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Contents: Heat Index | Summer Simmer Index | Heat health conditions | City Averages | Exercise | Wind Chill | Sample Hourly tables

How hot or cold you feel (Apparent temperature) is affected by not only absolute air temperature, but things like wind speed (which carries heat away from you body) and humidity (which affects the body's natural cooling mechanism thru evaporation of perspiration). Attempts to compute indexes to measure this are still evolving as of 2006. The major ones are listed below.

Humans are physically adapted to a narrow range of temperature, with the metabolic mechanism functioning best at air temperatures around 77°F (25°C). Source: Biometeorology at answers.com

There are several different heat indexes.

Since 1923 heating and ventilating engineers, used effective temperature (ET) for determining comfort level. People will feel uncomfortable at effective temperatures above 81°F (27°C) or below 57°F (15°C); between 63°F (17°C) and 77°F (25°C) they will feel comfortable.

Temperature Humidity Index (THI) developed for the US Weather Bureau [now the National Weather Service (NWS)] (Thom, 1959). It computed temperatures in relation to what it felt like at 100% humidity, so the THI is lower than the actual temperature.
The formula is:
THI = 0.8°Ta + ((relative humidity/100)*(Ta - 14.3)) + 46.4, where Ta = temperature, °C.
THI is still used in some tables used for livestock.

Heat Index (HI) or Apparent Temperature (AT) (Steadman, 1979), became the norm adopted by the NWS in the 1980's. Steadman used a model of an average man (5 feet 7 inches, 147 pounds) walking at about 3.1 miles per hour in a light breeze, wearing long pants and a short-sleeved shirt.
It computes temperature relative to a normal relative humidity of 20-40%.

More recently, John W. Pepi, has come up with something called the Summer Simmer Index (SSI) based on temperature relative to a dry 10-20% humidity.

AccuWeather RealFeel® Temperature
The RealFeel Temperature takes into account the effects of multiple parameters, including ambient temperature, Dew point (humidity), wind speed, Sun angle for a given date, Solar intensity as defined from the solar index, percent of sky cover, precipitation intensity/type, elevation and atmospheric pressure.
The secret algorithm is patented.

See RealFeel at accuweather.com and Real Feel Temperature at allExperts.com.

It is generally 3-10° higher than NWS Heat Index (HI) (with 0-5 mph winds), although with very hot (98°) weather and 10 mph winds it can be the same or lower than the HI.

The Wet Bulb Globe Temperature (WBGT) It was developed in the late 1950s for the US Marine Corps in South Carolina. Because its use is recommended in the Standard, ISO 7243, it is often used in Occupational Health and Safety guidelines for working in hot environments. It has been advocated for use in sports requiring continuous exertion, such as the marathon.
It combines three readings 1. Black globe thermometer (temperature inside a black globe, which measures the effect of radiation and wind), 2. Natural wet-bulb temperature (Temperature with a damp cotton over the bulb), 3. Shade air temperature . See: About the WBGT and Apparent Temperature Indices at the Bureau of Meteorology in Australia.

Temperature/Humidity/Sun/Wind (THSW) Index - Takes into account wind and solar radiation on the heat index.

Examples:
Conditions RH Temp
°F
HI SSI
Hot Humid 65% 90 103 111
Hot Dry 20% 105 104 112
Mild 50% 75 75 77
RH = Relative Humidity %
HI = NWS Heat Index
SSI = Summer Simmer Index
ReelFeel = AccuWeather index - Appears to be similar to SSI when there is no wind but closer to the HI with a 10 mph wind.

Relative Humidity:
Humidity itself simply refers to the amount of water vapor in the air. However, the amount of water vapor that the air can contain varies with air temperature and pressure. Relative humidity takes into account these factors and offers a humidity reading which reflects the amount of water vapor in the air as a percentage of the amount the air is capable of holding.
Relative humidity (RH) changes with temperature, pressure, and water vapor content. For example on a humid Aug. day in Bridgewater NJ the RH at 3PM with an air temperature of 91° was 55%, by 11pm the air temperature had dropped to 76° and the RH was up to 89%.

