Gonna be a scorcher in Austin today

[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).

[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]

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

[bekeselassie]

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

[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.

[La_Spigola_Loca]

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.

I suppose that JMR knows more on this topic.

A 7% or 30% drop in air resistance still doesn't tell me how much of the power input goes to overcoming air resistance. But ponder this- even if you find 4.5% too little (I personally don't, but it's very difficult to estimate such a thing from pure experience- ever tried to evaluate the viscosity of water you were swimming in? we are not really accurate measuring equipments :wink, I think 10% would be a gross overestimation, and even 10% would give a difference betweeen running 10.00 and 9.99 uncder conditions of 0% and 100% humidity, respectively.

[26mi235]

If you have any figures for me, that is, total force required to overcome air resistance, at a given speed and for a rider of a given weight, along with the ratio of the rider 's "surface area" (facing the wind) to the total surface area of the rider + bicycle, they're more than welcome. I had to use an "accepted" drag coefficient for "humans" of 0.45 kg/m (at a stretched position). Again, any value better than that is more than welcome.

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.

I suppose that JMR knows more on this topic.

[La_Spigola_Loca]

If you have any figures for me, that is, total force required to overcome air resistance, at a given speed and for a rider of a given weight, along with the ratio of the rider 's "surface area" (facing the wind) to the total surface area of the rider + bicycle, they're more than welcome. I had to use an "accepted" drag coefficient for "humans" of 0.45 kg/m (at a stretched position). Again, any value better than that is more than welcome.

[26mi235]

[quote=La_Spigola_Loca]

My most simplified calculation for 10.00 at 0% humidity gives 9.95 at 100% hunidity.

Eeep! I meant 9.995.
Let's do this again. The average force of your "average elite" sprinter, say a 70 kg guy, is around 4000 N, applied about 25% of the time of the race. Let's simplify this and talk averages:
100m, 10.00 at 0% humidity.
4000 N *.25 = 1000 N
Let's assume 10 m/s is quick enough for a drag impact, where the force ius proportional to the square of the body's velocity, or F(drag) = qV^2, where V is the body's veocity, and q is the drag coefficient. q for your average guy (say 1.75 to 1.80, 70 to 75 kg) is about 0.45 kg/m. Now 0.45 kg/m * (10 m/s)^2 = F(drag) =~45 N, applicable throughout the race. So we can assume the guy applies on average 1045 N, out if which 45 are F(drag).
Now F(drag) is linearly dependent on air density. On going from 0% humidity to 100% we'd be changing air density by about 2% (at 20°C and atmospheric pressure, the difference is between 1.204 kg/m^3 and 1.176 kg/m^3), or decrease F(drag) by around 45 N *.02 =~0.9 N, which our sprinter can now put into his speed generating force, increasing it from 1000 N to 1000.9 N.
10.00/(1000.9/1000)^0.5 = 9.9955. So, not really a measurable difference there.[/quote:14fsjl3l]

Without going through the math in the limited time that I have it is my impression that at 100m speeds of 10m/sec, the wind resistence is greater than 4.5% of the effort. Now the figure that I remember from bicycling is that at 20mph most of the effort is overcoming wind resistence. At 10m/sec there is more running friction than other bicycle frictional losses. However, it would take a lot for the wind resistence to drop to 4.5%

[La_Spigola_Loca]

My most simplified calculation for 10.00 at 0% humidity gives 9.95 at 100% hunidity.

Eeep! I meant 9.995.
Let's do this again. The average force of your "average elite" sprinter, say a 70 kg guy, is around 4000 N, applied about 25% of the time of the race. Let's simplify this and talk averages:
100m, 10.00 at 0% humidity.
4000 N *.25 = 1000 N
Let's assume 10 m/s is quick enough for a drag impact, where the force ius proportional to the square of the body's velocity, or F(drag) = qV^2, where V is the body's veocity, and q is the drag coefficient. q for your average guy (say 1.75 to 1.80, 70 to 75 kg) is about 0.45 kg/m. Now 0.45 kg/m * (10 m/s)^2 = F(drag) =~45 N, applicable throughout the race. So we can assume the guy applies on average 1045 N, out if which 45 are F(drag).
Now F(drag) is linearly dependent on air density. On going from 0% humidity to 100% we'd be changing air density by about 2% (at 20°C and atmospheric pressure, the difference is between 1.204 kg/m^3 and 1.176 kg/m^3), or decrease F(drag) by around 45 N *.02 =~0.9 N, which our sprinter can now put into his speed generating force, increasing it from 1000 N to 1000.9 N.
10.00/(1000.9/1000)^0.5 = 9.9955. So, not really a measurable difference there.

[tafnut]

My guess is any benefit is only measurable at velocities much higher than any sprinter can run.

I meant to posit that as well in my response. The 'aid' HAS to be less than .01 sec (IVHMO).

Empirical evidence suggests otherwise.

I just used JRM's algorithm and using 10.00 on a 25C day with 1000 mb, zero wind, zero altitude I get zero difference between 100% humidity and 0% humdity. Am I doing something wrong?

[tlb747]

The perfect weather for a track meet is 55 degrees, overcast and no wind. 8)

I think both sprinter/jumper types and spectators prefer something a bit warmer.

Yeah, and I would be the exteme example because I love the heat. Perfect track'n'field conditions FOR ME is a least 90 degrees, with hardly any wind and humidity. I remember the 1998 Texas Relays (last held at the football stadium) was quite humid for me because I was running in Arizona at the time. As soon as I walked out of the Austin Airport, is was like the humdity just slapped me in the face. It was so bad that I can see it, slowly falling to the ground, almost like fog. The city is known to be a "Clean Air" city, but I know it as a "Slap in my face humid city."

[donley2]

Here is JRM weighing in on the previous thread

http://www.trackandfieldnews.com/discussion/viewtopic.php?t=15273

[mike renfro]

The perfect weather for a track meet is 55 degrees, overcast and no wind. 8)

Not if you are sitting in the stands, shorts, sandals, tshirt optional. Give me the suntan days!

[DrJay]

No doubt temperature affects longer distances, but the 1500? I don't know for sure, but here's something to consider. Even if the race distance doesn't seem to come into play so much in the heat, how about the warmups plus the race? For an 800 or mile my typical warmup was a mile-and-a-half before my accelerations and such. It took that much before I really felt everything was flowing just right.

In late spring, I'd step to the starting line already dripping sweat sometimes. So could it be argued that if we include the warmups then not only the 1500 is affected by the heat, but maybe the 800 as well?

Good point. I think it's up in the air as to whether the 1500 is slowed by hot temps, but if it's not, you can bet the athletes feel it worse in the 15 minutes or so after a fast (near PR) race. I bet if you measured core temps 5 or 10 minutes after the finish of a 1500 on an 85F day, you'd find some values in the 102-103 range, with said warmup setting things up for that.