Meteorological Tables

  1. Wind Chill
  2. Heat Index

August, 2001 Press Release:

New Wind Chill Temperature Index The NWS is planning to implement a replacement Wind Chill Temperature (WCT) index for the 2001/2002 winter season. The reason for the change is to improve upon the current WCT Index used by the NWS and the Meteorological Services of Canada (MSC, the Canadian equivalent of the NWS), which is currently based on the 1945 Siple and Passel Index. For over a year, there has been discussion within the NWS and Meteorological Services of Canada (MSC), about updating the WCT. During the Fall of 2000, the Office of the Federal Coordinator for Meteorological Services and Supporting Research (OFCM) formed a special group consisting of several Federal agencies, MSC, the academic research community (Indiana University-Purdue University in Indianapolis (IUPUI), University of Delaware, and University of Missouri), and the International Society of Biometeorology to evaluate the existing wind chill formula and make necessary changes to improve upon it. The group is called the Joint Action Group for temperature Indices (JAG/TI) and is chaired by the NWS. The goal of JAG/TI is to internationally upgrade and standardize the index for temperature extremes (e.g., Wind Chill Index).

After the October 2000 and February 2001 meetings, the JAG/TI reached agreement on a new wind chill formula, discussed a process for scientific verification of the new formula, and developed plans for implementation of the new formula. The new WCT index was presented at the JAG/TI meeting in Toronto, Canada on August 2, 2001. The JAG/TI formula will make use of advances in science, technology, and computer modeling to provide a more accurate, understandable, and useful formula for calculating the dangers from winter winds and freezing temperatures. In addition, clinical trials have been conducted and the results of those trials have been used to verify and improve the accuracy of the new formula. Standardization of the WCT Index among the meteorological community is important, so that an accurate and consistent measure is provided and public safety is ensured. Our goal is to implement the new wind chill formula in Canada and the United States during the same time frame in order to have a consistent WCT Index for North America.

Specifically, the new WCT index will: use wind speed calculated at the average height (5 feet) of the human body's face instead of 33 feet (the standard anemometer height); be based on a human face model; incorporate modern heat transfer theory (heat loss from the body to its surroundings, during cold and breezy/windy days); lower the calm wind threshold to 3 mph; use a consistent standard for skin tissue resistance; and assume the worst case scenario for solar radiation (clear night sky).

For example, assuming an air temperature of 5 degrees and a wind of 30 mph... Old WCT = -41 New WCT = -18

To reflect the new formula, National Weather Service Forecast Offices have adjusted the threshold values of Wind Chill Temperatures that trigger Wind Chill Warnings and Wind Chill Advisories.

In 2002, adjustments for solar radiation (i.e., the impact of sun) for a variety of sky conditions (sunny, partly sunny and cloudy) will be added to the calculation model.


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Here's the formula:

Wind Chill (F) = 35.74 + 0.6215T - 35.75(V0.16)
                     + 0.4275T(V0.16)
Where V = the wind speed value in mph and
      T = the temperature in F

Note: Frostbite occurs in 15 minutes or less at wind chill values of -18 or lower

Compare new wind chill to old wind chill

=

Index Index


Heat Index Table

HEAT INDEXaffects on the human body
130 or aboveheat stroke highly likely with continued exposure
105 to 130heat stroke likely with prolonged exposure
90 to 105heat stroke possible with prolonged exposure


RELATIVE

HUMIDITY

AIR TEMPERATURE (F)
70 75 80 85 90 95 100 105110115120
30677378849096104113123135148
35677379859198107118130143
406874798693101110123137151
456874808795104115129143
506975818896107120135150
556975818998110126142
6070768290100114132149
6570768391102119138
7070778593106124144
7570778695109130
8071788697113136
8571788799117
90717988102122
95717989105
100728091108