Dew Point:
Dew Point is the temperature to which air must be cooled, at constant barometric pressure, for water vapor to condense into water, called dew (i.e. 100% relative humidity).

20°+C (68°F) Oppressive
18°C (64°F) Sticky
16°C (61°F) Humid
13°C (55°F) Comfortable
10°C (50°F) Refreshing
You can also use dew point to predict the minimum overnight temperature. Provided no new fronts are expected overnight and the afternoon Relative Humidity >= 50%, the afternoon's dew point gives you an idea of what minimum temperature to expect overnight, since the air cannot get colder than the dew point anytime.

Formula for calculating dew point.

Examples:
Temp RH Barometric
Pres
Dew
Point
HI
Denver, CO 95° F 15% 30.14 in. 40°F 91
Seattle, WA 65° F 65% 30.09 in. 53°F 65
Miami, FL 84° F 77% 30.07 in. 76°F 93
Phoenix, AZ 102° F 14% 29.90 in. 44°F 98
St. Louis, MO 89° F 38% 30.09 in. 60°F 89
It had been raining in Miami, hence the high humidity.
See more city data below.

Which city has the driest air?
Phoenix has the most arid (dry) air. The dew point is about 58° away from the current temperature.

Which city would be the most comfortable to exercise in?
In Phoenix, even though the humidity is very low, the temperature is extremely high. Seattle would probably be the most comfortable to exercise in because of the lower temperature. Even though it has a "relatively" high humidity, the lower temperature means that less moisture is in the air.
Source: Education Programs on Weather at arc.nasa.gov
See: Using Temperature-Dew Point Tables at NOAA.gov

Yet other indexes are used for measuring stress in farm animals.
See: Index Comparisons at SummerSimmer.com
Heat Stress Index at NOAA.gov
Method, system, and software for calculating a multi factor temperature index. at FreePatentsOnline.com

Heat Index Risks: National Weather Service (NWS) Heat Index Risks:

  • Less than 80 degrees--Comfortable by most standards
  • 80 to 90 degrees--Caution: fatigue with prolonged exposure and activity, use caution
  • 90 to 104 degrees--Extreme Caution: sunstroke, heat cramps, heat exhaustion possible
  • 105 to 129 degrees--Danger: sunstroke, heat cramps, heat exhaustion likely
  • Over 130 degrees--Extreme Danger: will result in heatstroke or sunstroke quickly.
Technically, Heat Index is calculable only when air temperature is above 68° F (20° C) because it is a measure of heat stress, which is not significant at lower temperatures. Below 57° F (13.9° C), the Heat Index is equal to outside temperature.

Heat Index (HI):
Relative to normal (20-40%) humidity.
Air
°F
Relative Humidity %
0 10 20 30 40 50 60 70 80 90 100
125 111 123 141                
120 107 116 130 148              
115 103 110 120 134 152            
110 100 105 112 122 136 152          
105 97 100 104 112 121 134 149        
100 94 95 97 102 109 118 129 143      
95 89 90 92 94 99 105 113 123 134    
90 85 86 86 88 91 94 100 106 113 122 132
85 81 82 82 83 84 86 89 93 97 102 107
80 77 78 79 79 80 81 82 83 84 86 87
75 69 70 72 73 74 75 76 77 78 79 80
70 64 65 66 67 68 69 70 70 71 71 72
With Prolonged Exposure and/or Physical Activity:
Caution:
Fatigue possible
Extreme Caution:
Sunstroke, muscle cramps, and/or heat exhaustion possible
Danger:
Sunstroke, muscle cramps, and/or heat exhaustion likely
Extreme Danger:
Heat Stroke or Sunstroke likely
Exposure to direct sunlight can increase the Heat Index by up to 15°F.
The National Weather Service issues a heat advisory for HI values ≥ 105°
They issue a heat warning when the HI ≥ 105°F for more than 3 hours per day for 2 consecutive days, or a HI of more than 115°F for any period of time.

