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Key factors fall in line to form frost

by Gary Castor | December 2, 2021 at 4:00 a.m.

Editor's note: Meteorologist Maddie Est will be writing a weekly column explaining weather trends and topics.

Temperatures in the 70s this past week were a welcome change from the chilly mornings of late November when most people awakened a few minutes early to defrost cars before leaving to start their day.

Those early morning frost-scraping duties are likely to return this week.

Most people are most familiar with the frost that collects on their windshields and lawns, but frost can cause a nightmare for farmers if it occurs before harvest. If a frost damages the farmer's crops, they could be wasted and unusable for profit. Because of this, knowing how frost forms and when it is likely to occur can be incredibly important.

Frost formation depends on several key factors that must fall in line for frost to form on surfaces like cars, crops or trees.

First, there must be enough moisture in the air to condense or become a droplet that is visible. Condensation occurs when the air temperature is lower than the dew point temperature or the temperature at which water vapor will become liquid. When this moisture condenses and the air temperature is cool enough for the water to freeze, frost will begin to form.

If the air is not cool enough to freeze the water droplets, dew will collect on surfaces instead of frost. Light wind can also aid in frost formation as a lack of mixing in the atmosphere by the wind will cause the cooler air to settle near the surface.

While all frost develops in a similar manner, there are actually a number of different types of frost that can occur. The most common types of frost that are seen in Missouri are advection and radiation frost.

Advection frost usually forms when a cold wind blows over the surface of objects or natural scenery. Advection frost is characterized by frozen spikes along the areas impacted by this freeze.

Radiation frost, or hoarfrost, is composed of tiny ice crystals that collect on objects and surfaces. Radiation frost usually occurs on calm, cold and clear nights when temperature inversions are more prone to happen.

While it can be difficult to forecast when a frost event will occur prior to a few days before the event, climatic averages can help groups such as farmers know when the typical first frost occurs. In Missouri, the northern portion of the state can expect to see their first frost during the second week of October.

Here in Central Missouri, our first frost climatically can be expected during the third week of October. Finally, the southern portion of the state that is not included in the Ozark Plateau will typically experience a frost during the last week of October. The Ozark Plateau usually experiences frosty conditions in mid-October due to the higher elevations in respect to the surrounding southern portions of Missouri.

Frost occurs throughout the majority of the fall, winter and early spring seasons in Missouri. While frost events usually do not cause harm, early or even late frost events can cause irreparable damage to crops and gardens across the state.

As such, knowing average dates of when the first and last frosts can be expected to occur can save gardeners and farmers alike from a few extra headaches.

Maddie Est has worked as a broadcast meteorologist and marine meteorologist since graduating from the University of Missouri in 2021. She has worked with the Missouri Climate Center and conducted research on atmospheric blocking while at MU.

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