The Extreme Heat Climatology of WFO St.

Louis (LSX) CWAMark F. Britt

National Weather ServiceSt. Louis, MO

Why Study Heat?Extreme Heat is the biggest killer in MO & IL 1995-

2006 (StormData and State of Missouri).

Event Type IL MO Total

Heat 917* 353** 1270

Flood/Flash Flood 13 49 62

Tornado 32 33 55

Cold 45 6 51

Snow and Ice 20 2 22

Tstm Wind 16 9 25

Lightning 13 8 21

*583 of these deaths occurred in the Chicago Metro area in July 12-16, 1995.**852 deaths in Missouri from 1980-2005

Why Am I Here?Gain a basic understanding of heat index climatology that can:

Answer media queriesProvide background information when forecasting extreme heat days.

Why Am I Here?Examine the Heat Index (HI) equation.Examine how often advisories and warnings are required. Examine HI distributions:

AnnuallyMonthlyHourlyTemperatureMinimum TemperatureDewpointTemperature vs. DewpointWind DirectionWind Speed

Examine composites of the advisory and warning HI days.

DatabaseThirty four years (1973-2006) of hourly temperature and dewpoint data collected by the National Climatic Data Center (NCDC) and the Midwest Climate Center (MCC).Did examine hourly data from 1930-1973 on days with a maximum temperature >102o F. However, this database did not yield any additional significant information.

Methodology

. Some defintions: A heat advisory hour (HAH) is when the HI at one

of these sites is between 105oF and 114oF. Because of the 5oF assumption for the city of St. Louis, a HAH is between 100oF and 109oF using the STL observation.

A heat warning hour (HWH) is when the HI at one of these sites is 115oF (110oF for the city).

A heat advisory day is a calendar day (CDT) with at least 3 HAH (not necessarily consecutive).

An intensity warning day is the same as an advisory day, only using HWH.

A duration warning day occurs in a stretch when there are at least 3 consecutive heat advisory or warning days.

MethodologyThis is a climatology study. It only looks at actual observations from STL, COU, and UIN. This study does not look at what headlines were actually issued.

Most of the distributions in this study include all HAH or HWH, regardless if they occurred on advisory or warning days (i.e. included HI hours on days with only 1 or 2 HAH or HWH).

Any reference to advisory or warning days means that there were at least 3 HAH or HWH on that day.

Heat Index Equation“No true equation for HI exists.” (Rothfusz, 1990)The “equation” we use today is actually the result of a multiple regression analysis (“a round-about way”) performed on a table developed by Steadman (1979). Steadman performed extensive biometeorological studies using parameters which can be individually modeled. Using assumptions for most of these parameters, the “equation” is reduced to a relationship between the dry bulb temperature and humidity.

Steadman’s ParametersVapor pressure. Ambient vapor pressure of the

atmosphere. (1.6 kPa)Dimensions of a human. Determines the skin's surface

area. (5' 7" tall, 147 pounds)Effective radiation area of skin. A ratio that depends

upon skin surface area. (0.80)Significant diameter of a human. Based on the body's

volume and density. (15.3 cm)Clothing cover. Long trousers and short-sleeved shirt is

assumed. (84% coverage)Core temperature. Internal body temperature. (98.6°F)Core vapor pressure. Depends upon body's core

temperature and salinity. (5.65 kPa)Surface temperatures and vapor pressures of skin and

clothing. Affects heat transfer from the skin's surface either by radiation or convection. These values are determined by an iterative process.

Steadman’s AssumptionsActivity. Determines metabolic output. (180 W m-2 of skin

area for the model person walking outdoors at a speed of 3.1 mph)

Effective wind speed. Vector sum of the body's movement and an average wind speed. Angle between vectors influences convection from skin surface (below). (5 KNOTS!!!)

Clothing resistance to heat transfer. The magnitude of this value is based on the assumption that the clothing is 20% fiber and 80% air.

Clothing resistance to moisture transfer. Since clothing is mostly air, pure vapor diffusion is used here.

Radiation from the surface of the skin. Actually, a radiative heat-transfer coefficient determined from previous studies.

Convection from the surface of the skin. A convection coefficient also determined from previous studies. Influenced by kinematic viscosity of air and angle of wind.

Sweating rate. Assumes that sweat is uniform and not dripping from the body.

Heat Index Equation

This equation was obtained by multiple regression analysis and there is a ±1.3 degree °F error:

HI = -42.379+2.04901523T+10.14333127R - 0.22475541TR - 0.00683783T2- .05481717R2+ .00122874T2R+0.00085282TR2- 0.00000199T2R2

Where:T = ambient dry bulb temperature (°F)R = relative humidity (integer %).

