How does running in heat/humidity and cooler weather affect the body?

Changes in temp 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?  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.

I guess I continue to be the odd duck.  I run BETTER in humid weather.  Probably because my running is so mental.  The best 2 mile time I ever did (12:03) in the military was on a hot humid morning at Ft. Gordon, GA.  Fast forward 20 years and 40 pounds, and I can't run fast in NJ because of the cool weather.  Yeah, that's it. 

I know a lot of racing is who shows up, but in my first 2 5k's this year, I was BOP for the first one, and MOP for the second one, and only got better by a few seconds.  However, the second race was "Africa Hot," and a lot of the runners wilted, while I felt great in the heat and only had to deal with my inability to run fast. 

I can't stand the humidity.  When I push too hard in the August hot and humid weather, I risk bonking.  Whereas yesterday I had my best 10k tempo run this year and it was 60 degrees out, cool and crisp.  So for me, 85 degrees and humid vs.  60 degrees and cool means a 3 to 4 minute difference in a 10k run. 

I read a while ago, too, a study about what temps are best to run in and the avg runner is fastest between temps of 55 and 65 degrees.    

OK, so that time shave-off really could have been attributed to the weather.  I don't know whether to be glad about that or depressed?

thanks for the comments.  

all i know is that when i stopped at 9.5, i felt awesome and as though i could have went another 3 or 4 miles with relative comfort.   actually really excited about my run tomorrow.   looking to go an even 10 miler.

Weather makes a huge difference, for some it is more extreme than others.  Lucky me, I live in the south (texas), and it's hotter than crap here 80% of the year... the bad thing is my body hates hot weather and acts accordingly... I'll sweat buckets, my heart rate will sky rocket and I have to slow down a lot.  Combined with 80-90% humidity and it's just a bad deal all around. 

I'm looking forward to the fall weather which should be here in a couple weeks.

My personal theory and philosophy, that I use to help motivate me through hot and humid runs, is that running in those conditions will make me that much faster when the cooler weather comes around.  So I believe regardless if you trained much over the summer, yes - you will be faster in the Fall ... but if you trained a lot and hard throughout the ugly humid summer weather, you will be even faster come Autumn. 

The varying weather is why training by HR is so critical.

Look at say 150 HR in the heat I am running 9:00 miles;  I am getting the same fitness with its cool, my HR is 150 and I am running 8:00miles. 

Based on the 5:21 I ran cold (no warm up) in 90+ degree temps on a rolling course after racing a bike race in the morning he has me training at 8:57 for my long runs????  That is a bad day recovery pace for me!

Yet another reason to not be such a fan of his methodology ............

Some days the heat really hits me and can barely manage sub 10:00's.  Other days it seems to have no impact at all and can crusie for miles at 8:30's staying in Z2.  I do think that longer runs are more adversely affected due to the body heating up and dehydration impact over time.  However, there remains much variability on the impact for me.

I'm waiting for Scout7 to chime in with "slow down or go faster, depending upon how you feel."

...or something to that effect...  

High humidity is usually not a factor where I live, however, I can attest to the slowing power of heat.

In a 5k this past August near the San Bernardino foothills, I ran a 27:15. My average HR was over 200 bpm. Normally, I can run 8:30 miles during training, staying in the 160 bpm range. The difference? I do most of my training runs around sunset (60's to 70's, breezy, not too dark or light, basically perfect). When the gun went off for this race at 8:00, it was already pushing 95 degrees.

If it is hot and humid I will be about 60" per mile slower if not more.

I choose races based on what the weather will be like. I'd rather race in 40's than 80's. It just how I am and always have been even as a kid.