Author Topic: Why was it so cold?  (Read 1734 times)

Why was it so cold?
« on: August 30, 2019, 11:42:50 am »
The conditions that caused reported temperatures down to 2 degrees C will be mysterious to many. So what happened? Crudely, when it's still and cloudless, warm air rises due to convection. This is then replaced by colder air from a higher altitude. That cold air is denser, which is why it falls. In windier conditions it mixes, and when it's cloudy, the air column is capped at the cloud level.

Once the cold air hits the ground, it acts as a fluid, and rolls down the easiest channels. In Brittany there are abundant hedge-banks, and they channel the air towards streams and roads, where the flow is least impeded. The cold air then ponds in lower areas, which contain dense, and cold, fog.

So the coldest air is flowing towards you as you climb, and you will descend into very cold, and wet, air. The result is that it's coldest at 5.30am or so. Those conditions were in place in 2003, 2015 and 2019. Reporting is better now, as GPS's have thermometers.

For the science-minded, its a mixed temperature inversion and katabatic effect. For those who like modelling and maths, here's a paper.

Quote
The formation of cold-air pools in valleys and sink-holes during clear nights is a well-known process that has been intensively studied by means of field experiments (e.g., Clements et al. 2003; Steinacker et al. 2007; Price et al. 2011; Lareau et al. 2013) and numerical simulations (e.g., Zängl 2005; Burns and Chemel 2014; Vosper et al. 2014). The strength and depth of the temperature inversion that develops during night time strongly influence the development of the boundary layer during daytime (Schnitzhofer et al. 2009) and therefore also the exchange of heat, moisture, momentum, and pollutants with the free atmosphere above the valley.

https://journals.ametsoc.org/doi/pdf/10.1175/JAMC-D-15-0091.1

I live in a bit of a frost hollow, so I'm aware of these processes. Curiously, the UK capital of these conditions is Rickmansworth.
https://www.independent.co.uk/news/uk/home-news/ice-age-returns-to-hertfordshire-1281725.html

FifeingEejit

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Re: Why was it so cold?
« Reply #1 on: August 30, 2019, 01:03:13 pm »
Knew that the coldest time of day is just before dawn from riding through it but didn't know the sciency bit.
ta

Re: Why was it so cold?
« Reply #2 on: August 30, 2019, 01:34:37 pm »
I've had plenty of opportunities to observe these effects on endurance cyclists. The Espley roundabout on the Mersey Roads 24 sits in a frost hollow, so these conditions are apparent during high atmospheric pressure.

We've recorded the performance of  all participants during the night since we started marshalling there in 2007. I've also filmed riders, and that gives an indication of responses to the cold, and the influence on speed. The key to a good ride is consistency during the night, and prompt shedding of additional layers as the temperature rises. The nearby Shawbury RAF station has weather records, which give a general guide to conditions on the course.

The coldest time is usually at sunrise. There's an interesting 'clearance cycling' effect. That's where the mist is burnt off by the early morning sun, which then causes evaporation, until the humidity hits the dew point, and the mist returns. The most we have seen is five of those cycles.

2008 exhibited those conditions. Hot during the day, cold at night, with mist. This video demonstrates how riders then wear too much to do well in the early morning.


rr

Re: Why was it so cold?
« Reply #3 on: August 31, 2019, 12:05:30 am »
I call it Buncefield weather, it is not chance that large industrial explosions tend to happen around dawn on cold, still, usually winter mornings.

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Re: Why was it so cold?
« Reply #4 on: August 31, 2019, 12:33:28 pm »
There's a point to all this detail. LEL has a number of hill ranges where these various effects occur. The Lincolnshire and Yorkshire Wolds, the Howardian Hills, The North Pennines, and the Southern Uplands. All can be like this. I made this short video from PBP 2015 material as a warning.

https://www.youtube.com/watch?v=UyhEzl30nJM

mattc

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Re: Why was it so cold?
« Reply #5 on: August 31, 2019, 12:37:41 pm »
Has never ridden RAAM
---------
No.11  Because of the great host of those who dislike the least appearance of "swank " when they travel the roads and lanes. - From Kuklos' 39 Articles

FifeingEejit

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Re: Why was it so cold?
« Reply #6 on: August 31, 2019, 07:06:12 pm »
Average white band, from somewhere better known for the Haar rolling in off the sea  ;D

vorsprung

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Re: Why was it so cold?
« Reply #7 on: August 31, 2019, 09:03:43 pm »
It wasn't cold

I mean obviously it is *colder* at the hours around dawn but it was typical weather for Northern France in August.  Mainly hot.

