The high temperatures over 40°C that the riders found on the Spanish roads these days have surely influenced on their performances: increases in body temperature and dehydration brought the most common problems in these first 10 days of racing.

Together with the metabolic heat produced by muscles working there is the radiation heat coming from sun exposition: we have barely seen areas with a bit of shade in the semi-desert landscape of south-western Spain.

An effort producing 60Kcal is enough to cause an increase by 1°C in body temperature: a few minutes would therefore be sufficient to reach decidedly dangerous body temperatures (above 40-41°C), were it not for compensation mechanisms that allow heat dispersion.

CONVECTION dispersion takes place when skin temperature is higher than air temperatue: the thin layer of air touching the skin heats up and the skin cools down, reducing the body temperature.
Obviously this mechanism is impaired if the environmental temperature exceeds 38-40° C.

The EVAPORATION of sweat is an efficient mechanism (if air humidity is not too high): every gram of sweat that evaporates takes away 0.6 Kcal, whereas the sweat that falls to the ground or that impregnates the clothes has no effect on heat reduction.
It is known that heavy perspiration exposes to the risk of dehydration.

CONDUCTION dispersion, that is the contact of the organism with fluids of colder temperature, can be determining, given the possibility to pour over the body such fluids often enough or to ingest cool beverages
(10-15°).

The maximum perspiration in an athlete was measured at 3.7 litres/hour on marathon runner Alberto Salazar (Physician Sportsmed., 14:73-81,1986)
The fastest rate of gastric emptying (therefore of fluids absorption) is at 1-1.5 litres/hour, and this drops under heavy effort. 
Obviously in extreme heat conditions the risk of dehydration is very high even though the athlete takes as many fluids as he can absorb: every 1% of body weight loss (i.e. loss of water) means an increase in body temperature by 0.2-0.4 °C.
Fluids deficit reduces plasma volume, stroke volume and increases heart rate. Also red blood cells lose vlume and functionality.

A reduction of 2% in body weight due to dehydration impairs performances by up to 8-10%;
thhis means that every 30' of continuous effort, there is a loss of 2' (Med.Sci.Sports Exerc., 17:456-461, 1985).

So far we've seen a great show in the race, also because of an interesting and varied course since the very first stages.
We had also some excellent uphill performance by the race favorites: Di Luca on the Covatilla and Vinokourov on the Alto de la Cobertoria expressed VAM values of about 1680 m/h.
There is a great balance among the favorites, with 5 riders enclosed in a gap of 90 seconds: eventually the smartest rider in managing the race will prevail over the others.