Heat and Humidity

Cartoon earth sweating from heat In a normal year, about 175 Americans succumb to the demands of summer heat. This is more deaths than hurricanes, lightning, tornadoes, floods, and earthquakes combined. Only the cold of winter takes a greater toll.

Human bodies dissipate heat by varying the rate and depth of blood circulation, by losing water through the skin and sweat glands. When blood is heated above 98.6 degrees, the heart begins to pump more blood, blood vessels dilate to accommodate the increased flow, and the bundles of tiny capillaries threading through the upper layers of skin are put into operation. The body’s blood is circulated closer to the skin’s surface, and excess heat drains off into the cooler atmosphere. At the same time, water diffuses through the skin as perspiration.

The skin handles about 90 percent of the body’s heat dissipating function. Sweating, by itself, does nothing to cool the body, unless the water is removed by evaporation-and high relative humidity retards evaporation. Heat energy required to evaporate the sweat is extracted from the body, thereby cooling the body. Heat disorders occur when the body loses its ability to shed heat through circulation and sweating. When heat gain exceeds heat loss, or when the body can no longer compensate for fluids and salt lost through perspiration, the core temperature of the body begins to rise, and heat-related illness may develop.

Symptoms for common heat related illnesses

Symptoms for common heat related illnesses are listed below. In particular, know the differences between heat exhaustion and heatstroke. HEAT STROKE IS A SEVERE MEDICAL EMERGENCY. SUMMON EMERGENCY MEDICAL ASSISTANCE OR GET THE VICTIM TO A HOSPITAL IMMEDIATELY. DELAY CAN BE FATAL.

Sunburn

Redness and pain. In severe cases swelling of skin, blisters, fever, headaches.

Heat Cramps

Painful spasms usually in muscles of legs and abdomen possible. Heavy sweating.

Heat Exhaustion

  • Profuse perspiration.
  • Cool, moist skin.
  • Rapid respiration.
  • Body temperature may be normal, or slightly below normal, or as high as 102F.
  • Possible giddiness.

Heatstroke

  • Absence of perspiration.
  • Hot, dry, red or mottled skin.
  • Slow deep respiration.
  • Extremely high body temperature (104F or above, rectally).
  • Mental confusion, disorientation, delirium, irrational behavior, feeling of euphoria or impending doom, diminished level of consciousness or abrupt loss of consciousness.
  • Signs of shock.

Heat Index

The NWS 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. As an example, if the air temperature is 95F (found on the left side of the table) and the RH is 55% (found at the top of the table), the HI (how hot it really feels) is 110F. This is at the intersection of the 95F row and the 55% column. The orange color indicates you should be aware that eat exhaustion is likely and heatstroke is possible.

The Heat Index
Temp Relative Humidity (percentage)
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
135F 120 126
130F 117 122 131
125F 111 116 123 131 141
120F 107 111 116 123 130 139 148
115F 105 107 111 115 120 127 135 143 151
110F 99 102 105 108 112 117 123 130 137 143 150
105F 95 97 100 102 105 109 113 118 123 129 135 142 149
100F 91 93 95 97 99 101 104 107 110 115 120 126 132 138 144 150
95F 87 88 90 91 93 94 96 98 101 104 107 110 114 119 124 130 136 140 150
90F 83 84 85 86 87 88 90 91 93 95 96 98 100 102 106 109 113 117 122 126 131
85F 78 79 80 81 82 83 84 85 86 87 88 89 90 91 93 95 97 99 102 105 108
80F 73 74 75 76 77 77 78 79 79 80 81 81 82 83 84 85 86 87 88 89 90
75F 69 69 70 71 72 72 73 73 74 74 75 75 76 76 77 77 78 78 79 79 80
70F 64 64 65 65 66 66 67 67 68 68 69 69 70 70 70 70 71 71 71 71 72
Heatstroke risk extremely high Heat exhaustion possible
Heat exhaustion likely, heatstroke possible Fatigue possible

Simple precautions can greatly reduce the risk of heat-related illness and death. Some of these are:

  • Dress for summer. Lightweight light colored clothing reflects heat and sunlight, and helps your body maintain normal temperatures.
  • Protect individuals and the environment from direct sunlight by drawing shades, blinds and curtains.
  • Stay out of the sun. Sunburn makes it harder for your body to release heat and cool down.
  • Do NOT leave children, elderly adults, or pets in parked cars.
  • Reduce, eliminate, or reschedule your strenuous activities to the coolest part of day.
  • Keep air circulating and use air conditioning when possible.
  • Avoid using fans directed on individuals when the temperature exceeds approximately 100F. Fans can actually increase heat stress when the Heat Index exceeds 100F.
  • Take baths, use cool cloths to the head or feet, and/or cool compresses to the groin or underarm area to cool the body. Placing hands and wrists in cool water can help prevent body temperature from accelerating.
  • Do not take salt tablets unless specified by a physician.
  • Do not drink alcoholic beverages.
  • Monitor individuals for early warning signs of dehydration, heat exhaustion and heat stroke.

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