The Track is HOT!

Humidity, Heat and Speed



Why track records are falling at Athens

"I wasn't looking at a world record here, but the conditions were hot and dry, and that's when the track is always fastest," said Olympic champion Anna Meares after her Olympic and world record-breaking ride in the women's 500m time trial. As it happens Meares was half right - hot, humid weather provides the best conditions for breaking records on the track. As John Stevenson explains, it's all a matter of gas physics.

Why are track records falling at Athens, a sea-level velodrome that wasn't expected to be fast? In short, it's the heat and humidity. Hot air is less dense, and therefore easier to push through and increased humidity involves the air being partly replaced with less dense water vapour. Combine the two - especially as hot air can contain more water vapour than cold air - and conditions are right for records. Read on for a more detailed explanation.

In cycling, the rider's biggest enemy - after his or her competitors - is the air. In pure against-the-clock events such as the kilo, 500m and pursuit, the factor that can make a difference between breaking a world or Olympic record and not is the resistance from the mixture of gases that riders have to punch their way through on their frantic dash to the finish line.

Assuming it's not windy - the case for indoor velodromes and conditions at the Athens track events so far have been less affected by wind in the partially-open velodrome than was feared - the air property that matters most is its density. Fluids such as air and water are harder to move through the more dense they are, which is why it's a lot harder to ride a bike through a river than along a road.

The difference between the density of the air at one time and another is less extreme, but it's still significant when you're measuring a cyclist to a thousandth of a second over four kilometers.

Three things affect air density: altitude, temperature and humidity. To understand what's going on here, it's convenient to imagine the molecules and atoms that make up the air as being 'point masses' - that is, specks of matter with mass but no volume. The heat energy of the gas molecules causes them to whiz about all over the place like a Greek sprinter evading a drug test, and in the process the bulk gas expands to fill whatever container it's in - or until it's constrained by some other factor like the Earth's gravity.

Gravity keeps the atmosphere in place and weight of the atmosphere compresses the air at sea level to the density we're all familiar with. As you go higher there's less air above you to compress the air around you, its density is lower and you go faster for a given effort. This is why track records have traditionally been set at altitude. Arnoud Tournant's 58.87 second record for the kilometer was set at the La Paz velodrome in Bolivia at an altitude of 3,417m, for example.

Temperature has a similar effect. By increasing the speed at which the air molecules move, a higher temperature causes the bulk gas to occupy more volume, if it can. The Athens velodrome isn't airtight (a good job too, as all the spectators who turned out to have been patiently waiting for the track racing to start before showing their faces last night would have suffocated if it were) so higher temperatures mean less dense air, and faster times.

The effect of humidity is perhaps the least obvious. Humidity is a measure of the amount of water vapour the air contains - in other words, it's an indication of the composition of the air. Perfectly dry air consists mostly of oxygen and nitrogen, with a bit of an inert gas called argon and a trace of carbon dioxide. For our purposes we can ignore everything but the nitrogen and oxygen, which make up 78 percent and 21 percent of the air, respectively. As the humidity increases a small but significant amount of water vapour is added to the mix.

The air's density is affected by its volume, as we've seen with the effect of altitude and temperature, but also by its average mass. Because air behaves as a bunch of point masses whizzing about, any given number of gas molecules will occupy the same volume at a given temperature and pressure.

The mass of a molecule of gas is determined by the masses of the atoms that come together to form it. A water molecule is made up of two atoms of hydrogen and one of oxygen - the good old chemical formula H2O. Hydrogen atoms are the lightest atoms around, so their 'atomic weight' is considered to be 1 and the weight of all other atoms is expressed proportional to a hydrogen atom. (Actually the baseline is 1/12 of the weight of a carbon-12 atom, but for our purposes it's the same thing). An oxygen atom has an atomic weight of 16 and a nitrogen atom a weight of 14.

There are two atoms in the oxygen and nitrogen molecules in the air, so their molecular masses are 32 and 28 respectively. The molecular weight of water, however, is just 18: 1 + 1 + 16. That means that as you increase the humidity, water molecules displace some of the oxygen and nitrogen molecules. The volume stays the same, so the density of the air goes down and, once again, times decrease.