Some Indian riders said it was cold but they are from a very hot place
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Re: Why was it so cold?
« Reply #8 on: September 01, 2019, 12:02:35 am »
I'd tend to agree that it was normal for PBP, and consistent with conditions in 2003 and 2005, which is 50% of the PBPs I've experienced. It just so happens that the 'full-value' 90 hour riders pass through the most notable frost hollows in the early morning.

The only indication I had of temperatures prior to 2015 were the displays on pharmacies, and in 2003 those near Loudeac indicated 6 degrees C at 6am. Newer GPS devices have thermometers, so there's more information. It would be interesting to see the data from a GPS recording temperature.

Prior to 2015 there were few riders from tropical climates.

Re: Why was it so cold?
« Reply #9 on: September 02, 2019, 12:36:34 am »
It wasn't cold

Oh, I think it was a bit bracing overnight. Its go so far as to say it was chilly.

vorsprung

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Re: Why was it so cold?
« Reply #10 on: September 02, 2019, 08:52:48 am »
It wasn't cold

Oh, I think it was a bit bracing overnight. Its go so far as to say it was chilly.

All I am saying is that during the hours around dawn (regardless of the time of year) it is a low point of temperature.   It's true that there are always places that have reverse temperature gradients with cold settling in valleys.  But overall it was not unusually cold for Northern France in Brittany.  Any UK rider who had the usual clothing for riding in the UK through the night at this time of year would be just fine.  I had leg warmers, a Helly, a buff, long finger gloves and a goretex coat.  I was aiming to not be riding 3am-5am (this plan didn't work out :) )

I suppose it's possible that a few UK riders will have made it through their qualifiers without ever riding at dawn and so were not aware of how cold it gets at that time.  It's also possible that some riders might think that La Belle France is so loverly and warm that there would be no low temperature at this point

To say it was actually "cold" during the event is incorrect.  It was, it seems to me, pretty average. A 20 or 30 degree swing of temperature is normal

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marcusjb

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Re: Why was it so cold?
« Reply #11 on: September 02, 2019, 09:13:25 am »
But overall it was not unusually cold for Northern France in Brittany.

Several Bretons I talked with thought is was unusual for August.
Right! What's next?

Ooooh. That sounds like a daft idea.  I am in!

Feanor

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Re: Why was it so cold?
« Reply #12 on: September 02, 2019, 09:39:46 am »
Newer GPS devices have thermometers, so there's more information. It would be interesting to see the data from a GPS recording temperature.

Here you go:


PBP-Profile by Ron Lowe, on Flickr

https://www.strava.com/activities/2651210722/analysis

Re: Why was it so cold?
« Reply #13 on: September 02, 2019, 10:24:21 am »
It being effing cold and effing cold weather being not unusual for the time and place are not exclusive propositions.

It was effing cold.

FifeingEejit

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Re: Why was it so cold?
« Reply #14 on: September 02, 2019, 10:27:41 am »
Never below 5? That's no cold!

It appears RWGPS cleared my temperature data (which was dodgy anyway) when I did the elevation correction.
There were a few sections where the data looked reliable but it was mostly junk, Strava retained it but I'm not sure it quite hit -40...
Closer to full value.

Zed43

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Re: Why was it so cold?
« Reply #15 on: September 02, 2019, 10:32:39 am »
It certainly felt colder than the 10 degrees that was forecasted, and on the Tuesday night (well, early morning) my toes were clumps of ice (should have brought overshoes).

