The question isn’t a new one, but it arose this morning when discussing the feats of the Austin Marathon and Half Marathon runners. The forecast was a muggy one, with AM patchy fog in the area:
Fog itself is not a sort of precipitation, as most people think: it’s actually a type of atmospheric suspension, which is created by water vapor condensing into tiny water droplets that are light enough to remain in the air. Fog is most likely when there are fewer than 4 degrees that separate the air temperature and the dew point temperature. Note the places where fog exists when air and dew point temperatures are compared (as an example from this morning):
Where the dew points are generally within 4 degrees of the air temperature, you can see that visibility is reduced, due to fog formation. This example demonstrates that fog, made from water vapor, was present this morning. Water vapor is the real item we’re after.
A good blog on all things weather comes to us from Meteorologist Jeff Haby, who produces TheWeatherPrediction.com. Here is an interesting read on the ways that an object traveling on the Earth can be affected by the weather. Haby notes air pressure, air temperature, wind direction/speed and humidity, using the example of a baseball that has been hit and is traveling away from home plate.
(“Texas’ Kacy Clemens hits an RBI single against Houston in the fourth inning of an NCAA college baseball tournament super regional game in Austin, Texas, Friday, June 6, 2014. (AP Photo/Michael Thomas)”)
The chemical composition of a molecule of air allows for anywhere between 0.01% – 4.24% of breathable air be made up of water vapor. Check that out here. Below is a table of what air molecules are made from, but the gist is that Oxygen (O2) and Nitrogen (N2) are “heavy” at a molecular level, while water vapor is “light.”
(This image is from Wikipedia, which certainly doesn’t know everything, but usually gets its tables right!)
Combined, O2 and N2 make up about 99% of an air molecule. If there’s more water vapor in the air, it forces a displacement of N2 and O2 from the area (of equal pressure, temperature and volume) around it. Thanks to Avogadro’s number (it’s nerdy: check out the definition here), the number of molecules has to be the same. So lighter ones (water vapor) push the heavier ones (dry O2 and N2 molecules) out.
That means that, regardless of the way moist air FEELS sticky and sluggish, it is actually LIGHTER than dry air. If the air is lighter, there is less friction to force an object to slow down, like baseballs … or, in the case of today, humans.
The Austin Marathon and Half Marathon were also helped by a relatively mild air mass. Warmer air = lower air density. Higher temperatures force a parcel of air to expand, which increases volume and lowers density. We like to think about it like this: if you heat up plastic, sealed package of, say, broccoli (gross!) in the microwave, that package will expand ( = increased volume). If you were then to take that puffy package and measure how many pieces of broccoli were in each cubic inch of space, you’d find that they were fewer broccoli bits than when the package was cold and small. The density of air molecules works the same way!
BOTTOM LINE: with all other things being equal, a mild, moist air mass over Austin today may have helped propel the Austin Marathon and Half Marathon runners just a little bit faster. (Congratulations to all the finishers and winners of the race!)