Since we are running around like headless chickens, what's the worst conditions you've ever run in

Beijing HASH race summer 1991.

90 million degrees, 99% humidity - your sweat could not evaporate so you just cooked as you ran.

but the beer they poured over me at the end was awesome - especially the one that went down my throat 

Tie

Three hour trail run in an ice storm sleet + freezing rain
and
July gnat hatching, 90 degrees, 90 percent humidity, and the air was nearly black with gnats.I t took me hours to find all the little carcasses

I ran in Lamoni IA once, it was really hot and pretty miserable, and some guy on a combine nearly ran me into a ditch.

Nitrates, eh...I guess that's why I felt like a piece of bacon.

Hey Hey Hey....you might just learn something

Last year I ran my 3hr training run in 7F and snow falling with biting wind.  The dog didn't enjoy that one much!

my first tri wsa in a downpour remnants of a hurricane (yuck)

last week I ran in 34 degree wind, rain slop (a first), Sunday I did 33 deg and moderately high winds.  I'll take that over 98 degrees with 80% humidity though ...done a few of them too

Different extremes .....

A few weeks ago I did a 15 miler starting at around 4:30 in the morning.  My little weather station thingy said 8 degrees and there was a light wind.  The key thing is that my water bottle I was carrying and drinking from turned into a slushie about 12 miles into it and I couldn't drink from it as it wouldn't come out of the spout anymore!

The other is when I was stationed in Saudi during Desert Storm and went out for an afternoon run in late April (when I finally had a day off after 4 months straight - no joke there).  Only went for about 5 miles, but got back to my tent and was totally wiped.  Couldn't figure out why until I was listening to AFN and they said the local temp was about 135 ........ oops!!  That would explain a lot!

ha ha ha ha ha

Touche.

Mine started in tropical (and I mean equatorial, none of this Hawaii BS) heat, which was interrupted by intermittent torrential rain and rapid pending darkness. Meanwhile, I was alternately lost in forest and endless rice paddies, and there were some gorges--the only way to cross 'em was on a double bamboo pole . Oh--and no water.

Please do not ask how I found myself in those circumstances, but it might have had something to do with an international and insane running-ish club.

If you know that you are going to be racing in a specific condition it is always good to acclimate yourself to that condition.  If I recall correctly, it takes about 2 - 3 weeks to adapt to atmospheric/temperature differences.

So if you are planning to race in Baja Mexico but train in Canada it may make sense to do some indoor stuff with controlled higher temps to acclimate yourself.

As far as cold?  Colder air is more "dense" in oxygen as molecules slow down their movement and condense.  But with the level of changes we are talking about there is not a huge affect.  For me personally, the ideal racing temp for running seems to be right around 55 - 60 degrees.  Cool enough to not overheat, but warm enough for shorts and a tech tee without excessive sweating that I notice.

Cold: 4 degrees and foggy. We were costed in ice by the end from the moisture in the air.

Hot: 104. but it was a dry heat. Like 17% RH.