Note: weatherunderground.com is a good place to check the day's dewpoint, it's not listed in all weather sites and apps!
I wanted to share this great bit of info I found on a RW training forum, it was written by a runner who is also a meteorologist. By adding the temperature and dewpoint, you get an idea of whether your run/race will suck or not:
Temp+dewpoint of 90-100 is probably ideal, but anything below 130 is pretty much OK to go for the fast time…maybe 120 for longer distances…say 1/2 marathon or above. Between 130-150 will see a gradual slowdown in pace, and if it’s above 150, forget pace altogether and just focus on finishing.
Here’s another poster’s way of looking at it (that the meteorologist agreed with):
Dewpoint <55*F: Go for it!Dewpoint in the 60s…it’ll be tough for racing, training runs OKDewpoint in the low 70s…hard training will be toughDewpoint in the upper 70s….anything other than a recovery run will be a struggleDewpoint in the 80s…even a recovery run is tough
This approach is good science, for it considers the cooling effects of sweat. You can't cool off without evaporation of sweat, and sweat won't evaporate in the air is saturated with water. Also % humidity is a number we think of when calculating the heat index of a day, but humidity comes from dew point.
and... for my run home today, temp = 97deg, and dew point is 60deg = ugh.