Graham Handrichard wrote:
"Graeme Obree was twice individual pursuit world champion and two-time holder of the one-hour distance record, and is a man known for his analytical approach. ‘When you’re waiting for a downpour and it’s calm – that’s the sweet spot,’ he says. ‘The three perfect conditions are high temperature, high humidity and low atmospheric pressure. Humid summer evenings tend to be quicker, when you can almost smell the water in the air. You don’t get those conditions very often, and when they come together it’s like “wow”, you get your best form of the season. That’s the night when you should use your fastest wheels.’"
http://www.cyclist.co.uk/in-depth/917/are-you-faster-when-it-rains
The thing that you are missing is that it is not the relative humidity that matters a bit. 100% RH and 50 degrees means that the dew point is 50 degrees and the amount of water in the air is pretty low. So, since it is 67 and the RH at ground level is probably not 100% (that is happening at a higher elevation the causes the moister to precipitate) so the dew point is probably no more than 65. The difference in H2O in the air between the level Orbee is talking about (at least 75 for dew point and probably close to 80, is very large and the help for thinning the air is much lower with the dew point of 65. The real lesson is that for these types of issues, relative humidity is useless as a concept to use because it misleads people - it is the dew point that is the main factor here.
Also of importance is that as the temperature rises the air gets thinner (when it gets too hot in Phoenix they have to ground some of the planes that do not have enough lift to take off because the air is too thin). In addition, notice that Orgee also mentions low pressure because that means that the air is less dense. Of course, another way to diminish the density is to go to a higher elevation - think Beamon in Mexico City, and note to additional things, a huge thunderstorm started right after is jump, and the actual wind was almost certainly (90% chance) higher than 2.0 mps because the guy running the anniometer at the time had things screwed up and was rounding down all of the readings to the next lower integer, so the wind was anywhere in the range of 2.00 to 2.99 mps.