Difference in actual and Met Office temperatures

[horizon]

Last week I camped in a small campsite in a river valley about 5 miles from Exeter. At 5.00 am my temperature gauge attached to my bike outside read 6 deg C. The night was still (virtually but not completely windless) and clear, hence the low temperature in August. I immediately checked the weather on the BBC Meteo site and that read 11 deg C. for the following hour. I realise that there are reasons for this discrepancy but I'd very much appreciate some views, explanations etc regarding it. I presume the Met Office would have been the same as the BBC.

[rualexander]

BBC stopped using the Met' Office a couple of years ago, they now get all their weather from Meteo.
Temperatures can vary considerably even in a small local area, you were in a river valley which will be cooler than elsewhere.

[gbnz]

Aside from which, the forecast weather rarely matches weather experienced. Any number of rides in torrential downpours on days alleged to be bone dry rather proves the point.

As do those rides on "calm" days such as yesterday, when cycling into the wind reduced speed to 10mph, while turning around increased it to 27mph. I suppose believing in the Met is rather like most religions - no basis in fact, a system relying purely on belief

[reohn2]

Local temperatures vary and are influenced by local phenomena eg; a valley indicates a stream or river near by that lowers temps.If it rained heavily for a few days previously damp ground has a similar effect.If you camped on the north facing side could also effect temps locally due to it being in shade most of the day,etc,etc....

Forecast can only ever be a general guide,especially on the edge of a land mass such as the UK is.

[Mick F]

Yep.
Very true.

Here's a typical temp trace from a ride recently. Out and back, exact same route.

The elevation trace is corrupted a tad as it was windy, and it effects the barometric pressure readings as the vent is at the front of the Garmin Montana. Headwind on the way home.
I'm raising this subject in the tech section shortly.

[ChrisF]

Last week I camped in a small campsite in a river valley about 5 miles from Exeter. At 5.00 am my temperature gauge attached to my bike outside read 6 deg C. The night was still (virtually but not completely windless) and clear, hence the low temperature in August. I immediately checked the weather on the BBC Meteo site and that read 11 deg C. for the following hour. I realise that there are reasons for this discrepancy but I'd very much appreciate some views, explanations etc regarding it. I presume the Met Office would have been the same as the BBC.

5am in August is before sunrise, and on a still clear night will always be the coldest part (heat escapes the atmosphere all through the night). But the forecast for the folowing hour, 6am, will include when the sun has risen so there will be new warmth from the sun.

[Audax67]

I remember one ride we did years ago when it was -4°C all the way except for one stretch of a couple of hundred yards where the road crossed a marshy bit and it went up to around 2°C.

[Brucey]

if the air is still and you are in a clearing in an otherwise wooded valley, with a shallow slope to it, you might be in a 'frost hollow'. These places can see temperatures five degrees or more lower than forecast under these conditions. One of my colleagues used to live in such a place and even though the valley was basically shallow (and set into an otherwise nearly flat landscape by most people's standards) they would swear and curse in the morning that they had to deal with frost on their car on a regular basis. I only lived a few miles away and yet there was an overnight frost less than half as often where I was.

I think what happens is that when it is cloudless, you can get a temperature inversion in the night, i.e. the air is warmer at ground level, and in the absence of any wind, convection cells are established which can cause cold air to pour downwards, but only in certain areas. In a frost hollow, the cold air runs down the valley sides but is (at ground level) obstructed from running down the length of the valley. This means the frost hollow literally fills up with a pool of the coldest air. As the night goes on the temperature of the air in the sky becomes lower and lower and the cold air convecting downwards gets colder too.

If there much of a breeze going then the air at ground level gets churned up and temperature predictions are liable to be more accurate, with smaller local variations.

cheers

[reohn2]

I remember one ride we did years ago when it was -4°C all the way except for one stretch of a couple of hundred yards where the road crossed a marshy bit and it went up to around 2°C.

Marsh gas(?)

[pwa]

Weather forecasts have not yet reached a level of local detail where you can say exactly what you will get at any particular spot. On flatland you stand a good chance of reasonable accuracy, but in hills you don't. The top of a hill will always get different weather to the bottom of the same hill, the north side will get something different to the south side. Within my own region I know of two locations just twelve miles apart as the crow flies, yet one gets roughly twice the rain of the other. And if I cycle towards Bridgend early in the morning I can pass through the village of St. Brides Major with my bare knees feeling comfortable, but by the time I have plunged down into the valley below, just half a mile down the road at most, the temperature has dropped several degrees. I can really feel it. In winter the Highways people put warning signs for ice on the road down in that frost hollow.

[Mick F]

Agree.
Look at this one. Cycling home from Gunnislake Station at quarter to four in the afternoon.
Mile and a quarter with a drop of 400ft.
20odd deg up in the sunshine and 10deg down here in the gloom.

[Audax67]

I remember one ride we did years ago when it was -4°C all the way except for one stretch of a couple of hundred yards where the road crossed a marshy bit and it went up to around 2°C.

Marsh gas(?)

Just decomposition I suppose. It was amazing how much warmer it felt, though.

[ChrisButch]

As well as the reasons already cited, met office forecast temperatures assume measurement in a standard weather station - a Stevenson screen, which is a vented wooden box painted white to reflect solar radiation, between 1.5/2m above the ground, not overshadowed by vegetation etc etc. Even small variations in this environment can make quite a difference. Also worth remembering that cheap portable thermometers can be inaccurately calibrated.

[Cyril Haearn]

Lots of places were named for their temperatures, Bwlch Oerddrws is the Cold Door Pass and on the Migneint are the Bryniau Poethion, warm hills

[Mike Sales]

Lots of places were named for their temperatures, Bwlch Oerddrws is the Cold Door Pass and on the Migneint are the Bryniau Poethion, warm hills

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