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July 14, 2010 / Dave Gorham

Tour de France: Hot AND Cold

An update from Le Tour de France: It’s cold!

Tour de France racers yesterday on a very long 130-mile stage, which included summiting the Col de la Madeleine. Photo: Yahoo / AFP.

True enough, western (and now central) Europe have been in the throes of a heat wave this past week. Riders of the Tour de France have been taking extra precautions while they battle the heat. It’s even been reported that Lance Armstrong has his own unique brew of “water” that is formulated specifically for his body. This “super water” replenishes exactly what he needs (electrolytes, potassium, sodium, et al) as efficiently as possible; no other Tour racer has access to his formula. (If you’re tired of seeing Lance Armstrong pop-up ads with the huckster-ish “Are you tired of being tired?” then get ready for the post-Tour marketing blitz for his new elixir.)

Back to the weather. Cold? During a July heat wave?

I’m a bit misleading. I’m suggesting that the entire route of the Tour is cold, when if fact it’s only the mountain tops. Temperatures yesterday at the summit of the Col de la Madeleine (2,000 meters; 6,562 feet) were in the low-mid 50Fs (10-12C). How is this possible? Doesn’t hot air rise? Shouldn’t a heat wave translate to lots of hot air rising? Yes. And no.

Yes, hot air rises, but no it doesn’t translate to warmer mountain tops. And to clarify: “hot” air is only hot relative to the temperature of other air parcels around it. If you place a parcel of 60F air next to a parcel of 50F air, the 60F air parcel is considered “hot” (or hotter) and will rise. What happens with a dry air parcel (dry = non-saturated, or temperature higher than the dew point) rising with altitude is a bit more complicated.

Warm air molecules are warm because they’re compressed. As molecules vibrate in a compressed state, they bump against their neighbors and generate heat. As air rises and the pressure drops, the molecules have more room to move while bumping into their neighbors less and less; less heat is the result. Combine higher altitude with continually lower pressures and the molecules really begin to shake off their heat. This explains why mountain tops are typically cooler than the valleys below. I remember a wise guy in one of my early meteorology classes asking about cooling molecules beginning to shiver — wouldn’t the increased vibrating create heat — even at altitude? No!

Lifting a parcel of air and its release of heat is called the adiabatic lapse rate it’s one of the foundations of meteorology. A dry air parcel will lose almost 10C (9.8C) of heat for every 1000 meters of gained altitude. Lift a 20C parcel of air 2000 meters from sea level and it will shed almost 20C of heat, all other factors being equal. This would certainly explain the cooler temperatures at the top of the Col de la Madeleine yesterday (and every day).

This view of the Col de la Madeleine is from the 2001 Tour de France: spectators and officials are bundled up against the cold in July. Photo:


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