Hi,

I don't have quantitative numbers but will offer these points:

• loss of momentum - rain is 600 times denser than air and it doesn't 'flow around' the vehicle. Put your hand out in the rain while driving (use slower speeds, not highway) and you can feel the effect.
• tire loss of momentum - the splash is a loss of energy. Just drive into a standing puddle of water and the car will all but come to a halt. A water tray is used to stop rocket sleds, FAST.
• cooling - water evaporative cooling takes a lot of heat out of every warm, exposed surface including radiators, engine blocks and transmissions. These parts see an effective, lower temperature. Our hybrids seem especially sensitive to cooler temperatures.
• humidity - the water vapor does not burn so engine power is somewhat reduced and the throttle has to open wider. In mitigation, H(2)O vapor weights less than O(2) and N(2) but not enough to make up for the power loss at a given throttle setting.

In aviation, we have forumulas for calculating the air density which includes humidity. Temperature is the greater componet but humidity is there:

http://wahiduddin.net/calc/calc_da_rh.htm

Bob Wilson

Operation Iraqi Oil Freedom:

Automatic, stock, project car.

My other 1500 cc car:

Automatic, stock, backup car.
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