By Claire Glossop
BBC Sport website user
I've just returned from cycling across the Pyrenees in 10 days and covered most of stages 14 to 16 on my trip.
There are several factors that determine how hard it is to climb a col (mountain) and it can be much easier up one side than another.
Most climbs are gentler than UK roads - but vastly longer. The key trick is to know your col well so you know where it hurts most.
Riders can expect breathtaking views in the Pyrenees
If you're me, at this point, you can stop and enjoy the view and eat a banana for energy and, if you're Lance Armstrong, this is the bit when you attack.
I've cycled some of these climbs a number of times and it really does help to know the route well.
The killer factors are the gradient, how many other cols you've had to ride that day (or day before) and the weather - the heat really gets you as it is hot work climbing, especially in a helmet.
Climbing a steep col in the summer heat is a bit like microwaving your brain - it feels like it is swelling and the smell of hot tar stays with you. I'm going to have nightmares about the smell for years to come.
And some of these climbs can be exposed - weather can change very quickly from hot to rain and the wind (especially on the Port de Pailheres - stage 14 this year) can be strong. It nearly blew us off our bikes.
Riding this route many of the roads were being resurfaced in preparation - but graffiti had already begun to appear. Someone has very accurately identified the worst parts of the Col de Marie Blanque ascension (stage 16) and written c'est dur (it's hard) in several places.
And it is very steep (I was glad to be descending the side the Tour goes up). But later that day the riders get to follow perhaps the most beautiful road in the Pyrenees - on the Aubisque (between Aubisque and towards Col de Soulor).
Any fan can ride the same route, albeit it much slower
The road is carved into the rock shoulder of the mountain like a balcony, offering expansive views. I can't wait to see the peloton whiz along it. The mountain views around the Col d'Aubisque are also some of the most spectacular on the Tour.
The first Pyreneen mountain stage is pretty easy - the Port de Pailheres. It was my first 2,000m col and tough in blustery wind, but the pros will be up it in 40 minutes as opposed to my 90.
It has a couple of steep sections - notably on the hairpin ramps above the ski station of Mijanes. There are no really steep sections and so, while it's nearly as high as the Tourmalet, it's not as hard. Then a mountain-top finish at Ax - but not too high.
The second day (stage 15) is a rollercoaster. The Col de Portet d'Aspet has a deceptively steep section below the summit and a descent that has been notorious since the death of Fabio Casartelli there in 1995.
The Col de Mente rises quickly and steeply - it's not a picnic although there is a café at the top (although I doubt the riders can stop for an ice cream like I did).
A beautiful sweeping descent - again the scene of a famous accident when Luis Ocana crashed out of the 1971 Tour wearing the yellow jersey ahead of Eddy Merckx. So unusual was it for anyone to lead Merckx the French have put up a memorial.
Makes you think carefully as you descend - all these memorials - and I was only doing 50-60km/h which was pretty terrifying - the Tour will be going much faster.
Climbing these cols for me was tough - you just have to keep turning the pedals and scoffing the power gels.
Taking enough water when it's hot and carrying warm gear in case it rains are a drag that the pros don't have to contend with but then again at least I didn't have to race up these monsters - to get to the top was sufficient challenge for me.