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Heat Index
What is Heat Index?
Heat Index uses temperature and relative humidity to determine how hot the air actually "feels." When humidity is low, the apparent temperature will be lower than the air temperature, since perspiration evaporates rapidly to cool the body. However, when humidity is high (i,e., the air is saturated with water vapor) the apparent temperature "feels" higher than the actual air temperature, because perspiration evaporates more slowly.
Formula:
The formula for heat index is based upon the lookup table presented by Steadman (1979). This table was derived with a complicated set of measurements. The table can be mathematically analyzed to produce an approximate equation for the heat index:
Heat Index = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 6.83783x10-3T2 - 5.481717x10-2R2 + 1.22874x10-3T2R + 8.5282x10-4TR2 - 1.99x10-6T2R2
Where:
T = ambient dry bulb temperature
R = relative humidity
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Steadman Heat Index Table
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Relative Humidity (%)
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Temp °F
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0
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5
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10
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15
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20
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25
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30
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35
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40
|
45
|
50
|
55
|
60
|
65
|
70
|
75
|
80
|
85
|
90
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95
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100
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110
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100.3
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102.1
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104.6
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108.0
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112.0
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116.8
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122.3
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128.6
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135.7
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109
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99.7
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101.3
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103.6
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106.6
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110.4
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114.9
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120.1
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126.0
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132.7
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108
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99.2
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100.5
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102.6
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105.4
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108.8
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113.0
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117.9
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123.5
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129.8
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136.7
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107
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98.6
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99.7
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101.6
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104.1
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107.3
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111.2
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115.7
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121.0
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126.9
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133.5
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106
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98.0
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98.9
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100.6
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102.8
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105.8
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109.4
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113.6
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118.6
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124.2
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130.4
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137.3
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105
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97.4
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98.2
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99.6
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101.6
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104.3
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107.6
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111.6
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116.2
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121.5
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127.4
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133.9
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104
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96.8
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97.4
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98.6
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100.4
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102.9
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106.0
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109.6
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114.0
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118.9
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124.4
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130.6
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137.4
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103
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96.1
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96.6
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97.6
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99.2
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101.5
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104.3
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107.7
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111.7
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116.4
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121.6
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127.4
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133.8
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102
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95.5
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95.8
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96.6
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98.1
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100.1
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102.7
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105.9
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109.6
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113.9
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118.8
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124.3
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130.3
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136.9
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101
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94.8
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95.0
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95.7
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97.0
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98.8
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101.1
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104.1
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107.5
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111.5
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116.1
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121.2
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126.9
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133.1
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100
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94.1
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94.2
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94.8
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95.8
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97.5
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99.6
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102.3
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105.5
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109.3
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113.5
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118.3
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123.6
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129.5
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135.9
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99
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93.5
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93.4
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93.8
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94.8
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96.2
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98.2
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100.6
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103.6
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107.0
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111.0
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115.5
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120.5
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126.0
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132.0
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138.5
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98
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92.8
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92.6
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92.9
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93.7
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95.0
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96.7
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99.0
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101.7
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104.9
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108.6
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112.8
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117.4
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122.6
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128.2
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134.3
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97
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92.0
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91.8
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92.0
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92.7
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93.8
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95.4
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97.4
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99.9
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102.9
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106.3
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110.2
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114.5
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119.3
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124.5
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130.3
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136.4
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96
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91.3
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91.0
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91.1
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91.6
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92.6
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94.0
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95.9
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98.2
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100.9
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104.0
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107.6
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111.7
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116.1
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121.0
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126.4
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132.1
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138.3
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95
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90.6
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90.2
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90.2
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90.6
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91.5
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92.7
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94.4
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96.5
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99.0
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101.9
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105.2
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108.9
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113.1
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117.6
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122.6
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128.0
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133.8
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140.0
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94
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89.8
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89.4
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89.3
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89.7
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90.4
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91.5
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93.0
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94.9
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97.2
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99.8
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102.9
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106.3
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110.2
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114.4
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119.0
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124.0
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129.4
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135.2
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141.3
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93
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89.0
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88.6
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88.5
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88.7
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89.3
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90.3
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91.6
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93.4
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95.4
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97.9
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100.7
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103.8
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107.4
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111.3
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115.5
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120.1
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125.1
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130.5
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136.2
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142.3
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92
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88.3
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87.8
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87.6
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87.8
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88.3
