Heat vs Humidity (Read 682 times)

If you knew the day was going to be like below and you were going to be running in the shade for 95% of the time which part of the day would you run? Would you do the morning when the temperature is lower but the humidty is higher or later when the temp is higher but the humidty is a lot lower? Which is more important for running? I almost think I would rather have higher temp with lower humidty. I hope this readable below. I just did a copy and past. Not sure if it matters but it would be a long run of 16 - 20 miles. As you can tell by the time of this message I chose early and high humidty just because I am used to my Sunday run. Time Temp Hum 6am 64°F 90% 7am 64°F 90% 8am 67°F 84% 9am 71°F 73% 10am 79°F 62% 11am 84°F 52% 12pm 88°F 46% 1pm 90°F 43% 2pm 92°F 40% 3pm 93°F 39% 4pm 94°F 38% 5pm 94°F 38% 6pm 92°F 42% 7pm 91°F 48% 8pm 88°F 53%

Personally, I'd make the same choice you did and pick the morning. The thing with humidity is that in terms of moisture in the air, you should be more concerned about the dewpoint. For example, say the dewpoint for the day is 61 degrees. This doesn't change much throughout the day, so the AMOUNT of moisture in the air is almost the same. The relative humidity is basically the dewpoint divided by the temperature. So the relative humidity does "go down" as the day warms up and the temperature increases, but the amount of moisture in the air is more or less the same. The dewpoint is the level where if the temperature dropped to this point the air would be completely saturated with moisture (100% humidity, i.e. fog or raining). It may feel stickier early in the day because the actual temperature is closer to the dewpoint, but there is not much more moisture in the air at 6 am than at 3 pm. You will feel a little more comfortable humidity-wise in the afternoon because the difference is larger, but it's more of an apparent humidity relief, not a true drop in the moisture.This is true most of the summer across the US, unless you have a cold front come through which can clear out a lot of the moisture in the air. I hope this made sense! Generally, dewpoints below 60 generally indicate a comfortable setting. Dewpoints above 60 produce an uncomfortable setting and the higher the reading the more unbearable it becomes.

I'd go with the morning too. Shaunna, thanks very much for the explanation of humidity and dewpoint. I've wondered about that from time to time and never bothered to look it up. Handy having a meteorologist around here

Shaunna: I like a lot of information so that was great. I had to read it a couple of times to understand it though : I I made the decision using a much easier approach today. My run this morning, at a slow 9:30 pace for 20 miles was not nearly as bad as I would have thought. Now in the afternoon I wouldn't have wanted to run 6 miles. So easy decision. Temperature over Humidity anyday!

You may also want to think in terms of heat index, which is a means of calculating apparent temperature as a function of temperature and relative humidity/dewpoint. This link shows a table and calculator. Note that the calculator doesn't work for temps less than 80°F. At that point, the effect of humidity is relatively minor, and the "heat index" isn't meaningful. Accuweather uses a more advanced calculation called RealFeel, which also takes local sun angle, cloud cover, wind speed, etc. as well as temp and humidity into account to give a very accurate idea of what it "feels" like outside. Their calculation is patented and they don't reveal the formula. Going back to your data, I added the heat indices for temps 80°F and above: Time Temp Hum Heat Index 6am 64°F 90% N/A 7am 64°F 90% N/A 8am 67°F 84% N/A 9am 71°F 73% N/A 10am 79°F 62% N/A 11am 84°F 52% 86°F 12pm 88°F 46% 90°F 1pm 90°F 43% 92°F 2pm 92°F 40% 94°F 3pm 93°F 39% N/A (Index N/A for <40%rh) 4pm 94°f 38% n/a 5pm 94°f 38% n/a 6pm 92°f 42% 95°f 7pm 91°f 48% 96°f 8pm 88°f 53% 92°f relative humidity in the 40% range boosts the heat index by a few degrees, but into a very uncomfortable range (mid 90's). although it's muggy at 6am, it's a cool muggy -- much better conditions to run in than mid-day, shade or not. 4pm="" 94°f="" 38%="" n/a="" 5pm="" 94°f="" 38%="" n/a="" 6pm="" 92°f="" 42%="" 95°f="" 7pm="" 91°f="" 48%="" 96°f="" 8pm="" 88°f="" 53%="" 92°f="" relative="" humidity="" in="" the="" 40%="" range="" boosts="" the="" heat="" index="" by="" a="" few="" degrees,="" but="" into="" a="" very="" uncomfortable="" range="" (mid="" 90's).="" although="" it's="" muggy="" at="" 6am,="" it's="" a="" cool="" muggy="" --="" much="" better="" conditions="" to="" run="" in="" than="" mid-day,="" shade="" or=""></40%rh) 4pm 94°f 38% n/a 5pm 94°f 38% n/a 6pm 92°f 42% 95°f 7pm 91°f 48% 96°f 8pm 88°f 53% 92°f relative humidity in the 40% range boosts the heat index by a few degrees, but into a very uncomfortable range (mid 90's). although it's muggy at 6am, it's a cool muggy -- much better conditions to run in than mid-day, shade or not.>