From the National Weather Service
North American summers are hot; most summers see heat waves in one section or another of the United States. East of the Rockies, they tend to combine both high temperature and high humidity although some of the worst have been catastrophically dry.
Based on the latest research findings, the NWS has devised the "Heat Index" (HI), (sometimes referred to as the "apparent temperature"). The HI, given in degrees F, is an accurate measure of how hot it really feels when relative humidity (RH) is added to the actual air temperature.
To find the HI, look at the Heat Index Chart. As an example, if the air temperature is 95°F (found on the left side of the table) and the RH is 55% (found at the top of the table), the HI-or how hot it really feels-is 110°F. This is at the intersection of the 95° row and the 55% column.
It is important to note that the HI values were devised for shady conditions with light winds. 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.
The heat index can be related to the threat of heat-related ailments, particularly for people in the higher risk groups, such as infants and the elderly. The risks are color-coded on the accompanying chart.
•Heat Index of 130F or Higher: Heatstroke/sunstroke highly higher likely with continued exposure.
•Heat Index of 105F - 130f: Sunstroke, heat cramps or heat exhaustion likely, and heatstroke possible with prolonged exposure and/or physical activity.
•Heat Index of 90F - 105F: Sunstroke, heat cramps and heat exhaustion possible with prolonged exposure and/or physical activity.
•Heat Index of 80F - 90F: Fatigue possible with prolonged exposure and/or physical activity Note on the HI chart the shaded zone above 105°F. This corresponds to a level of HI that may cause increasingly severe heat disorders with continued exposure and/or physical activity.
|Source: National Weather Service|