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Re: [RC] Heat index - Jon . LindermanThe simplest way to describe a heat index is the "feels like" temperature listed on most weather web sites. The heat index is some form of what we call a WBGT, or wet bulb globe thermometer. This type of apparatus measures the radiant heat, (simply put is it sunny?), the water vapor pressure of the air (humidity) as well as any wind (convection), and the ambient temperature. About 20% of the heat index comes from the radiant effect of the sun, 10% from the actual air temperature, and a whopping 70% from the humidity and any effect of air movement. When the combination of these factors exceeds 90 degrees the risk of heat injury is increased, beyond 102ish and heat exhaustion is possible, and very serious heat related injury such as heat stroke and death occur beyond 130. The reason for humidity being such a huge part of the equation is that sweating works to cool only when it evaporates. Dripping sweat off of a body does not cool. If water vapor can not move into the environment on a still humid day, the ability to lose heat is impaired and a body stores heat. Laws of physics tell us that gases move from high concentration to low concentration. When its humid the vapor pressure created by sweat does not move as freely into the environment saturated with water vapor, and basically we are screwed! The "its dry heat" really is no joke. 90 degrees and 10% humidity and the efective temperature is 85 Mid-80's and 60% humidity drives the effective temperature to more than 90. A 90 degree day with 80% humidity, not uncommon in the midwest, puts us into a much higher risk category at nearly 115 degrees! So you can see humidity kills far more than simply warm air temperatures, especially if we try to work/ride. I rode the Endure for the cure in July where it was sunny, humid, and nary a breeze the whole weekend. Ambient temperature was in the high-80's, but with the humidity and lack of breeze it was easily in the high 90's (maybe higher?) and I used lots of cooled water on my horse & had many hands helping me at th finish to cool him out. Another factor in addition to the sun is other forms of radiant heat. Some sections of rides go over these large moved tracks where the sun refeclects of the dry grass similar to reflecting off the pavement, sand, snow, or water, this also adds to the radiant heat effect. The idea of moving out in the sun and recovering in the shade bears some merit since losing the radiant effect of the sun when we duck into the woods drops the heat stress, but if its still and muggy in the woods its not a huge relief. Best bet is to move where I grew up...San Diego, CA, best weather in terms of heat and humidity in the US. Now of course I am trapped in Ohio where I can ride muggy rides within Ohio and all bordering states! Jon K. Linderman, Ph.D., FACSM Assistant Professor of Health and Sport Science University of Dayton 300 College Park Dayton, OH 45469-1210 Voice:(937) 229-4207 FAX: (937) 229-4244 http://homepages.udayton.edu/~lindermj/
I have heard the term heat index before and wonder if someone could explain it to me. I am thinking it is the sum when you add the air temperature and the humidity percent. Is this true? And once you have this heat index number, then how do you use it? What steps do you take with your riding the higher the number is? And what number is the cut off for not riding at all? Thanks, Laura, where humidity is never much of a factor out West, but heat sure is
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