1. What is the difference between Dew Point and Relative Humidity?
Relative humidity is the ratio of the water vapor in the air compared to the water vapor needed to put into the air to achieve complete saturation. Dew point is the temperature at which water condenses and is a combination of relative humidity and temperature. At a constant dew point when the temperature goes up the relative humidity goes down. Likewise, when the temperature goes down the relative humidity goes up. This explains why in the summer we see the highest humidity levels in the early morning or late evening. As the sun comes up and the air temperature begins to rise the relative humidity will drop.
Just knowing the humidity in percentage doesn’t tell the whole story because it’s relative to the actual air temperature. Warmer air can hold more moisture than cool air. The higher the dew point the more uncomfortable it feels. For this reason, dew point is a better indicator of how “sticky” it is outside compared to relative humidity.
2. Dew Point Comfort Index with corresponding Relative Humidity at 90F (32C)
On the most oppressive days, most casual observers will describe the conditions as 90 degrees with 90% humidity. In most parts of the world this is very unlikely to occur. The chart below illustrates the Relative Humidly with an air temperature of 90F (32C) at corresponding levels of Dew Point.
3. How does Heat Index factor into the discussion?
According to NOAA, the Heat Index is a measure of how hot it really feels when relative humidity is factored in with the actual air temperature. To find the Heat Index temperature, look at the Heat Index chart below. As an example IMPORTANT: Since heat index values were devised for shady, light wind conditions, exposure to full sunshine can increase heat index values by up to 15°F. Also, strong winds, particularly with very hot, dry air, can be extremely hazardous, if the air temperature is 90°F and the relative humidity is 60%, the heat index–how hot it feels–is 100°F.
4. Why do you only provide the high dew point and not the low?
Unlike relative humidity, which can vary greatly, the difference between the high and low dew point is typically very slight, sometimes only 1 or 2 degrees. The only time when there might be as much as 10 degree difference is in a very dry climate. In such a case your body would not recognize the difference as it would seem dry throughout the day.
5. How do the relative humidity and/or dew point affect my performance?
This is best explained by a 2011 article found in Runner’s World. At the end of the story dew point is incorporated into the explanation, which most similar articles tend to overlook. http://www.runnersworld.com/race-training/training-heat
To briefly summarize, the closer the dew point is to the air temperature, the more saturated the air is and the less perspiration can evaporate and help the body cool itself, resulting in extra stress on the heart and lungs as the body attempts unsuccessfully to cool itself. Accordingly, the dew point provides a strong indicator of how you’ll feel during competition and a useful tool in predicting how much performance will be impacted.
6. What is a climate category and why should I care?
The Koppen Climate Category It is a 20th century climate classification system developed by German climatologist and amateur botanist Wladimir Koppen (1846-1940). It continues to be the authoritative map of the world climates in use today. We provide this so that triathletes have a way to compare similar race site climates across the globe. For example, if you live in Italy and race at Challenge Rimini this takes place in a Mediterranean climate. If you wish to race in similar conditions in the United States your best bet would be Ironman 70.3 California in Oceanside. Keep in mind these are general classifications and by no means does it indicate the weather/climate will be identical.
7. What is Solar Noon?
Solar noon is the approximate time when the sun is the highest above the horizon on that day. Typically it does not correspond with the high temperature of the day as that is most often late afternoon, around 3:00-4:00PM.