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89.2
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90.4
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91.9
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93.8
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96.0
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98.5
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101.4
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104.7
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108.3
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112.2
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116.4
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121.0
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126.0
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131.3
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136.9
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91
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87.5
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86.9
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86.7
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86.9
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87.3
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88.0
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89.1
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90.5
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92.2
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94.2
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96.5
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99.2
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102.1
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105.4
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109.0
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112.9
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117.1
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121.6
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126.5
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131.7
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137.1
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90
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86.6
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86.1
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85.9
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86.0
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86.3
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87.0
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87.9
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89.2
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90.7
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92.5
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94.6
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97.0
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99.7
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102.7
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105.9
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109.5
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113.3
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117.5
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121.9
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126.6
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131.6
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89
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85.8
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85.3
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85.1
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85.1
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85.4
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86.0
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86.8
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87.9
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89.3
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90.9
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92.8
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94.9
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97.4
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100.0
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103.0
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106.2
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109.7
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113.5
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117.5
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121.8
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126.3
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88
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85.0
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84.5
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84.3
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84.3
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84.5
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85.0
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85.7
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86.7
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87.9
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89.4
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91.0
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93.0
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95.1
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97.6
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100.2
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103.1
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106.3
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109.6
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113.2
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117.1
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121.2
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87
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84.1
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83.7
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83.5
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83.5
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83.7
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84.1
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84.7
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85.6
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86.6
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87.9
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89.4
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91.1
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93.1
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95.2
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97.6
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100.2
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102.9
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106.0
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109.2
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112.6
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116.3
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86
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83.3
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82.9
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82.7
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82.7
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82.8
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83.2
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83.8
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84.5
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85.4
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86.6
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87.9
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89.4
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91.1
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93.0
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95.1
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97.3
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99.8
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102.5
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105.3
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108.3
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111.5
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85
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82.4
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82.1
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81.9
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81.9
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82.0
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82.4
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82.9
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83.5
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84.3
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85.3
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86.5
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87.8
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89.3
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90.9
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92.7
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94.7
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96.8
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99.1
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101.6
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104.2
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107.0
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84
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81.5
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81.2
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81.1
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81.1
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81.3
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81.6
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82.0
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82.6
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83.3
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84.1
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85.1
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86.3
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87.5
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88.9
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90.5
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92.2
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94.0
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95.9
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98.0
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100.3
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102.7
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83
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80.6
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80.4
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80.3
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80.4
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80.6
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80.8
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81.2
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81.7
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82.3
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83.1
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83.9
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84.9
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85.9
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87.1
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88.4
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89.8
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91.3
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92.9
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94.7
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96.5
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98.5
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82
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79.7
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79.6
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79.6
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79.7
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79.9
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80.1
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80.5
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80.9
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81.5
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82.1
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82.8
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83.6
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84.4
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85.4
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86.4
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87.6
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88.8
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90.1
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91.5
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93.0
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94.5
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81
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78.7
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78.8
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78.8
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79.0
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79.2
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79.5
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79.8
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80.2
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80.7
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81.2
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81.7
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82.4
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83.1
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83.8
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84.6
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85.5
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86.4
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87.4
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88.5
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89.6
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90.8
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80
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77.8
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77.9
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78.1
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78.3
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78.6
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78.9
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79.2
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79.5
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79.9
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80.3
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80.8
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81.3
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81.8
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82.4
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83.0
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83.6
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84.2
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84.9
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85.6
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86.4
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87.2
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With Prolonged Exposure and/or Physical Activity:
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Extreme Danger: Heat Stroke or Sunstroke likely
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Danger: Sunstroke, muscle cramps, and/or heat exhaustion likely
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Extreme Caution: Sunstroke, muscle cramps, and/or heat exhaustion possible
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Caution: Fatigue possible
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The Davis implementation simply extends the range of use of this table to make it usable at temperatures beyond the scope of the table.
The formula Davis uses is also used by the US National Weather Service. Heat Index can also be used to determine indoor comfort levels.
Note, Heat Index has also been referred to as "Temperature-Humidity Index" and "Thermal Index" in some Davis products
REFERENCE
Steadman, R.G., 1979: The Assessment of Sultriness, Part 1: A Temperature-Humidity Index Based on Human Physiology and Clothing Science. Journal of Applied Meteorology, July 1979
"Media Guide to NWS Products and Services", National Weather Service Forecast Office, Monterey, CA, 1995.
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