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Temperature, Dewpoint, and Humidity Conversions

The old saying goes, "It's not the heat, it's the humidity". The way the human body reacts to warm temperatures is a function of both the actual temperature, and the moisture in the air. Low humidity lets the body cool off due to evaporation of perspiration. As the humidity rises, less perspiration evaporates, and it becomes more difficult to dissipate the heat.

The heat index is an attempt to measure the apparent temperature, or the way it feels outside. It uses a formula of both actual temperature, and absolute humidity. Generally, when the heat index rises above 90 degrees, people should avoid being outside for too long, and when the heat index is 105 degrees or higher, it's becoming dangerous. Note that the heat index is calculated as an 'in the shade' temperature, so if you are in the sun, it could actually feel warmer than the heat index indicates.

Another way to measure how hot it feels outside is to use the dew point temperature. The dew point temperature is an indicator of the absolute humidity — if the temperature drops to the dewpoint temperature, water vapor condenses to dew. No matter what the actual outside temperature is, most people begin to feel uncomfortable when the dew point approaches 70 degrees, and dewpoints above 70 degrees are opressive.

The table below shows the heat index and dew points for differing combinations of air temperature and relative humidity. To determine the heat index (and corresponding dew point), locate the column with the air temperature, and match it with the approprite row for humidity. For example, if it's 90 degrees and 55% relative humidity, the heat index is 97 degrees, and the dew point is 72 degrees. You'll likely feel uncomfortable. On the other hand, if it's 90, but the relative humidity is 30%, the heat index is only 88, and the dew point is 54. Most people wouldn't find this oppressive.

Keep in mind that different people react to heat and humidity differently, and that there is a real danger when you spend too much time outside during a heat wave.

  80 82 84 86 88
  HI DP HI DP HI DP HI DP HI DP
10% 78 18 80 20 81 22 83 23 84 25
20% 79 35 80 37 84 39 83 40 85 42
30% 79 46 80 48 82 49 84 51 86 53
40% 80 54 81 55 84 57 87 59 89 61
45% 80 57 82 59 84 60 87 62 89 64
50% 81 60 83 62 85 63 88 65 91 67
55% 81 62 84 64 86 66 89 68 93 70
60% 82 65 84 67 88 69 91 70 95 72
65% 82 67 85 69 89 71 93 73 98 75
70% 83 69 86 71 90 73 95 75 100 77
75% 84 71 88 73 92 75 97 77 103 79
80% 84 73 89 75 94 77 100 79 106 81
85% 85 75 90 77 96 79 102 81 110 83
90% 86 77 91 79 98 81 105 83 113 85
95% 86 78 93 80 100 82 108 84 117 86
100% 87 80 95 82 103 84 112 86 121 88
 
  90 92 94 96 98
  HI DP HI DP HI DP HI DP HI DP
10% 86 26 88 28 89 29 91 31 93 32
20% 86 44 88 45 90 47 93 48 95 50
30% 88 54 90 56 93 58 96 60 99 61
40% 93 62 94 64 97 66 101 68 105 70
45% 93 66 96 68 100 69 104 71 109 73
50% 95 69 99 71 103 73 108 74 113 76
55% 97 72 101 74 106 75 112 77 117 79
60% 100 74 105 76 110 78 116 80 123 82
65% 103 77 108 79 114 80 121 82 128 84
70% 105 79 112 81 119 83 126 85 134 87
75% 109 81 116 83 124 85 132 87
80% 113 83 121 85 129 87
85% 117 85 126 87 135 89
90% 122 87 131 89
 
  100 102
  HI DP HI DP
10% 95 34 97 35
15% 96 44 98 45 80-90 Degrees -- Caution
20% 97 52 100 53 91-104 Degrees -- Extreme Caution
25% 100 58 103 60 105-125 Degrees -- Danger
30% 102 63 106 65 125+ Degrees -- Extreme Danger
35% 106 68 110 70
40% 109 71 114 73
45% 114 75 119 77
50% 118 78 124 80
55% 124 81 130 83
60% 129 84 137 85
65% 136 86