Gonna be a scorcher in Austin today

[La_Spigola_Loca]

An interesting point to note- a temperature increase of 5 degrees has the same effect on air density as going from 0% to 100% humidity.

[bekeselassie]

The answer to my question ceased being interesting long ago. :roll:

[JRM]

Just back from a conference in Tucson on quantum mechanical effects in brain functions, and only now saw GH's invite to this thread. So, I'm joining in this conversation a bit late. Briefly, though the combination of humidity, temperature, and physical elevation above sea level all contribute to something called "density altitude". This is the effective altitude that one would have to be above sea level (at "standard" temperature, humidity and pressure conditions) to get that equivalent air density. So, changes in humidity, temp, etc..., amount to "corrections" to the altitude's venue. Higher altitude means lower air density, which means lower drag forces acting on sprinters, which means they can use reach higher speeds (and thus go faster).

For example, Mt. SAC is about 200m above sea level, but if it's very hot there, it will seem like it's much higher. Likewise, if it's chilly, it will seem to be lower than 200m.

1. Humidity decreases air density and has the effect of simulating a higher altitude venue. This is because the water molecules displace the oxygen and nigtrogen molecules in a given volume of air (gh posted something to this effect early in the thread), and thus decrease the number of these molecules (i.e. lower density).

2. Humidity variations have a very low effect. Between 25% and 100% humidity, the variations are less than 0.01s. 0% humidity is not a realistic condition for competition.

3. Temperature changes have a bigger, but still almost negligible, effect on times. A 20 degree (celsius) difference from 15 to 35 C will change times by only about 0.02s. This temperature difference can simulate an altitude change of about 500-600 metres.

3. Combined effects of humidity (and temperature) and altitude-related air density changes, however, do make an important difference. So, ideal sprint conditions would be a hot, humid day in Mexico City, which will provide a huge improvement over a cold, dry day at sea level.

The following links may be useful to various individuals:

Density altitude calculator: http://myweb.lmu.edu/jmureika/track/Den ... itude.html

Short report on density altitude effects in the 100m (not too technical):
http://arxiv.org/abs/physics/0505118

Longer report on density altitude effects in both 100 and 200m (a bit more technical, but you can skip the math and go straight to the results):
http://arxiv.org/abs/physics/0508223

[La_Spigola_Loca]

^So I did use a right drag coefficient... Like you say, a 20 degree difference between 15°C and 35°C corresponds to about 0.02 s or 20 ms, a 5 degree difference is worth about 5 ms, and going from 25% to 100% humidity is worth about 3.5 ms ( worth about a 4 degree difference).

[JRM]

Because wind resistence is so important in bicycle riding/racing there is a much more developed literature (and culture) on the topic. A long time ago there was a book (Bicycle Science?) put out by the MIT press that was fairly definative at the time, but I am sure there are better things available. Another comment I remember that is running specific is that the reduction in wind resistence when drafting behind a runner at sub-4 mile pace was 7%. The wind resistence should be just more than double at sprint speeds and I seem to remember that "drafting" reduces wind resistence by 30%, but certainly that depends on geometry and speed.

Wind effects are more pronounced in cycling (and speed skating) because they move very fast. Drag forces are a function of the athlete's speed *squared*. So, the ratio of the drag force acting on a sprinter (10 m/s) versus a middle distance runner (5m/s) is 100/25 = 4. So, doubling your speed means you have to fight 4 times the amount of drag. Cyclists and speed skaters have an even greater challenge.

However, once you hit a certain speed, something odd happens with the way the air flows around you. If you aren't moving very fast, the air flow around you is turbulent -- a bunch of lilttle vortices and eddies contribute to drag forces and slowing you down. However, at some critical speed, a transition occurs and the air flow becomes *extremely* smooth (called 'laminar' flow). No more turbulence, and the drag forces drop sharply.

That's why planes, cars, and ultimately cyclists are aerodynamically-shaped. It facilitates the transition to laminar air flow, so they don't have a lot of drag forces to fight (and can therefore go much faster).