Heat Index Chart

If you are looking for how to dress for Chicago weather, see The Chicago Weather Story
or the LOCAL FORECAST for Arlington Heights/Chicago. See also Chicago Weather Extremes.

In an average year only the winter's cold -- not lightning, hurricanes, tornadoes, floods, or earthquakes -- takes a greater weather-related death toll than the summer's heat and humidity. In an effort to alert you to the hazards of prolonged heat/humidity episodes, the National Weather Service devised the "heat index." The heat index (HI) is an accurate measure of how hot it really feels when the affects of humidity are added to high temperature.

The human body contains several mechanisms to maintain its internal operating temperature at 98.6° F. When threatened with above "normal" temperatures, the body will try to dissipate excesss heat by varying the rate and depth of blood circulation, by losing water through the skin and sweat glands, and, as a last resort, by panting. When weather conditions force the air temperature above 90° F and the relative humidity is high, the body is doing everything it can to maintain its normal temperature. Unfortunately, conditions can exceed the body's ability to cope with the combined affects of heat and humidity. At such times the body may sucumb to any of a number of heat disorders including sunstroke, heat cramps, heat exhaustion and heatstroke.

To use the heat index charts, find the appropriate temperature at the top of the chart. Read down until you are opposite the humidity/dewpoint. The number which appears at the intersection of the temperature and humidity/dewpoint is the heat index.

Heat Index Chart (Temperature & Dewpoint)

Dewpoint
(° F)

Temperature (° F)

90

91

92

93

94

95

96

97

98

99

100

101

102

103

104

105

65

94

95

96

97

98

100

101

102

103

104

106

107

108

109

110

112

66

94

95

97

98

99

100

101

103

104

105

106

108

109

110

111

112

67

95

96

97

98

100

101

102

103

105

106

107

108

110

111

112

113

68

95

97

98

99

100

102

103

104

105

107

108

109

110

112

113

114

69

96

97

99

100

101

103

104

105

106

108

109

110

111

113

114

115

70

97

98

99

101

102

103

105

106

107

109

110

111

112

114

115

116

71

98

99

100

102

103

104

106

107

108

109

111

112

113

115

116

117

72

98

100

101

103

104

105

107

108

109

111

112

113

114

116

117

118

73

99

101

102

103

105

106

108

109

110

112

113

114

116

117

118

119

74

100

102

103

104

106

107

109

110

111

113

114

115

117

118

119

121

75

101

103

104

106

107

108

110

111

113

114

115

117

118

119

121

122

76

102

104

105

107

108

110

111

112

114

115

117

118

119

121

122

123

77

103

105

106

108

109

111

112

114

115

117

118

119

121

122

124

125

78

105

106

108

109

111

112

114

115

117

118

119

121

122

124

125

126

79

106

107

109

111

112

114

115

117

118

120

121

122

124

125

127

128

80

107

109

110

112

114

115

117

118

120

121

123

124

126

127

128

130

81

109

110

112

114

115

117

118

120

121

123

124

126

127

129

130

132

82

110

112

114

115

117

118

120

122

123

125

126

128

129

131

132

133

Note: Exposure to full sunshine can increase HI values by up to 15° F


Heat Index Chart (Temperature & Relative Humidity)

RH
(%)

Temperature (° F)

90

91

92

93

94

95

96

97

98

99

100

101

102

103

104

105

90

119

123

128

132

137

141

146

152

157

163

168

174

180

186

193

199

85

115

119

123

127

132

136

141

145

150

155

161

166

172

178

184

190

80

112

115

119

123

127

131

135

140

144

149

154

159

164

169

175

180

75

109

112

115

119

122

126

130

134

138

143

147

152

156

161

166

171

70

106

109

112

115

118

122

125

129

133

137

141

145

149

154

158

163

65

103

106

108

111

114

117

121

124

127

131

135

139

143

147

151

155

60

100

103

105

108

111

114

116

120

123

126

129

133

136

140

144

148

55

98

100

103

105

107

110

113

115

118

121

124

127

131

134

137

141

50

96

98

100

102

104

107

109

112

114

117

119

122

125

128

131

135

45

94

96

98

100

102

104

106

108

110

113

115

118

120

123

126

129

40

92

94

96

97

99

101

103

105

107

109

111

113

116

118

121

123

35

91

92

94

95

97

98

100

102

104

106

107

109

112

114

116

118

30

89

90

92

93

95

96

98

99

101

102

104

106

108

110

112

114

Note: Exposure to full sunshine can increase HI values by up to 15° F

Source: National Weather Service

 Chicago Weather Extremes

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