2006 Year In Review

Date City STL COU UIN7/13 2 (101º)

7/15 2 (101º)

7/17 4 (103º) 4 (105º)

7/18 9 (112º) 5 (112º) 6 (106º)

7/19 8 (107º) 5 (107º) 7 (110º) 4 (110º)

7/20 12 (112º) 8 (112º) 7 (108º) 1 (105º)

7/26 3 (101º)

7/29 3 (103º)

7/30 8 (105º) 1 (105º)

7/31 10 (108º) 4 (108º) 1 (105º)

8/1 8 (105º) 1 (105º)

8/2 7 (103º) 2 (107º) 2 (105º)

8/9 5 (106º) 2 (106º) 2 (105º)

8/10 1 (101º)

(# of hour/day and max HI on that day)

Advisory Duration Warning

Intensity Warning

2007 So FarDate City STL COU UIN

7/9 1 (100º) None None None

7/18 1 (100º)

Top 10 Heat Index Days# STL (& City) COU UIN

1 7/13/1995 (120o) 7/13/1995 (116o) 7/23/2005 (122o)

2 8/18/1995 (118o) 8/29/1984 (115o) 7/11/1980 (120o)

3 7/14/1980 (118o) 8/11/1997 (115o) 7/8/1980 (118o)

4 7/12/1995 (117o) 8/18/1995 (114o) 7/7/1980 (118o)

5 7/1/1980 (116o) 7/30/1986 (113o) 7/12/1980 (118o)

6 7/13/1977 (116o) 6/27/1980 (113o) 7/19/1980 (117o)

7 7/13/2004 (116o) 7/1/1980 (113o) 7/14/1980 (117o)

8 8/17/1995 (116o) 7/12/1995 (113o) 8/16/1983 (116o)

9 7/4/1990 (115o) 8/19/1995 (113o) 7/4/1980 (115o)

10 7/29/1999 (115o) 7/17/2001 (113o) 7/23/2001 &

7/10/1980 (114o)

Heat Index Headlines

 

Days/Yr Needing Headline

Days/Yr NOT Needing

Headline*

City 13.94 3.82

STL 4.35 2.76

COU 3.03 2.24

UIN 2.44 1.44

*Days with only 1 or 2 hours meeting advisory criteria.

Heat Index Headlines

  City STL COU UIN

Advisory 12.62 4.24 2.97 2.29

Warning -- Intensity (>115oF for 3 hrs) 1.29 0.11 0.03 0.15

Warning – Length(3 consecutive days w/ adv. criteria) 7.68 2.03 1.18 0.85

Warning (Both) 1.00 0.08 0 0.12

Days Per Year

84 83 83 83 84 78 83 81

65 70 67 75 66 78 67 74

Max

Min

106 106 106 105 111 110 105 105

84 88 88 93 88 95 88 95

Max

Min

82 83 83 83 82 80 81 80

54 66 62 71 47 65 65 72

Max

Min

20 20 20 16 21 11 18 12

0 0 0 0 0 5 0 0

Max

Min

Composites Daily mean composites of several variables (500mb height,

850mb height and temperature, 925mb specific humidity, MSLP, 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).

Advisory composites were randomly selected from days that KSTL had 1.) a heat index at 21Z of 100 to 104oF and 2.) were not part of a duration warning (i.e. lower end advisories).Warning composites were randomly selected from days that reached intensity warning criteria.None of the dates are within 10 days of each other to prevent sampling the same pattern.

Composites500 mb Heights

Advisory Warning

CompositesPrecipitation Rate

Advisory Warning

Composites850 mb Heights

Advisory Warning

Composites850 mb Temperatures

Advisory Warning

Composites925 mb Specific

Humidity

Advisory Warning

CompositesMSLP

Advisory Warning

Composites1000mb Temperature

Advisory Warning

SummaryUsing the current criteria, a heat advisory is

needed for the city 3 to 5 times more often than the rest of the CWA.

Adding duration to the warning criteria significantly increases (5 to 20 times) the number of warnings needed.

Our “Heat Season” occurs from late June until late August, with the most intense part during mid-late July.

The peak HI for the day usually occurs between3-4 pm CDT, with “significant heat” lasting from Noon - 7 pm CDT.

SummaryMean temperatures for an advisory are in the mid

90s, and average dewpoints are in the lower-mid 70s.

Mean temperatures during a intensity based warning are in the upper 90s, with dewpoints in the mid-upper 70s.

Average minimum temperatures during warnings and advisories are in the mid-upper 70s.

Average wind direction during warnings and advisories are south to west, and average wind speeds differences are not significantly different between warnings and advisories.

SummaryA Comparison of 20 advisory days to 20 intensity warning days at KSTL indicate:

A large 500 mb high dominating the southern 2/3rds of the nation. Heights are higher during the warning days than the advisory days.

There is less precipitation during a warning day than a advisory day over the Midwest.

High pressure is located over the southeast U.S. (Bermuda High) with trough to the west from 850mb on down. This sets up southwesterly low level flow into IL/MO.

850mb and 1000mb temps are 3-4oC warmer on warning days than on advisory days.

Future WorkExamine an hourly dataset from

COU that goes back to 1946.

Compare this database with the Heat Health System.

Update current data every yearly.

ReferencesMesinger, Fedor, DiMego, Geoff, Kalnay, Eugenia, Mitchell, Kenneth, Shafran, Perry C., Ebisuzaki, Wesley, Jovi , Dušan, Woollen, Jack, Rogers, Eric, Berbery, Ernesto H., Ek, Michael B., Fan, Yun, Grumbine, Robert, Higgins, Wayne, Li, Hong, Lin, Ying, Manikin, Geoff, Parrish, David, Shi, Wei, 2006: North American Regional Reanalysis Bulletin of the American Meteorological Society, 87, 343-360

Rothfusz, L. P.,1990:The heat index equation (or, more than you ever wanted to know about heat index). NWS Southern Region Technical Attachment, SR/SSD 90-23, Fort Worth, TX.

Steadman, R.G., 1979: The assessment of sultriness. Part I: A temperature-humidity index based on human physiology and clothing science. J. Appl. Meteor., 18, 861-873.

U.S. Local Storm Events Database (1995-2004). www.ncdc.noaa.gov