Learning Weather - Part Two - Frost
Posted: Tue Nov 15, 2011 11:17 pm
In the second of the weather guides, I'm going to discuss frost. I want to make it as relevant to plants and gardening as possible.
First of all frost is a common blight in winter, and in general frost is an intechangeable word, but it can be sometimes confusing when you realise that there are different types of frosts, and a white frost doesn't necessarily mean the temperature has gone below 0C.
It often surprises people who wake up to a white frost that their temperature reading is above 0C, and for this reason, it is worth nothing first of all that there are multiple meanings to the word 'frost'.
The most important terms in Britain are;
- Air frost
- Ground frost
- Hoar frost.
Air Frost
Air frost is fairly straight forward it is the point at which the air at 2 metres above the ground (the official height for measuring temperature, globally), falls to 0C (32F). When the temperature reaches this figure, the water vapour in the air will turn to ice and so the term air frost is given.
Ground Frost
Ground frost is the point at which the ground surface; whether that be concrete, grass or a car roof, hits 0C. Quite often you will see hoar frost on the surfaces of various objects when the ground temperature hits 0C, therefore if this occurs you know that the ground temperature is below 0C.
Hoar Frost
Hoar frost (also known as white frost) is just an indicator that the ground temperature is below 0C, in hoar frost, ice crystals are arranged in a particular way on a surface, however even if the surface is below 0C, hoar frost doesn't always result as explained below.
So why is there sometimes no frost below 0C on the ground?
This is to do with dewpoints and also potentially cloud cover. Hoar frost is the calling card of radiation frost, that is when the ground loses its heat to a clear sky, with no thermal blanket the dew which forms on a surface often arranges itself into crystals, especially with a lowish dewpoint. However occasionally in situation where there is relatively high dewpoint, or under cloudy conditions, you will not get frost where the surface or ground temperature is below 0C, insteas you will get frozen dew. In these situations the crystals cannot arrange themselves into patterns, and therefore dew just develops as standard ice.
I hope this gives some insight into the main kinds of frosts we may receive in Britain.. again any questions let me know.
First of all frost is a common blight in winter, and in general frost is an intechangeable word, but it can be sometimes confusing when you realise that there are different types of frosts, and a white frost doesn't necessarily mean the temperature has gone below 0C.
It often surprises people who wake up to a white frost that their temperature reading is above 0C, and for this reason, it is worth nothing first of all that there are multiple meanings to the word 'frost'.
The most important terms in Britain are;
- Air frost
- Ground frost
- Hoar frost.
Air Frost
Air frost is fairly straight forward it is the point at which the air at 2 metres above the ground (the official height for measuring temperature, globally), falls to 0C (32F). When the temperature reaches this figure, the water vapour in the air will turn to ice and so the term air frost is given.
Ground Frost
Ground frost is the point at which the ground surface; whether that be concrete, grass or a car roof, hits 0C. Quite often you will see hoar frost on the surfaces of various objects when the ground temperature hits 0C, therefore if this occurs you know that the ground temperature is below 0C.
Hoar Frost
Hoar frost (also known as white frost) is just an indicator that the ground temperature is below 0C, in hoar frost, ice crystals are arranged in a particular way on a surface, however even if the surface is below 0C, hoar frost doesn't always result as explained below.
So why is there sometimes no frost below 0C on the ground?
This is to do with dewpoints and also potentially cloud cover. Hoar frost is the calling card of radiation frost, that is when the ground loses its heat to a clear sky, with no thermal blanket the dew which forms on a surface often arranges itself into crystals, especially with a lowish dewpoint. However occasionally in situation where there is relatively high dewpoint, or under cloudy conditions, you will not get frost where the surface or ground temperature is below 0C, insteas you will get frozen dew. In these situations the crystals cannot arrange themselves into patterns, and therefore dew just develops as standard ice.
I hope this gives some insight into the main kinds of frosts we may receive in Britain.. again any questions let me know.