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Using dew-point temperature for THI calculations

Kevin Janni, professor and Extension engineer, University of Minnesota
Erin Cortus, assistant professor and environmental quality engineer, South Dakota State University

April 11, 2015

Dew-point temperature is a measure of the amount of moisture in the air. It is a surface's temperature at which moisture in the air begins to condense onto the surface. Dew on the tractor cab in the morning indicates that the cab surface temperature dropped below the dew-point temperature overnight. Dew-point temperatures in the 50s are generally considered comfortable by most people; in the 60s it becomes more humid and uncomfortable, quite uncomfortable in the 70s, and oppressive when dew-points are in the 80s.

Relative humidity is a ratio of the amount of water in the air relative to the maximum amount of water that the air could hold at the same temperature. The maximum amount of water that the air can hold increases as temperature increases; roughly doubling every 20° F increase in air temperature. This means that relative humidity changes as air temperature changes even if the amount of moisture in the air remains the same. Figure 1 is a graph that shows how temperature and relative humidity can cycle during a 24-hour period even if the dew-point temperature is constant. Air temperature can change because of solar heating in the day and radiant cooling at night. One advantage of using dew-point temperature over relative humidity is that the dew-point temperature of air does not change as air temperature changes. The dew-point temperature used to create Figure 1 was 60° F. Dew-point temperatures do change as the amount of moisture in the air changes as air masses and rain storms move in and out of an area.

Figure 1. Air temperature and relative humidity cycling at a constant 60° F dew-point temperature and average 75° F air temperature.

Temperature humidity index (THI) is a common indicator of heat stress that combines the effects of temperature and the amount of moisture in the air. There are several THI equations and they have been around since the late 50s. Collier and others at Arizona State University reported in 2012 that lactating dairy cows producing more than 77 pounds of milk begin to have milk yield losses when the minimum THI is 65 or greater or when the average THI is 68 for more than 17 hours per day. A stress threshold is reached when average THI values range from 68 to 71. Within this range respiration rates exceed 60 breaths per minute, milk yield losses begin, reproduction losses become detectable, and rectal temperatures exceed 101.3° F. They define mild-moderate stress to be when average daily THI ranges from 72 to 79. Within this range respiration rates exceed 75 breaths per minute and rectal temperatures exceed 102.2° F. Moderate-severe stress is when average daily THI ranges from 80 to 89 and severe stress is when average daily THI values range from 90 to 98. Collier and others recommend that cooling practices be implemented before average daily THI values exceed 68. Table 1 summarizes the dairy cow heat stress levels based on THI.

Table 1. Dairy cow stress based on THI.

Heat stress level THI
None < 68
Threshold 68 - 71
Mild-moderate 72-79
Moderate-severe 80-89
Severe 90-99

Most THI charts give THI values using air temperature and relative humidity. The cyclic nature of relative humidity makes it more difficult to calculate an average 17 hour THI value. Since dew-point temperatures are an indication of the moisture in the air, they can be used instead of relative humidity to define the temperature humidity index (THI).

Table 2 gives THI values, comparable to the equation used by Collier, using air temperature and dew-point temperature. The table can be used by people who prefer to used dew-point temperature rather than relative humidity. So if the weather forecast indicates that the dew-point temperature is going to be 60° F today and the air temperature is going to start off at 65° F and rise to 95° F, the THI is going to start off around 64 and rise into the moderate-severe stress range with a THI of 81. On this day the minimum THI is less than 65 and the average THI will be around 73. These weather conditions would be expected to impact cow well-being and performance if cow cooling practices are not implemented. Since dew-point temperatures rarely reach 90° F, higher values are not presented.

Table 2. Temperature humidity index (THI) values when given air and dew-point temperatures.

Air temperature Dew-point temperatures (F)
(F) 35 40 45 50 55 60 65 70 75 80 85 90
65 62.4 62.7 63.0 63.4 63.8 64.4 65.0
70 65.2 65.6 66.1 66.6 67.3 68.1 6.90 70.0
75 67.8 68.3 68.9 69.5 70.3 71.2 72.3 73.5 75.0
80 70.3 70.8 71.4 72.1 73.0 74.0 75.2 76.6 78.2 80.0
85 72.6 73.2 73.8 74.6 75.5 76.5 77.8 79.2 80.9 82.8 85.0
90 74.9 75.5 76.1 76.9 77.8 78.9 80.1 81.5 83.2 85.2 87.4 90.0
95 77.1 77.7 78.3 79.1 80.0 81.0 82.3 83.7 85.4 87.3 89.5 92.1
100 79.3 79.9 80.5 81.2 82.1 83.1 84.3 85.7 87.4 89.2 91.4 93.9
105 81.5 82.0 82.6 83.3 84.2 85.2 86.3 87.7 89.2 91.0 93.1 95.5
110 83.6 84.1 84.7 85.4 86.2 87.1 88.3 89.5 91.0 92.8 94.8 97.1

THI values account for the combined effect of temperature and moisture in the air. Other factors also affect cow thermal comfort. They include air velocity blowing past the cow, solar heat gain (sunshine versus shade), and evaporative cooling from sweat or sprinkling cows' backs. These other factors correspond to common practices to cool cows in hot weather.

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