AN607

22

Rev. 1.7

8.2. Heat Index

The heat index (HI), also called “apparent temperature” combines air temperature and relative humidity to estimate

how “hot” a human will perceive the ambient conditions to be. The heat index is calculated for ambient

temperatures above 27 °C (81 °F) and dew points above 12 °C (54 °F) (relative humidity above 40%). Heat index

is calculated with the following equation2.

1. www.shorstmeyer.com/wxfaqs/humidity/humidity.html

2. www.srh.noaa.gov/images/epz/wxcalc/heatIndex.pdf

3. www.crh.noaa.gov/pub/heat.php

8.3. Wind Chill

Wind chill, also called wind chill temperature, wind chill factor or wind chill index, expresses the perceived air

temperature on skin exposed to wind. Wind chill is defined for temperatures at or below 10 °C (50 °F) and wind

speeds greater than 4.8 kilometers/hour (3.0 mph). Wind chill is calculated by the following equations4.

4. http://web.uvic.ca/~eos340/wind_chill.pdf

Table 2. Human Reaction to Heat Index, HI, in Shady Light Wind Conditions*

Heat Index, HI, °F

Human Impact

80 to 90

Fatigue possible with prolonged exposure

and physical activity

90 to 105

Sunstroke, heat cramps and heat exhaus-

tion are possible

105 to 130

Sunstroke, heat cramps and heat exhaus-

tion likely. Heat stroke possible

> 130

Heat stroke is highly likely with continued

exposure

*Note: See www.crh.noaa.gov/pub/heat.php.

HI

42.379

–

2.04901523T



10.14333127RH



0.22475541TRH



–

6.83783

10

3

–

T

2





–

5.481717

10

2

–

RH

2





–

1.22874

10

3

–

T

2

RH





8.5282

10

4

–

TRH

2





1.99

10

6

–

T

2

RH

2





Where:

HI = Head Index in degress Farenheit

T = Ambient temperature in degrees Farenheit

RH = Relative humidity in %

–

++

++

=

T

wc

35.74

0.6215T

a

35.75V

0.16

–

0.4275T

0.16

Where:

T

wc

Wind chill temperature in degrees Farenheit

T

a

Ambient temperature in degrees Farenheit

V

Wind velocity in miles-per-hour

=

=

=

++

=