Monday, December 10, 2018

It's Frost Season! But What Kind of Frost is That?

Autumn frost, photo by Donna Duffy

It’s the time of year when frost occurs almost daily. But what kind of frost is it? Just a few degrees or a slight change to the environment can separate air frost from ground frost, or hoar frost from glaze or rime. Following is a detailed look at what each term means, courtesy of The Weather Channel.

Hoar frost, photo by Donna Duffy

Hoar frost
Consisting of tiny ice crystals, hoar frost is formed through the same process as dew - but only when surface temperatures are below freezing point. 
Hoar frost follows a feather-like appearance, forming when the surface temperature reaches 0C before the dew begins to manifest on it.  Often more rounded frost particles appear, called 'white' frost, which is when the dew forms first and then freezes as a result. However, if there is any fog present, this usually prevents the formation of hoar frost since it lessens the potential for the cooling of the Earth's surface.

Air frost
Air frost happens when the temperature of the air plunges to or below the freezing point of water. The depth of air above the ground also must have increased to at least three feet above the ground. The combination then creates a layer of cold air hovering just above the ground that should hit 32F or below, and gradually becomes thicker and thicker as more heat energy fades away and the temperature continues to fall.

Grass frost
A grass frost is an un-official strain of ground frost. A grass frost takes place when natural surfaces, such as grass, freeze, while other surfaces such as tarmac and concrete pavements don’t. The reason why such man-made surfaces don't come into contact with this frost is due to their better ability to hold onto warmth.

Glaze or rime
Frost is often confused with rime or glaze. Rime is a rough white ice deposit which forms on vertical surfaces exposed to the wind. It is formed by supercooled water droplets of fog freezing on contact with a surface it drifts past. Glaze can only form when supercooled rain or drizzle comes into contact with the ground, or non-supercooled liquid may produce glaze if the ground is well below 32F. Glaze is a clear ice deposit that can be mistaken for a wet surface and can be highly dangerous. On the roads, we often refer to glaze as “black ice”.

Glaze, also called black ice, photo courtesy science.howstuffworks

Ground frost
Ground frost relates to ice formed on the ground, or on objects and trees, where the surface consists of a temperature below freezing point. On those occasions when the ground cools faster than the air, ground frost can occur without an air frost.

So there you have it! Everything you always wanted to know about frost!\