Cycling UK Forum

Thanks for the replies. It does matter as 5 deg difference is enough not to take certain items of clothing etc. I came up with three possible reasons (all covered above):

1. The gauge was wrong: it has in fact accurately matched other BBC forecasts/one hour snapshots so I'm happy with it.
2. It wasn't sited properly: I'm aware of the slatted box used for weather equipment but because the night was still, I'm wondering if that really did make much of a difference.
3. The temperature was localised: this seems to me the most plausible explanation (and people have posted various plausible scenarios above). And indeed the simplest discrepancy is between the warmth of a city like Exeter and the rural spot where I camped.

However (and this is a big however):

The "forecast" (and I do think we are talking about actual temperatures here, it was only one hour out) was obtained by entering the name of the village and BBC/Meteo happilly obliged to provide the said forecast. I presume that BBC/Meteo don't have a local weather station in the village and so are really providing a regional forecast based (probably) on Exeter. Generally both village and city will indeed have the same weather (rain, sunshine etc) but at times local conditions will vary - a lot. So my question is: why do they bother to identify searched-for places when it is obvious that the forecast is for a wider region and that local conditions could be critical (certainly with ice for example)?

We don't use the BBC, we use the Metoffice website and weather app. These give different forecasts for places a few km. apart. There is also a report of recorded temps. over the previous 24 hours but these are restricted to the relatively few places that monitor. Ours is Plymouth, for example, more than 30km. away. You don't need an observation station in a location to provide a forecast for it.

I don't think you'll ever get the accuracy you want from any forecast,

There's a ~3deg drop observable by taking a reading in my road (Hello from Exeter BTW!) between all the terraced houses and tarmac leaking heat, vs walking 500m and taking it in the nearby deserted graveyard! Elevation, tree cover and water will also affect temperatures, as will shelter/exposure to prevailing winds (however light). A great example riding over somewhere like Woodbury Common you can get +/- 5 degrees of more just in the dips and rises of the hedgerow lined lanes, it was especially noticeable on the Exmouth Exodus one year, riding along a road at about 4am and in the space of a couple of miles went through several 'pockets' of warm air no more than 50-100m long.

Local variations can be huge over the space of tens of meters, and remember that although there are plenty of weather stations and clever maths used to work out the 'in betweens' there won't be coverage to the level you'd need to get that accuracy. The nearest weather station may not even be in the place you're asking for a forecast on, and if it is it likely won't be near enough to cancel out local variations.

My advice... always pack for 5deg either side of what you expect, and maybe a bit more

The detailed location temperature forecasts derive from an algorithm which takes into account altitude, exposure, distance from coast etc, wind speed amd direction etc. Different forecasters use different algorithms based on different forecasting models. For instance if you look at windy.com, you can zoom in on the map and click on any location for a temperature at any specific time. Click on a location a few kilometres distant and it will show something different. If you play around with the display to show the various parameters (wind speed/direction etc) you can often see why this is. (Windy.com is one of the best for its visuals).

I seem to remember that the "standard" temperature measurement is not only inside a screen but also that the screen has to be placed above some sort of standard surface of a certain area (sorry can't remember the details it has been a while since I worked on some weather related software!). The temperature difference between being over say grass and tarmac can be pretty large.

From https://www.metoffice.gov.uk/weather/gu ... emperature it seems that the met office take four different types of temperature measurement - air, grass minimum, concrete minimum and soil minimum. From the OP's description, theirs is a fifth method. I would think that, crossing over with/repeating factors already mentioned in this thread, there will be air movement/breezes that could shift warm air or cool air and affect temperatures on a very local scale.

Mick F wrote:Yep.
Very true.

Here's a typical temp trace from a ride recently. Out and back, exact same route.Screen Shot 2019-08-24 at 09.19.51.png

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.

Does the Garmin use barometric pressure, or satellite info, to judge elevation?

If barometric pressure then it isn't going to be reliable unless you constantly re-calibrate v. "ambient" pressure, which varies greatly. Aircraft altimeters need calibrating at the start of every flight, during the flight, and prior to landing using information given over the radio, and I'm sure that for accurate - or even reasonably accurate - readings the same must be true of your Garmin if it works that way.

rmurphy195 wrote:
Does the Garmin use barometric pressure, or satellite info, to judge elevation?

If barometric pressure then it isn't going to be reliable unless you constantly re-calibrate v. "ambient" pressure, which varies greatly. Aircraft altimeters need calibrating at the start of every flight, during the flight, and prior to landing using information given over the radio, and I'm sure that for accurate - or even reasonably accurate - readings the same must be true of your Garmin if it works that way.

Interesting.
Maybe others would care to comment.
Barometric altitude, and a compass which works when stationary, are I think the only differences between the Garmin etrex20/x and 30/x.
I went for the 20/x.