The four different types of frost

As the winter season gets well and truly underway, temperatures are regularly falling below freezing overnight, leaving frosty mornings behind.

Hearing a risk of frost in a forecast is quite common in the colder months, but did you know there are different types of frost? These are hoar, air, grass and ground.

Ground and grass frosts are most common between autumn and spring, while air and hoar frosts are most common during the winter season.

An air frost occurs when the temperature of the air falls to or below 0C although it is usually defined as a temperature below 0C at a height of one metre above the ground. When you hear on the television that there will be a frost, they are most likely referring to an air frost.

A grass frost refers to frost on grass or vegetation. This is because vegetation is more sensitive to temperature changes so the frost can appear while roads or windows might remain frost-free.

A ground frost refers to ice on the ground, objects or trees that have a temperature below the freezing point of water. Sometimes you can get a ground frost without an air frost. This happens when the ground cools faster than the air.

Finally, there is hoar frost. This is a fine and needle-like frost that occurs when water vapour in damp air freezes directly and quickly on surfaces. This is what gives it its white appearance.

In general, frost forms when a surface cools below a dew point. This is where an air parcel gets cold enough to turn into water vapour. If temperatures are cool enough, the water vapour freezes on objects to create the frost.

Learn more about how frost forms by watching our Weather Explained video below: