Fire Weather Concerns for the Plains; Record Warmth Spreads

Dry and breezy conditions prevail across the High Plains where elevated to locally critical conditions continue through Thursday. Meanwhile, record warmth increases and spreads across the South, Southwest and into southern California into this last weekend of February. Meanwhile, a fast moving system moves across the Plains then off the mid-Atlantic by Thursday evening. Read More >

Pictures from around western Colorado and eastern Utah

Wind Chill

The NWS implemented a replacement Wind Chill Temperature (WCT) Index during the 2001/2002 Winter Season. The reason for this change was to improve upon the old WCT Index used by the NWS and the Meteorological Services of Canada, which was based on the 1945 Siple and Passel Index.

Wind Chill is the term used to describe the rate of heat loss from the human body resulting from the combined effect of low temperature and wind. As winds increase, heat is carried away from the body at a faster rate, driving down both the skin temperature and eventually the internal body temperature.

The Wind Chill Index is designed for heat loss in humans. It has no effect on inanimate objects.

Click here for Information on Frostbite and Hypothermia

Cartoon image of a winter wind being blown up by a cloud

Wind Chill, the measure of the rate of heat loss based on air temperature and wind speed, is not a true temperature, but it allows us to understand how heat is lost to the wind.

The old WCT formula was introduced by Antarctic explorer Paul A. Siple in his 1939 dissertation, "Adaptation of the Explorer to the Climate of Antarctica." During the 1940s, Siple (who coined the term "wind chill") and Charles F. Passel experimented with measuring the time it took to freeze 250 grams of water in different temperature and wind conditions. They developed empirical formulas relating that data to the rate of heat loss from exposed human skin.

The new WCT formula makes 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.

Specifically, the new WCT Index:

Uses wind speed calculated at the average height (5 feet) of the human face, instead of 33 feet (the standard anemometer height),
Is based on a human face model,
Incorporates modern heat transfer theory,
Lowers the calm wind threshold to < 3 mph,
Uses a consistent standard for skin tissue resistance,
Assumes the worst case scenario for solar radiation (clear night sky).

Wind Chill Chart
Temperature (°F)
Wind Speed (MPH)	calm	40	35	30	25	20	15	10	5	0	-5	-10	-15	-20	-25
	5	36	31	25	19	13	7	1	-5	-11	-16	-22	-28	-34	-40
	10	34	27	21	15	9	3	-4	-10	-16	-22	-28	-35	-41	-47
	15	32	25	19	13	6	0	-7	-13	-19	-26	-32	-39	-45	-51
	20	30	24	17	11	4	-2	-9	-15	-22	-29	-35	-42	-48	-55
	25	29	23	16	9	3	-4	-11	-17	-24	-31	-37	-44	-51	-58
	30	28	22	15	8	1	-5	-12	-19	-26	-33	-39	-46	-53	-60
	35	28	21	14	7	0	-7	-14	-21	-27	-34	-41	-48	-55	-62
	40	27	20	13	6	-1	-8	-15	-22	-29	-36	-43	-50	-57	-64
	45	26	19	12	5	-2	-9	-16	-23	-30	-37	-44	-51	-58	-65

"Calm-air" as used in wind chill determinations actually refers to the conditions created by a person walking briskly (at 4 miles-per-hour) under calm wind conditions.

Frostbite Time for Exposed Skin (minutes)

30 15 10 5

Wind Chill Formula: **

WCT (°F) = 35.74 + 0.6215T - 35.75(V ^0.16) + 0.4275T(V ^0.16)

where, T = Air Temperature ("F), V = Wind Speed (MPH)

(** - valid only for wind speeds > 4 MPH)

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