Frost Hollows

[TapA321 0]

Hi Guys,

According to www.weatheronline.co.uk a frost hollow is described as a low-lying area (e.g. a valley bottom or a smaller hollow) where frosts occurs more frequently than in the surrounding area. This is normally as after a dry, clear and cold night cold air drains down neighbouring slopes into a localized pocket from which it is slow(or unable) to escape. Frost hollows of larger scale (a valley or basin) are also known as cold pools. Cold pools are areas where cold air is trapped under an inversion under calm winter weather conditions.

This subject became of interest to me on my last flying lesson, which took place at 8:30am on 19th August after a cold, clear and still night. The airfield (White Waltham, Maidenhead) is located in a slight frost hollow, and the temperature difference between where I live in North Surrey and there was unbelievable. It had been a cold night, and according to my thermometer, the minimum temperature where I lived was 7.3C. It was pretty chilly when I left the house at 8am (12C according to the car theremometer, which is normally a little high), but then when I arrived at the airfield 30 mins later, it was much colder. According to their weather station, it was still 7C and the minimum that night was 4.7C.

One of the most well-known frost hollows in the UK is RAF Benson in Oxfordshire. Any members who live close to here will perhaps be able to confirm this.

According to the Metoffice, tonight is supposed to be clear across the south, so if you're interested, you might like to keep an eye on the Metoffice observations. Here are a few things to note:

1. Benson's temperature observations are likely to fall fairly rapidly after the Sun goes down: http://www.metoffice.gov.uk/weather/uk/se/benson_latest_weather.html

2. High Wycombe is 150m higher ABSL than Benson. Even though it is located at higher ground, on a clear night, the min. temperature is normally much higher than Benson: http://www.metoffice.gov.uk/weather/uk/se/high_wycombe_latest_weather.html

3. South Newington is another example. Watch the temperatures plummet in a clear night sky: http://www.meteogroup.co.uk/uk/home/weather/world-weather/weather-stations/obsid/99166.html (notice in the table of observations, the low temperatures observed on the night of 18th-19th August)

Just something that some may be interested in. I'm sorry if you don't find this as interesting as I do. Sod's law is that the pattern won't emerge tonight, I'm only going by the forecast.

Cheers,

George

[mutley 4,487]

I often experience this on my way to work on cold mornings. I have to climb a steep hill, probably 200ft + and half way up my wing mirrors mist up and we lose about 2 or 3 degrees C at the top it goes up again. Further on, I know that one bend in the road will be covered in frost or ice when all around can be clear. This is what I consider to be a Frost hollow. I will keep an eye out tomorrow, there won't be frost but leaving home at 06.45 I might come across it.

[ddavid 149]

Did I fall asleep, or is it still August?

Cheers - Dai.

[allardjd 1,853]

I can remember feeling distinct changes in temperature going up and down hills on a motorcycle on still evenings - cool in the valleys and perceptibly warmer when cresting the hills. If there's any wind to speak of it doesn't seem to happen.

John

[TapA321 0]

This is also mentioned in the Metoffice factsheets section entitled "Microclimates". An interesting read.

http://www.metoffice.gov.uk/media/pdf/f/k/No._14_-_Microclimates.pdf

Cheers,

George

[TapA321 0]

Benson Observations:

20h00 = 19.3C

21h00 = 16.8C

22h00 = 13.9C

23h00 = 12.5C

South Newington:

20h00 = 18.3C

21h00 = 17.3C

22h00 = 14.6C

23h00 = 11.8C

[Tim_A 995]

One of the most well-known frost hollows in the UK is RAF Benson in Oxfordshire. Any members who live close to here will perhaps be able to confirm this.

I live on the edge of the Benson MATZ, and I can conform it is a well known frost hollow. Benson often records the coldest nighttime temperatures in the UK.