Understanding How and Why Frost Forms

by Rex Trulove September 23, 2017, 8:28 pm 804 Views 22 Votes 35 Comments

Late in the year, when the days get shorter and the temperatures drop, gardeners are often faced with the impending threat of frost. While many plants are hardy and can handle a light frost, many garden plants are quite susceptible to frost. For example, tomatoes, peppers, eggplants, cucumbers, squash, and melons are all sensitive to frost and can quickly die in frosty conditions. However, before doing anything to guard against this, it is a good idea to have an idea of how and why frost forms.

It isn’t hard to understand the formation of frost and many people already know the basics, from observation.

When you make iced water, you might have noticed that droplets of water form on the outside of the glass. This happens because the surface of the glass becomes colder than the surrounding air. Even very dry air contains at least some moisture and when a surface is substantially colder than the air temperature, in this case, the surface of the outside of the glass, the moisture in the air condenses. This forms the water droplets.

This is how dew forms. The leaves of the plants become colder than the air temperature, allowing the moisture in the air to condense into dew. Naturally, dew and frost aren’t the same things, but they are quite similar.

Water begins to freeze at a temperature of 32 F or 0 C. Most people are taught this as children. Consider what happens when a surface, such as that of a leaf, reaches 32 F or 0 C. Dew can still form, but it immediately begins to freeze, creating ice crystals. This is frost.

Weather agencies often forecast the ‘dew point’, or the temperature at which dew is expected to form. If the dew point is 32 F or below, frost is likely. This is straightforward and easy to understand. What can be a little confusing, however, is that the surfaces at ground level, such as leaves, can be substantially colder than the air temperature around those surfaces. For this reason, it is possible for frost to form even though the air temperature remains a few degrees above freezing. As an example, frost commonly occurs here in Montana when the air temperature is 36 to 38 F.

In fact, this is why frost covers can help prevent frost from forming on garden plants. The material of the frost cover, whether fabric or plastic, is the surface that becomes cold enough for frost to form on, while the plants inside the frost cover remain above freezing. The frost cover becomes frosted, but the plants don’t.

This might sound like nothing more than trivia, but it has a very practical application for home gardeners who are faced with an early or late frost. By knowing all of this and paying attention to the predicted dew point, a gardener knows when to use a frost cover, even a makeshift one, and why they should use it. This can mean the difference between losing the crops for the year and prolonging the growing season by weeks or even longer.

Especially for people living in the north, this takes on special meaning. The climate has been getting colder every year for at least the last decade, which means that in the north, an early frost is becoming much more common. Quite often, the temperatures warm up to average temperatures for a few more weeks before again plunging. Knowing how frost forms and why makes it possible to have a growing season that is at least that much longer than it would have been without this knowledge.

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