Heat Index charts are based on shady conditions with light winds. Exposure to direct sunlight can increase heat index values by as much as fifteen degrees.
Heat dangers are increased with continued exposure to the excessive heat and/or physical activity. Elderly persons, small children, individuals in poor health, those on certain medications or drugs and persons with weight and/or alcohol problems are particularly susceptible to heat-related medical complications or illness.
NOAA’s Heat Alert procedures are based mainly on Heat Index Values. The Heat Index, sometimes referred to as the apparent temperature and given in degrees Fahrenheit, is a measure of how hot it really feels when relative humidity is factored with the actual air temperature.
To find the Heat Index, use the chart below.
For example, if the air temperature is 96°F (found on the top of the table) and the relative humidity is 65% (found on the left of the table), the heat index – how hot it feels – is 121°!
The National Weather Service will initiate alert procedures when the Heat Index is expected to exceed 105°- 110°F (depending on local climate) for at least 2 consecutive days.