I hear what you're saying, and it makes sense, sort of. I just took a look at today's readings for Oxford, MS and it's clear that the dewpoint has spent most of the daylight hours, from 7:15 AM to noon, in the vicinity of the 73.4 degrees max. That specific figure shows up repeatedly. It shows up early, when the temp was 73.4 degrees and the humidity was 100%. It shows up at noon when the temp was in the high 80s but the relative humidity was much lower.
Here's the link:
http://www.wunderground.com/history/airport/KUOX/2009/6/18/DailyHistory.html?req_city=NA&req_state=NA&req_statename=NABut here's my question: assuming NO sun (and of course there's sun today), isn't it LESS stressful at a given dewpoint to run at the higher rather than the lower temperature? Because at the lower temperature, with 100% humidity, you get zero evaporative cooling. At the higher temperature and lower humidity, you get quite a bit of evaporative cooling.
But of course you also get dehydration at that higher temperature, which you don't get (for obvious reasons) to nearly the same extent at the lower temperature.
And in the middle of the day you get direct sunlight, which places a heat-burden on the body that superadds to the temperature/humidity stress.
My subjective impression, from 6 years' experience in this climate using a HR monitor and paying close attention, post-run, to the temp, humidity, radiant sun, and dewpoint, is that there's a witching hour between 9 and 10 AM where running is toughest. The temps have risen, the relative humidity is noticeably higher than it will be at 2 PM, and the sun is coming on strong. If I'm trying to complete a 2-hour run in that window, I'm potentially in big trouble, no matter how well I hydrate during the run.
By the same token, I've started 2-hour runs at 5:30 when it was 95 degrees and moderately low humidity, with temps falling and the sun slowly angling in the sky. Not fun, but easier than the morning.