My impression is that when humidity is high (fog, or close to it) a lowish temperature feels much colder. And that this was one of the factors, at least on the Tuesday night.

frankly frankie

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Re: Why was it so cold?
« Reply #16 on: September 02, 2019, 10:46:49 am »
You think carrying overshoes for 1200km would be a fair trade-off for ~2 hours of toasty feet?
It's not dark yet but it's getting there.

FifeingEejit

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Re: Why was it so cold?
« Reply #17 on: September 02, 2019, 01:04:37 pm »
Was certainly colder than the forecast 10c, I was almost for leaving my warmers at home because of that forecast; but ended up wearing them on nights 2 and 3.
Only really took them because i prefer to have something between arms and rain jacket.

Re: Why was it so cold?
« Reply #18 on: September 02, 2019, 01:36:24 pm »
Was certainly colder than the forecast 10c, I was almost for leaving my warmers at home because of that forecast; but ended up wearing them on nights 2 and 3.
Only really took them because i prefer to have something between arms and rain jacket.

Same here-I left my merino T behind but kept the warmers.
Remember those comments about  those asian riders with full length longs and winter weight jackets?
It turns out they were better prepared than some of us were.

I had arm and knee warmers and thin merino gloves but even then I felt really cold on the section between Carhaix and Brest.
Sleep deprivation and riding fatigue accentuates the effect and I can understand why hypothermia victims just want to curl up and go to sleep.

On previous editions the mix of warm air and falling temperature has resulted in thick fog over the Roc at night.
That didn't happen this time, which was good for me as a specs wearer.

Re: Why was it so cold?
« Reply #19 on: September 02, 2019, 01:38:26 pm »
The record Diurnal temperature variation for the UK is over 29 degrees C in Altnaharra one February. That 's why I predicted these sort of conditions for the National 400 when it was in Northern Scotland.

Of more direct relevance is the second highest. The topography of the Cote D'Armor is similar to the Chilterns, and we need an August comparison.

Quote
No idea about other records, but the second biggest diurnal range I can find was 28.3C, from 1.1C to 29.4C, on 29 August 1936 at Rickmansworth.

The data for that day when Rickmansworth recorded that large diurnal range.

GMT Temp F
1am 38.8
2am 37.9
3am 36.2
4am 35.1
5am 34.4
6am 35.5
7am 46.7
8am 58.3
9am 70.7
10am 74.1
11am 78.0
12pm 80.9
1pm 83.0
2pm 84.5
3pm 84.5
4pm 83.1
5pm 79.7
6pm 69.2
7pm 56.8
8pm 51.1
9pm 47.7
10pm 46.6
11pm 45.7
12am 44.1

The lowest reading for the thermometers in the standard screen were 34.0F and the highest reading was 84.9F

The radiation thermometers ranged from 26.3F to 150.4F
https://www.theweatheroutlook.com/twocommunity/default.aspx?g=posts&t=19763

One interesting effect is the rapid rise in temperature from 6am to 9am. My feeling is that reflects the curtailment of the supply of cold air from higher areas. That rapid warming was apparent at PBP.

We planned our filming of PBP to allow a whole night in St Martin des Pres. My opinion is that the coldest place was probably where the road crosses the stream at Uzel at 5.30 am

https://www.google.co.uk/maps/place/22460+Uzel,+France/@48.2762405,-2.86507,127a,35y,131.88h,45t/data=!3m1!1e3!4m5!3m4!1s0x4811d95387938199:0xffbca8948541d4c3!8m2!3d48.280398!4d-2.840971?hl=en&authuser=0

That point ticks all the boxes. By stationing ourselves at St Martin we were able to ask about temperatures in that section.
Obviously PBP is 1200km, and riders are at different places at different times. A rigorous approach would have been to spend a night by the stream in Uzel, but St Martin is fun.





Re: Why was it so cold?
« Reply #20 on: September 03, 2019, 10:27:07 am »
I interviewed a couple of riders from Mamers at the finish. One reckoned the cold had caused his Shermer's neck. It's in French, well their bit is, I'm speaking my own version.

https://www.youtube.com/watch?v=fadx3AOD8Ms