With this record setting cold, central Kentucky is experiencing what’s known as a ‘hard freeze.’ But what exactly defines a hard freeze? –
A hard freeze refers to extended periods of temperatures below 28° (that temperature would feel like a heatwave at the moment). These conditions are also sometimes referred to as a ‘killing freeze.’ These weather conditions are essentially the same thing with one important difference: dormant plants can generally survive a hard freeze, but actively growing plants can suffer severe damage or death – that’s when it a hard freeze becomes a killing freeze.
It Starts With A Frost
If the super-cooled, freezing temperatures cool to the dew point (the temperature at which, when cooled to at constant pressure, condensation occurs; moisture will have to come out of the atmosphere as fog, frost, etc. Frost is usually found on exposed surfaces like your car – you can easily sweep frost off of your windshield as it is light and fluffy – not frozen hard.
The Beautiful Hoar Frost
We’ve had beautiful hoar frost (also known as hoarfrost) this year. But what is hoar frost? “Hoarfrost is a deposit of interlocking ice crystals (hoar crystals) formed by direct sublimation (the transition of a substance directly from the solid to the gas phase, without passing through the intermediate liquid phase) on objects, usually those of small exposed tree branches, plant stems and leaf surfaces.
Hoarfrost, therefore, is like dew, except when it’s cold enough outside to freeze water (that is when it’s 32°F or 0°C), and there’s moisture in the air, then hoar crystals (flat crystals that interlock together), form from the moisture in the air when it comes in contact with the edge of the object that is below freezing (or vice versa?). So if you get up early enough, especially after a clear cold night, you can experience the direct sublime beauty of hoarfrost in all its simplicity.
Light, Moderate or Hard Frost
As air temperatures cool, the ground also begins giving off heat. This is called radiational cooling as the heat ‘radiates’ from the surface. Generally, the clearer the sky, the more heat is given off. For a frost to kill plants, the earth has to lose enough heat so that the freezing temperature occurs at ground level. If the ground is still warm, it is possible for the frost to hover a bit above ground level. That’s what happens when the tops of your plants are killed by frost but the lower portions remain green. We tend to refer to this as a light frost.
A killing frost is the occurrence of temperatures cold enough to kill all but the hardiest vegetation. A killing frost tends to be the last frost in spring when tender first stems and leaves begin coming up, and the first in fall frost, when plants have not yet begun to lose leaves and flowers.
Then There Are The Freezes
A freeze can happen when the surface air temperature falls to 32° or below; frost may or may not form, depending on the moisture in the air. Freezes are usually caused by what is called advective cooling. Advection is the transference of some atmospheric condition, like heat, humidity, or cold, by the movement of an air mass. Think arctic blast; when one of these blows in, all the annual plants and tender perennials tend to call it a day.
Freezes can be quick or they can linger. If they are cold enough, even a short freeze can do a lot of damage. Even the hardiest perennials will be killed back to ground level by a long, hard freeze. A sure sign is when the ground crunches when you walk on it.
It’s interesting to note that frost and freeze advisories are only issued during growing seasons, both spring and fall. If the plants are dormant, it’s just called cold weather.
Call it what you will – cold, by any other name, is still cold – and baby it’s cold outside right now! Stay in if you can, and if you must venture out – stay safe.