Temperature, Dewpoint, and Humidity Conversions
The old saying goes, "It's not the heat, it's the humidity". The way the human body reacts to warm temperatures is a function of both the actual temperature, and the moisture in the air. Low humidity lets the body cool off due to evaporation of perspiration. As the humidity rises, less perspiration evaporates, and it becomes more difficult to dissipate the heat.
The heat index is an attempt to measure the apparent temperature, or the way it feels outside. It uses a formula of both actual temperature, and absolute humidity. Generally, when the heat index rises above 90 degrees, people should avoid being outside for too long, and when the heat index is 105 degrees or higher, it's becoming dangerous. Note that the heat index is calculated as an 'in the shade' temperature, so if you are in the sun, it could actually feel warmer than the heat index indicates.
Another way to measure how hot it feels outside is to use the dew point temperature. The dew point temperature is an indicator of the absolute humidity — if the temperature drops to the dewpoint temperature, water vapor condenses to dew. No matter what the actual outside temperature is, most people begin to feel uncomfortable when the dew point approaches 70 degrees, and dewpoints above 70 degrees are opressive.
The table below shows the heat index and dew points for differing combinations of air temperature and relative humidity. To determine the heat index (and corresponding dew point), locate the column with the air temperature, and match it with the approprite row for humidity. For example, if it's 90 degrees and 55% relative humidity, the heat index is 97 degrees, and the dew point is 72 degrees. You'll likely feel uncomfortable. On the other hand, if it's 90, but the relative humidity is 30%, the heat index is only 88, and the dew point is 54. Most people wouldn't find this oppressive.
Keep in mind that different people react to heat and humidity differently, and that there is a real danger when you spend too much time outside during a heat wave.