"Changes in temperature and humidity do two things .......

Temperature change alters the density of the air. More dense, more oxygen with each intake of breath. i.e. colder weather makes you faster. One of the main reasons you see the majority of marathons in the Spring/Fall.

Humidity is a different story. The bulk of your energy that you use in running is actually lost as heat within your body. The only way for your body to shed heat is through the skin. Hence sweating. Cooler air and wind will allow this to happen faster. High humidity however will not. If the air around your body is already saturated, then it will be really hard for your sweat to evaporate off of the skin. So you do not cool down as well. Your bodies reaction? Heart Rate (HR) goes up to try and get more oxygen to the muscles and get things cooler, and your pace drops so you will stop generating as much heat within. "

http://www.beginnertriathlete.com/discussion/forums/thread-view.asp?tid=133926&posts=16

Got this from the internet while reading some articles on running and training.

today, 12km run, excruciating.

Humidity and temperature really spent me.

tired