There is a great article in Running Times this month called It’s Not the Heat, Nor the Humidity.  You can’t read it online, but the ladies over at The Bitchy Runners have summarized it and created a replica of the dew point chart that you should check out.

I incorporate race training into my run commute.  It’s not that exciting, but it can be pretty intense based on which route I am running at the time (hills vs. flat). Sometimes during the summer, it REALLY sucks the life out of me; almost to the point where I am worthless the rest of the day. I always chalked it up to humidity, but after reading the RT article, I realized that what I should really be keeping an eye on is the dreaded dew point.

Wikipedia defines dew point as:

…the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into water. The condensed water is called dew. The dew point is a saturation temperature.

Part of a body’s cooling process is sweating.  If the dew point is high, the air cannot accept very much additional water vapor into it.  So a runner’s sweat will remain on his/her skin longer which in turn slows down the cooling process.  If you can’t thermoregulate properly, you’ll feel like a crap sandwich (and a soggy one at that).

Here is what Atlanta’s last 24 hours looks like (from the National Weather Service):

I could only swing 2 miles this morning at 7:00 am (dew point = 73, temp = 77).  While the dew point was high and I was running much slower than usual, I cut my run short by 3.5 miles (took the train) so I didn’t wear myself out as I have to run an additional 4 – 8 miles after work.

On days like this, just forget about training.  Take it easy and shut your stopwatch off so you aren’t tempted to pick up the pace.  Carry water with you and regularly sip it throughout your run.  And just listen to your body – if you’re sucking, stop running and walk for a bit.

Be careful out there!