Source: NOAA's Heat Index (Apparent Temperature) Chart on their Magazine site and Heat Index Calculator. The calculator based on the formula below gives slightly lower results for some values. We used the calculator values for 80° and above.
The formula at NOAA
T = Temp °F,   rh = Relative humidity
HI = -42.38 + 2.049*T + 10.143*rh - 0.2248*T*rh - .0068378*T2 - .05482*rh2
+.001229*T2*rh +.0008529*T*rh2 -.00000199*T2*rh2
doesn't work below 80°.

The Weather Channel uses values similar to those above except for temperatures below 80°. The values below are observed "feels like" values from the weather channel.
Air
°F
Relative Humidity %
0 10 20 30 40 50 60 70 80 90 100
78           79 79 80 80 81 81
75           75 75 75 75 75  

The ReelFeel = AccuWeather index has higher "feels like" values.

RH = Relative Humidity %
HI = NWS Heat Index
ReelFeel = AccuWeather index Uses wind, clouds, ...

Hourly fluxuation tables (examples for NJ and Sacramento).

The Summer Simmer Index below also has higher values.

See average relative humidity for selected locations below.

See Also:
The Heat Index Explained at the Weather Channel
The Marine Corps. Heat Index and Physical Exercise table uses a Wet-Bulb Globe Temperature Index (WBGTI), which takes into account four variables: air temperature, humidity, radiant heat and air movement.
Dr. Stephen M. Pribut's Running In The Heat page.
Wind speed and solar radiation adjustments for the temperature-humidity index for measuring livestock distress.

Summer Simmer Index (www.summersimmer.com/ssi.htm)
"The New Summer Simmer Index - a Comfort Index for the New Millennium" was presented to an international audience at the 80th annual meeting of the AMS at Long Beach, California, on January 11, 2000.
Computes temperature relative to a dry (10-20% relative humidity) environment.

The New Summer Simmer Index

Temp
(° F)

Relative Humidity (%)

5 10 20 30 40 50 60 70 80 90 100
70   66 69 70 71 72 73 73 74 74 75
75   73 75 76 76 77 78 79 80 81 82
80   78 80 82 83 84 86 88 91 93 97
85   83 86 87 88 92 96 100 104 109 117
90   88 92 94 97 100 106 115 123 135 *
95   94 99 103 107 112 123 135 149
100   100 104 112 117 128 137 *
105 102 106 112 119 131 142 *
110 108 112 119 131 144 *
115 113 117 128 142 *
120 118 124 138 *

Copyright 1999.  John W. Pepi.  All Rights Reserved.

Heat Related Conditions:

  1. Heat syncope: Fainting or near fainting following dizziness, usually while running or after a sudden change in position. Caused by a drop in blood pressure as the brain is deprived of oxygenated blood.
  2. Heat cramps: Tightening or spasms of active muscles, without loss of consciousness. Caused by an electrolyte imbalance.
  3. Heat exhaustion: Dizziness, fatigue, nausea and vomiting, which may be accompanied by irrational behavior of belligerence and some muscle cramping. Loss of consciousness may occur.
  4. Heat stroke (sun stroke): Acute medical emergency. Extremely high body core temperature 106-108° Fahrenheit, no perspiration (hot, dry skin), disorientation, muscle twitching, convulsions, coma and possible death.
Condition Symptoms Response Care
Heat Exhaustion
  • Normal or slightly elevated body
  • temperature
  • Pale, clammy skin with profuse
  • perspiration
  • General weakness with possible
  • headaches
  • Nausea and/or vomiting
  • Dizziness and/or fainting
Stop activity immediately. Get victim to a cool, dry environment and drink plenty of fluids. Do not resume activity until normal fluid balances re-established (1-2 days)
Heat Stroke
  • High body temperature (106°F or
  • higher)
  • Hot, red and dry skin conditions
  • Rapid and strong pulse
  • Victim may be unconscious
Requires immediate medical attention. While waiting for medical response, get victim to cool environment.

Exercise in hot weather:
You have to be especially careful when exercising in hot humid weather.
One measure is the comfort index = T+RH/4
When the comfort index is above 95 and the wind speed is low, it may be necessary to acclimatize to the conditions before attempting very strenuous activity; the presence of wind allows higher values to be tolerated.

The weather channel gives a comfort index from 1-10 when you access your area from the fitness guide.

