hill climbs in bicycling

The Tour de France, the famous annual race in France, rates each climb within each day's stage, in an attempt to characterize the difficulty of the climb as it will be experienced by a rider in the race. A Tour official drives over the route months prior to the race and places each climb in one of 5 categories (one of which is HC, “outside categories,” for the most difficult climbs). This official takes into account such factors as:

the average grade
the steepest grade
the increase in altitude from the bottom to the top of the hill
the absolute altitude of the summit
the condition of the road
where in the stage the climb falls. A hill at the end of the stage, when riders are tired, might be placed in a higher (smaller number) category than the same hill would receive if it were the first climb in the stage.

Here are some examples of categories assigned to climbs in the 2007 Tour de France.

Category	Examples
Category	Name	Length in km	Average grade
HC (hors categorie)	Port de Larrau	14.2	8%
	Port de Pailhéres	16.8	7.2%
	Col d'Galibier	17.5	6.9%
	Port de Balés	19.2	6.2%
	Col d'Iseran	15	6%
1	Col de Menté	7	8.1%
	Col de Peyresourde	9.7	7.8%
	Col de Marie-Blanque	9.3	7.7%
	Col de la Colombière	16	6.7%
	Col d'Télégraphe	12	6.7%
	Col de la Pierre St.-Martin	14	5.2%
2	Col de Portet d'Aspet	5.7	6.9%
	Col de Port	11.4	5.3%
	Col de St.-Sarraile	9	5.2%
3	Côte de Corlier	5.9	5.5%
4	Côte de Pujos	1.3	6.4%
	Côte Peguin	4.3	4.1%
	Côte du Pal de Pailhès	4.5	3.6%
	Côte de Parniers	3.9	2.9%

This system of rating climbs has been picked up by cyclists elsewhere, but there seems to be no objective specification for arriving at a rating. In any particular location, however, a category 1 will be more difficult than a category 2, and so on. In some areas cyclists use a hill rating (HR) system in which HR4 is the most difficult climb and HR1 the easiest.

An old story is that the Tour ratings are made by driving an old Citroen up the hill. A hill that the car can climb in third gear is a category 3; in second gear, a category 2; and so on to a hill so steep that the car can't even climb it, which is an HC.