The Wind Chill Climatology of the WFO LSX CWA

Mark F. BrittNational Weather Service

St. Louis, MO

The Why and How

Gain a basic understanding of Wind Chill (WC) climatology that can help with:

Answer media queries

Forecasting extreme cold days

The Database covers 35 winters (1973/74-2007/08) of hourly temperature and wind observations from the airports at STL, COU, & UIN collected by the National Climatic Data Center (NCDC) and the Midwest Climate Center (MCC). It does not refer to any advisory or warning that was in effect.

Wind ChillCalculates wind speed at an average height of five feet (typical height of an adult human face) based on readings from the national standard height of 33 feet (typical height of an anemometer) Is based on a human face modelIncorporates modern heat transfer theory (heat loss from the body to its surroundings, during cold and breezy/windy days) Lowers the calm wind threshold to 3 mph Uses a consistent standard for skin tissue resistanceAssumes no impact from the sun (i.e., clear night sky).

Wind Chill Formula

Wind Chill (ºF) =35.74+0.6215T-35.75(V0.16)+ 0.4275T(V0.16)

Where:

T = ambient dry bulb temperature (°F)

V = wind speed (mph)

A Few Definitions

Advisory Criteria: -15o to -24o FWarning Criteria: < -25o FHeadlines: Either Advisories or WarningsDays: Calendar Days in UTC

Top 10 WC Events at STL

Rank# DatesColdest

WC < 0 F < -15F < -25F

1 1/19-21/1985 -48 53 40 18

2 1/9-11/1982 -44 56 38 28

3 12/21-26/1983 -41 112 84 38

4 1/15-19/1977 -38 88 54 27

5 12/21-24/1989 -37 82 59 38

6 2/10-12/1981 -36 41 20 10

7 2/1-5/1996 -35 64 43 13

8 1/27-31/1977 -34 77 35 15

9 1/16-18/1982 -32 54 31 7

10 1/17-20/1994 -29 66 24 8

Other CWA Coldest WC

UIN COU

Rank # Date UIN Date COU

1 1/10/1982 -51 1/20/1985 -44

2 1/20/1985 -50 12/25/1983 -44

3 12/24/1983 -46 12/22/1989 -43

4 1/16/1977 -43 1/10/1982 -41

5 1/22/1989 -41 1/28/1977 -36

6 2/3/1996 -38 1/16/1977 -36

7 1/28/1977 -38 2/11/1981 -35

8 1/18/1994 -37 12/15/1989 -35

9 1/12/1974 -37 2/3/1996 -31

10 12/15/1985 -36 1/14/1979 -30

Earliest and Latest Occurrences

STL COU UIN

Earliest Latest Earliest Latest Earliest Latest

of < 0F11/8

(1991)3/24

(1974)11/3

(‘74 & 91)3/24

(1974)11/3

(1991)4/14

(1991)

-15 to -24F

12/6 (1977)

3/4 (1978)

12/2 (1985)

3/8 (1996)

12/2 (1985)

3/8 (1996)

< -25F12/15 (1989)

2/11 (1981)

12/15 (1989)

2/12 (1988)

12/9 (1977)

3/4 (1978)

WC Hours and Days at STL

 # of

Hours

Average # of

Hours/Year # of Days

Average # of

Days/Year

-15o to-24 F 760 22 85 3

< -25o 244 7 33 1

Composites Mean composites of several variables (500mb height,

850mb height and temperature, as well as 1000mb temperature, and precipitation rate) were generated using the North American Regional Reanalysis Dataset from the NOAA/ESRL Physical Sciences Division, Boulder Colorado. (http://www.cdc.noaa.gov). Composites include 20 separate days valid when advisory or warning criteria were met.Twenty days were chosen between 1985 and 2005 when UIN had a minimum wind chill between -15 and -20°F (i.e. advisories).Twenty days were chosen between 1979 and 2003 when UIN had a minimum wind chill < -25°F (i.e. warnings).

Composites500 mb Heights

Advisory Warning

Composites850 mb Heights

Advisory Warning

Composites850 mb Temperatures

Advisory Warning

Composites1000 mb Heights

Advisory Warning

Composites1000 mb Temperatures

Advisory Warning

SummaryFor the past 35 winters, the three

coldest wind chill years occurred in the late ’70s.

Our “Wind Chill Season” lasts from November to March, with headlines required from December to early March.

On average, STL will require 3 advisories and 1 warning per year.

SummaryFor the past 35 winters, the three

coldest wind chill years occurred in the late ’70s.

Our “Wind Chill Season” lasts from November to March, with headlines required from December to early March.

On average, STL will require 3 advisories and 1 warning per year.

SummaryThe coldest wind chills occur at night.

Temperature appears to be a bigger discriminator for advisories vs. warnings than wind speed.

Wind chill headlines tend to occur with west or northwest winds.

SummaryComposite means depict northwesterly

flow at 500mb between a polar vortex over southeastern Canada and ridging over western North America.

Temperatures at 850mb and 1000mb are noticeably colder during warning days than advisory days.

Northwesterly flow is noted near the surface with high pressure is centered over the central plains.