Average Relative Humidity and Heat Index (HI):
July average afternoon humidity and temperature ° F
Note: It will be more humid and cooler in the mornings.

Location RH Temp °F HI
Record Ave
Houston 65% 106 94 114
Dallas-Fort Worth 55% 115 96 112
New Orleans 69% 102 91 108
Jackson, MS 65% 107 91 105
Phoenix 20% 122 104 103
St. Louis 61% 105 91 103
Jacksonville 59% 103 91 101
Nashville 63% 107 89 99
Atlanta 59% 105 89 97
San Juan, PR 67% 98 87 96
Bakersfield 23% 118 97 95
Washington, D.C. 54% 104 89 94
Miami 63% 98 87 94
Raleigh 58% 105 88 94
Sacramento 30% 114 94 93
Honolulu 52% 95 88 92
Albuquerque 27% 107 92 90
Los Angeles 69% 112 84 90
Location RH Temp °F HI
Record Ave
Philadelphia 54% 106 86 89
Chicago 61% 104 84 88
New York 55% 105 85 88
Pittsburgh 54% 101 85 87
Reno 19% 108 91 87
Denver 34% 105 88 86
Minneapolis-St. Paul 60% 108 83 86
Salt Lake City 22% 102 89 86
Boise 21% 111 89 85
Providence 56% 104 83 85
Concord, NH 51% 102 83 84
Boston 57% 104 82 84
San Diego 67% 111 76 76
Seattle-Tacoma 49% 100 75 75
San Francisco 60% 106 71 71
Juneau 67% 90 64 64
Source for humidity: www.census.gov/compendia/statab/tables/07s0384.xls
Note: Record High is for any month. In San Diego it was Sept., in San Francisco it was June.

Hottest temperature in North America - 134 Deg F at Greenland Ranch, now known as Furnace Creek Ranch in Death Valley, California in 1913.

Hourly fluxuation tables (examples for NJ and Sacramento).

See Also:
Relative Humidity at CityRating.com
Average Relative Humidity at ncdc.noaa.gov
NOAA's Mean Heat Index Forecasts


Wind Chill:
The apparent temperature felt on exposed skin due to the combination of air temperature and wind speed.

The exact definition of wind chill has been controversial because it is a composite index, because animate and inanimate bodies behave differently, and because wind chill reports have a major impact on winter tourism.

The US and Canadian formulae are best suited to extremely cold climates. Other formulae such as the Steadman Wind Chill Index (developed by Australian environmental scientist Robert Steadman it takes into account the metabolic heat generation of the human body) have been developed for temperate climates, but are less well known.

In 2001 the National Weather Service (NWS) implemented the new wind chill index, used by the US and Canadian weather services, which is determined by iterating a model of skin temperature under various wind speeds and temperatures.
Twc = 35.74 +0.6215*Ta - 35.75*V0.16 +0.4275*Ta*V0.16
where Twc is the wind chill and Ta is the air temperature in °F and V is air speed in mph.

Wind Chill Chart
Wind
speed
MPH
Air Temperature °F
40 35 30 25 20 15 10 5 0 -5 -10 -15 -20 -25 -30 -35 -40 -45
5 36 31 25 19 13 7 1 -5 -11 -16 -22 -28 -34 -40 -46 -52 -57 -63
10 34 27 21 15 9 3 -4 -10 -16 -22 -28 -35 -41 -47 -53 -59 -66 -72
15 32 25 19 13 6 0 -7 -13 -19 -26 -32 -39 -45 -51 -58 -64 -71 -77
20 30 24 17 11 4 -2 -9 -15 -22 -29 -35 -42 -48 -55 -61 -68 -74 -81
25 29 23 16 9 3 -4 -11 -17 -24 -31 -37 -44 -51 -58 -64 -71 -78 -84
30 28 22 15 8 1 -5 -12 -19 -26 -33 -39 -46 -53 -60 -67 -73 -80 -87
35 28 21 14 7 0 -7 -14 -21 -27 -34 -41 -48 -55 -62 -69 -76 -82 -89
40 27 20 13 6 -1 -8 -15 -22 -29 -36 -43 -50 -57 -64 -71 -78 -84 -91
45 26 19 12 5 -2 -9 -16 -23 -30 -37 -44 -51 -58 -65 -72 -79 -86 -93
50 26 19 12 4 -3 -10 -17 -24 -31 -38 -45 -52 -60 -67 -74 -81 -88 -95
55 25 18 11 4 -3 -11 -18 -25 -32 -39 -46 -54 -61 -68 -75 -82 -89 -97
60 25 17 10 3 -4 -11 -19 -26 -33 -40 -48 -55 -62 -69 -76 -84 -91 -98
Frostbite Times 30 minutes 10 minutes 5 minutes
See: Wind Chill Chart at the NWS
and answers.com

