Can you predict if a cycling route is warm or cold?

An exploration into predicting what rides are going to be colder than others.

5 mins
Written by
Steve Owens

Building on my previous post about Digital Elevation Models (DEMs) and Open Street Map (OSM) data I wanted to do a short exploration piece, looking at what else we can do with DEMs in the context of cycling. 

One question that jumped out to me a few weeks ago in the middle of a Huli ride was - “Can I figure out what route will give me the most sunlight/warmth?” (It was a cold day!)

Using a DEM and the Huli cycling road/trail network, let’s see what we can figure out. So thinking about this, especially in winter, and particularly in Scotland or other higher latitude areas, we can say that the sun is pretty low in the sky and you’re limited in daylight. The fact that the sun is low in the sky means that roads and trails that are obscured by hills are going to be in shade for longer, and therefore colder. So can we use our DEM to figure this out? Well, yes otherwise this would be a pretty short post…

The Basics

The cotext is a cold Sunday morning in the middle of January, where we set off just after 8am. The specific date was 16th January, and using a handy wee tool that tells you all your sunlight info, you can get a feel for how muc sunlight is on offer for the day, and some high-level info:

Sun info for 16/01/2022 in Milngavie

So 7.41 hours of daylight, with dawn at 7:52 and sunrise at 8:36, and the sun at the highest point in the sky at 12:27. The temperature was forecast to be 4°C at 8am, rising to 7°C by midday

So great, that gives us a little bit of info, but doesn’t really help with the specific route I was planning to do. For that, I turn to QGIS and do some initial analysis on the DEM. QGIS has a lot of built in functionality that will help such as tools to calculate slope aspect and slope angle.

Slope Aspect

  • In the image below, generally 🟩 = South (in sunlight), 🟥 = North (in shade).
  • Relevance to route - South facing paths/roads will get the sun sooner than their North Facing counterparts, and will therefore be warmer. This is why the North Face of mountains will be the side that keeps snow longest. 
  • Fun fact - If you like the brand The North Face, you can read why they are named so here, it’s all related! Quoting from their website - “We are named for the coldest, most unforgiving side of a mountain”. 
Slope aspect for Scotland extract

Slope Angle

  • In the image below, 🟩 = 0°, 🟥 = anything over 40°, and varying shades of 🟩 and 🟨 for everything in between. This is interesting as it gives us a clear picture of where steep areas are. In a future post, I’ll revisit this to look at how that can help us find great descents/climbs and what is hike-a-bike or not!
  • Relevance to route: The slope angle, along with the elevation change from the top to bottom of the slope, helps you figure out how far a shadow will reach across the land. Generally, shallow slope = small reach, steep slope = big reach.
Slope angle for Scotland extract

Bringing the pieces of the puzzle together

With these bits of information we can figure out exactly when we’ll see/feel sunlight. I actually found an incredible little tool called shademap built by @truted that plays out a GPX with the shade map updating along the way, so you can see exactly when I was in or out of shadow. Really incredible tool and I’m in awe of the processing that’s gone into this to create such a seamless feel!

Plotting my ride through the shadows

And luckily I even took a few pics demonstrating it working. In this first pic we're in shade, and the second one is the first bit of sunlight on us for the ride - good enough at the time for me to stop and take a picture!

8:48 - This is cool as you can actually see the sun hitting the face of Dumgoyne, as predicted by shademap 😄

Science in action

8:57 - Looking back into the sun

Don't look back into the Sun... 🎶

A future Huli routing option?

So coming back to the point of the post - how do I use this information to bring sunlight into route choice. In the context of my route, then perhaps I should have done this route in the other direction to maximise sun-time as the latter part of the route gets sun first. But what if Huli could do something more with this? What if we could suggest the route that will give you the most sunlight/keep you warm? Similarly, on the flip side while dreaming of summer, you could use the same principal to find routes that are coolest? I don't think I'll be using that option in Scotland in a hurry, so perhaps one for warmer climes 😂

Who knows - Perhaps in the future Huli will have an option to select the warmest/coolest ride, wouldn’t that be nice? 

See ya on the (warm) trails. 


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One more fun fact before I go...

One of my other hobbies, backcountry skiing ⛷, requires a lot of understanding around slope angle and aspect. Angle is really important as avalanches are most prone on slopes greater than 30°, and aspect is important for understanding snow stability. This has to be paired with wind data to get a full understanding of conditions. If you are interested then the Scottish Avalanche Information Service provide some quality information and also provide avalanche reports daily throughout the winter. As a proxy indicator for the lack of Scottish Snow this season, there have only been 17 avalanches so far compared to a total 212 last year, so cross your fingers and toes and pray to the Snow gods for a better end to the season.

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