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Heat Index

Heat Index

Temperature vs. Humidity

How to read the chart: find the temperature on the left hand side, then move to the right until you find the column for the approximate relative humidity. That number will be the temperature that it will "feel" like. Example: A temperature of 95 and relative humidity of 50% will "feel" like 107 degrees.

  0% 5% 10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60% 65% 70% 75% 80%
115 103 107 111 115 120 127 135 143 151                
110 99 102 105 108 112 117 123 130 137 143 151            
105 95 97 100 102 105 109 113 118 123 129 135 142 149        
100 91 93 95 97 99 101 104 107 110 115 120 126 132 136 144    
95 87 88 90 91 93 94 96 98 101 104 107 110 114 119 124 130 136
90 83 84 85 86 87 88 90 91 93 95 96 98 100 102 106 109 113
85 78 79 80 81 82 83 84 85 86 87 88 89 90 91 93 95 97
80 73 74 75 76 77 77 78 79 79 80 81 81 82 83 85 86 86
75 69 69 70 71 72 72 73 73 74 74 75 75 76 76 77 77 78
70 64 64 65 65 66 66 67 67 68 68 69 69 70 70 70 70 71


IMPORTANT: Heat index values were devised for shaddy, light wind conditions. Exposure to full sunshine can increase values by up to 15 degrees! Also, strong winds, particularly with very hot, dry air, can be extremely hazardous.

Or, if you know the temperature and the dewpoint, figure out the heat index and relative humidity (RH) using this calculator.

Air Temperature (ºF) =
Dew Point (ºF) =

Heat Index = º F
Relative Humidity = %

Dew Point vs. Humidity

The dew point is the temperature the air needs to be cooled to in order to achieve a relative humidity (RH) of 100%. At this point the air cannot hold anymore water in the gas form. If the air were to be cooled even more, water vapor would have to come out of the atmosphere in the liquid form, usually as fog or precpitation.

The higher the dew point rises, the greater the amount of moisture in the air. This directly effects how "comfortable" it will feel outside. Many times, relative humidity can be misleading. For example, a temperature of 30 and a dew point of 30 will give you a relative humidity of 100%, but a temperature of 80 and a dew point of 60 will only give you a relative humidity of 50%. It would feel much more "humid" on the 80 degree day with 50% relative humidity than on the 30 degree day with a 100% relative humidity. This is because of the higher dew point.

So if you want a real judge of just how "dry" or "humid" it will feel outside, look at the dew point instead of the RH. The higher the dew point, the muggier it will feel.

General comfort levels that can be expected during the summer months:

  • less then or equal to 55: dry and comfortable
  • between 55 and 65: becoming a bit "sticky" with muggy evenings
  • greater then or equal to 65: lots of moisture in the air, becoming oppressive

Heat Index/Heat Disorders

Heat Index Possible heat disorders for people in higher risk groups
130 or higher Heatstroke/sunstroke highly likely with continued exposure.
105-130 Sunstroke, heat cramps or heat exhaustion likely, and heat stroke possible with prolonged exposure and/or physical activity.
90-105 Sunstroke, heat cramps and heat exhaustion possible with prolonged exposure and/or physical activity.
80-90 Fatigue possible with prolonged exposure and/or physical activity.

Heat Disorder Symptoms First Aid
Sunburn Redness and pain. In severe cases swelling of skin, blisters, fever, and headaches. Ointments for mild cases if blisters appear and do not break. If breaking occurs, apply dry sterile dressings. Serious, extensive cases should be seen by a physician.
Heat Cramps Painful spasms usually in muscles of the legs and abdomen possible. Heavy sweating. Firm presure on the cramping muscles, or gentle massaging to relieve the spasm. Give sips of water. If nausea occurs, discontinue use.
Heat Exhaustion Heavy sweating, weakness, skin cold, pale and clammy. Pulse thready. Normal temperature possible. Fainting and vomiting. Get victim out of sun. Lay down and loosen clothing. Apply cool, wet cloths. Fan or move victim to air conditioned room. Sips of water. If nausea occurs, discontinue use. If vomiting continues, seek immediate medical attention.
Heat Stroke (sunstroke) High body temperature (106 F or higher). Hot dry skin. Rapid and strong pulse. Possible unconsciousness. Heat stroke is a severe medical emergency. Summon emergency medical assistance or get the victim to a hosiptal immediately. Delay can be fatal.

Move the victim to a cooler environment. Reduce body temperature with cold bath or sponging. Use extreme caution. Remove clothing, use fans and air conditioners. If temperature rise again, repeat process. Do not give fluids.

Safety Tips

  • Slow down. Strenuous activities should be reduced, eliminated, or rescheduled to the coolest time of the day. Individuals at risk should stay in the coolest available place, not necessarily indoors.
  • Dress for summer. Lightweight, light-colored clothing reflects heat and sunlight, and helps your body maintain normal temperatures.
  • Foods (like proteins) that increase metabolic heat production also increase water loss.
  • Drink plenty of water or non-alcoholic fluids. Your body needs water to keep cool. Drink plenty of fluids even if you don't feel thirsty.
  • Do not drink alcoholic beverages.
  • Spend more time in air-conditioned places. Air conditioning in homes and other buildings markedly reduces danger from the heat. If you cannot afford an air conditioner, spending some time each day (during hot weather) in an air conditioned environment affords some protection.
  • Don't get too much sun. Sunburn makes the job of heat dissipation that much more difficult.




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