Col du Tourmalet - East Climb from Sainte-Marie de Campan.

Velocity Ascended, Metres per hour Vm/h usually referred to as VAM, coined by Dr Michele Ferrari is the speed of elevation gain per hour in Metre per hour. This is a term used in cycling to rate how fast an individual gains elevation while climbing an incline. Ferrari also stated that every one percent increase in average gradient increases VAM by 50. For example, a 1650 VAM on a climb of 8 percent average grade is a performance equivalent to a VAM of 1700 on 9 percent average grade. Ambient conditions (e.g. friction, air resistance) have less effect on steeper slopes (absorb less power) since speeds are lower than on gentler slopes
The acronym VAM is not truly expanded in English, where many think the V stands in some way for vertical, and the M represents meters, for instance "Vertical Ascent Meters/Hour." Ferrari says,
I called this parameter Average Ascent Speed (‘VAM’ in its Italian abbreviation from Velocità Ascensionale Media).
A direct translation of "velocità ascensionale media" is "mean (average) ascent velocity" leading to an expansion of the acronym in English as Velocity, Ascent, Mean.
VAM is calculated the following way: VAM = (metres ascended x 60) / Minutes it took to ascend
A standard unit term with the same meaning is Vm/h, vertical meters per hour; the two are used interchangeably.
The relationship between VAM and relative power output is expressed as follows: [3]
Relative power (Watts/kg) = VAM (meters/hour) / (Gradient factor x 100)
This gradient factor ranges between 2.6 for a gradient of 6% and 3.1 for a gradient of 11%
To work out the gradient factor take 2 + (% grade/10)

Examples:

1800+ Vm/h: Lance Armstrong - and Marco Pantani of olden days
1650-1800 Vm/h: Top 10 / Tour de France GC or mountain stage winner.
1450-1650 Vm/h: Top 20 / Tour de France GC; top 20 on tough mountain stage.
1300-1450 Vm/h: Finishing Tour de France mountain stages in peloton
1100-1300 Vm/h: The Autobus Crew