A common misconception about humidity is that it can be at 90% when the temperatures are in the 90s. That will likely NEVER happen here. The humidity always tends to go down during the afternoon as the dew point temperature separates from the actual temperature. If you look at the graph, a humidity of 90% with a temperature of 90 would cause a heat index of 122, which is again not likely around here. Believe it or not, it feels very humid when the humidity is actaully only around 45-50% during the afternoon.
The easiest way I find to figure out the heat index is by connecting the temperature with the dew point temperature. Dew points help measure the amount moisture in the air, thus the humidity. The dew point is the temperature at which dew or water droplets would form. So, look at the water droplets on the outside of your glass of ice water. The air around the cool glass obviously got down to the dew point temperature.
These charts are easy to use if you ever want to find out how extreme the heat will feel. I hope this also shows why we sometimes mention dew points on television. We use them to show how dry or humid the air is, and for many other reasons as well.
During this extended heat wave you'll surely hear us mention the high heat index and this is the simple method we use.
Here is a heat index calculator ... http://www.crh.noaa.gov/jkl/?n=heat_index_calculator
(you may find some differences from the calculator to the graph)