Mt. Washington NH has some of the highest winds in the country. During the winter summit speeds average 44 mph. Winds reach hurricane vorce (73 mph) 110 days a year and crack 100 mph 63 days a year. In April 1934, a wind gust of 231 mph hit the summit, an all-time surface wind speed record.

Frostbite and Hypothermia:
Bare skin freezes at wind chill of -31°F and colder. Additionally, prolonged exposure of skin to any freezing temperatures can cause injury.
A wind chill advisory is issued when the forecast projects a wind velocity of at least 10 mph (16 km/hr) producing a wind chill temperature of -15°F or lower for 3 hours or more.

Hypothermia occurs when body temperature falls below 95 degrees Fahrenheit. Hypothermia can occur at temperatures at, or below 45 degrees Fahrenheit, or even in the 50's when the weather is also wet and windy. Most cases of hypothermia occur during the spring, summer and fall. Determine this by taking your temperature. Warning signs include uncontrollable shivering, memory loss, disorientation, incoherence, slurred speech, drowsiness, and exhaustion. Get medical attention immediately. If you can't get help quickly, begin warming the body SLOWLY. Warm the body core first, NOT the extremities.
Generally conductive heat loss accounts for only about 2% of overall loss. However, with wet clothes the loss is increased 5x. Perspiration (sweat) from heavy exercise will hasten heat loss in cold weather.
Wearing a hat is essential as up to 50% of body heat can be lost though the head.
Cold weather dehydration can lead to total body core cooling.
Condition Symptoms Response Care
Frostbite A white or grayish-yellow skin area Skin that feels unusually firm or waxy Numbness
  • Get into a warm room as soon as possible.
  • Unless absolutely necessary, do not walk on frostbitten feet or toes--this increases the damage.
  • Immerse the affected area in warm--not hot--water (the temperature should be comfortable to the touch for unaffected parts of the body).
  • Or, warm the affected area using body heat. For example, the heat of an armpit can be used to warm frostbitten fingers.
  • Do not rub the frostbitten area with snow or massage it at all. This can cause more damage.
  • Don't use a heating pad, heat lamp, or the heat of a stove, fireplace, or radiator for warming. Affected areas are numb and can be easily burned.
Hypo-
thermia
Adults:
Shivering/exhaustion
Confusion/fumbling hands
Memory loss/slurred speech
Drowsiness

Infants:
Bright red, cold skin Very low energy

  • Get the victim into a warm room or shelter.
  • If the victim has on any wet clothing, remove it.
  • Warm the center of the body first--chest, neck, head, and groin--using an electric blanket, if available. Or use skin-to-skin contact under loose, dry layers of blankets, clothing, towels, or sheets.
  • Warm beverages can help increase the body temperature, but do NOT give alcoholic beverages. Do not try to give beverages to an unconscious person.
  • After body temperature has increased, keep the person dry and wrapped in a warm blanket, including the head and neck.
  • Get medical attention as soon as possible.
Source: CDC Winter Weather FAQ Links:
Outdoor Action Guide to Hypothermia And Cold Weather Injuries

