn, take a moment to appreciate the science behind it. That beautiful white coating is the result of temperature, moisture, atmospheric conditions, and the movement of energy between Earth and the atmosphere. It is another example of how even the simplest weather events have fascinating stories to tell.
While we cannot stop the seasons from changing, understanding what is happening outside our windows can help us prepare for whatever Mother Nature has planned next.
Until next time, keep looking out the window, stay weather aware, and remember that sometimes the smallest changes in temperature can make the biggest difference.