Coldest Av. Wind Chill (excl Alaska)(January)
Rnk Location Wind
chill
temp wind
mph
°F Rnk
1 Mt. Washington, NH -37 -4.6 2 46
2 International Falls, MN -27 -10 1 9
3 Fargo, ND -22 -3.6 4 13
4 Duluth, MN -20 -2.2 6 12
5 Caribou, ME -19 -1.6 9 11
6 Bismarck, ND -18 -1.7 8 10
7 WIlliston, ND -18 -1.8 7 10
8 Aberdeen, SD -18 -0.6 10 11
9 Saint Cloud, MN -17 -2.4 5 8
10 Alamosa, Co -16 -3.9 3 6
11 Rochester, MN -16 2.6 13 14
12 Glasgow, Mt -14 1.2 11 10
13 Huron, SD -14 2.3 12 11
14 MInneapolis-St.Paul, MN -13 2.8 14 11
15 Sioux Falls, SD -13 3.3 15 11
16 Waterloo, IA -10 5.4 18 11
17 Sault Ste. Marie, MI -10 4.6 16 10
18 Green Bay, WI -9 5.8 19 11
19 La Crosse, WI -8 5.3 17 9
20 Sioux City, IA -7 7.7 24 11
21 Madison, WI -7 7.2 21 10
22 Valentine, NE -7 6.6 20 9
23 Norfolk, NE -7 8.4 26 12
24 Burlington, VT -6 7.5 23 10
25 Houghton Lake, MI -5 8.4 27 10
26 North Platte, Ne -5 8.6 30 9
27 Casper, WY -5 12 47 16
28 Great Falls, MT -4 11.6 44 15
29 Ely, NV -4 9.4 32 10
30 Concord, NH -4 7.4 22 7
Highest Wind Speed (excl. Alaska)
Rank Location Speed mph
Max Jan
1 Mt. Washington, NH 231 173
2 Galveston, TX 100 53
3 Fort Myers, FL 92 40
4 Blue Hill, MA 92 76
5 Sioux City, IA 91 56
6 Buffalo, NY 91 91
7 Providence, RI 90 46
8 Portland, OR 88 54
9 Miami, FL 86 46
10 West Palm Beach, FL 86 48
11 Redding, CA 85 70
12 Lihue, HI 84 37
13 Great Falls, MT 82 65
14 Newark, NJ 82 52
15 Flint, Mi 81 45
17 Allentown, Pa 81 55
18 Casper, Wy 81 58
19 Baltimore, Md 80 63
20 Scottsbluff, Ne 80 53
21 Guam, Pc 80 64
22 Billings, Mt 79 66
23 San Juan, Pr 79 25
24 Williamsport, Pa 78 66
25 Pendleton, Or 77 49
26 Huron, Sd 77 57
27 Madison, Wi 77 68
28 Blue Canyon, Ca 76 67
29 Des Moines, Ia 76 66
30 Worcester, Ma 76 60
31 San Angelo, Tx 75 44
32 Victoria, Tx 75 49
33 Wallops Island, Va 75 70
34 Sacramento, Ca 74 60
35 Bridgeport, Ct 74 67
Sources: Normal Daily Minimum Temperature, Maximum Wind Speed, Wind- Average Wind Speed

What is the Average Wind Speed in My Region? at awea.org
Wind Energy Atlas at National Renewable Energy laboratory nrel.gov Winter | Spring ! Summer | Autum


Terms:
Apparent Temperature (AT) - Heat Index
Black-globe humidity index (BGHI) - temperature humidity index using black globe temperature which accounts for radiant heat. Used by livestock producers.
Dew-point is the temperature to which air must be cooled for saturation (100% relative humidity) to occur, providing there is no change in water content.

Temperature/Humidity/Sun/Wind (THSW) Index - Takes into account wind and solar radiation on the heat index.
Wet Bulb Globe Temperature (WBGT) - Used in workplace safety requirements by OSHA and others.

See:
Weather Terms
Weather Parameter Definitions at cox.net
"Exercising in the Heat and Sun"; June 2007 Research Digest of the President's Council on Physical Fitness and Sports

Links:
Cattle Today
NOAA National Data Center Tables
See: Apparent Temperature Indices at the Bureau of Meteorology in Australia.
Climate Information
Air properties - temperature, density, specific heat, thermal conductivity, expansion coefficient, kinematic viscosity
Apparent Temperature Indices at the Bureau of Meteorology in Australia.
THOM E.C. 1959 "The Discomfort Index" Weatherwise 12 pp 57-59


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last updated 9 Jun 2008