SOUTHERN ILLINOIS UNIVERSITY CARBONDALE
Assessing Equivalent
Temperature Trends in
Major Eastern US Cities
Master’s Thesis Proposal
12/9/2014
Advisor: Dr. Justin T. Schoof
By: Mercedes L. Gomez Jacobo
Mercedes L. Gomez Jacobo
TABLE OF CONTENTS
Introduction……………………………………………………………3
Literature Review………………………………………………………4
Problem Statement and Research Questions………………………….9
Preliminary Methodology……………………………………………..9
Conclusion……………………………………………………………12
References…………………………………………………………….12
Annotated Bibliographies
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Mercedes L. Gomez Jacobo
Introduction
Extreme heat events in the United States are responsible for more deaths on average than
all other fatal weather events combined (National Weather Service, 2014). During the years
1999-2009 the United States experienced extreme heat events that claimed 7,233 lives, which is
an average of 658 heat-related deaths per year (US Department of Health and Services,
morbidity and mortality report, 2013). Many of these deaths often occur in large cities which
tend to house their own microclimates by creating their own set of thermal, radiative and
moisture conditions (Oke, 1997). The urban heat island effect refers to the warmer air
temperatures that occur in cities when compared to their rural neighbors (Oke, 1986). Urban
regions and their unique microclimates are of importance because it is where human activities
display the changes they create in the atmosphere most (Oke, 1997). Large cities have copious
amounts of asphalt and concrete, these impervious surfaces enable cities to modify the local
hydrologic cycle. A thermodynamic metric called equivalent temperature (𝑇𝐸 ) allows us to
quantify the amount of energy in a parcel of air by using temperature, dew point and pressure.
High temperatures coupled with high humidity contribute to human heat stress, for this
reason it is important to analyze changes in heat wave events using metrics that take both
humidity and temperature into to account. 𝑇𝐸 is the temperature that an air parcel would have if
all associated water vapor were condensed and the resulting latent heat is used to increase the
temperature of the parcel (Schoof et al., 2014). Equivalent temperature (°C) allows us to quantify
and separate the moist and dry components which contribute to its magnitude; this makes it a
good metric for assessing heat waves (Schoof et al. 2014). When 𝑇𝐸 is high both the temperature
and dew point are high because it is equally dependant of both variables along with pressure. By
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Mercedes L. Gomez Jacobo
contrast another common metric is the heat index, however the heat index is highly dependent of
temperature (or apparent temperature) and can be high, even when the humidity isn’t. The 𝑇𝐸
metric allows us the get a more accurate measure of the moisture in the air and also provides a
measure of lower atmospheric energy content (Pielke, 2004).
This study will analyze equivalent temperature trends in 21 of the largest cities in the
Eastern US in order to better understand the relationship between temperature and 𝑇𝐸 trends. The
cities chosen for this study are all located east of the 94th meridian; we refer to this area as the
Eastern US. The cities are both in coastal and continental regions which in 2010 were home to
over 24 million Americans (US Census Bureau, 2010).
Literature Review
Urban Heat Islands
The urban regions of the United States (US) are representative of the growth and
development experienced since the industrial age. Cities offer economic opportunity, cultural
diversity, centralized business districts, extensive road systems, complex infrastructure and a
variety of jobs for booming populations. Cities also have unintended impacts and consequences
such as magnified heat waves due to urban heat islands(Li and Bou-Zeid, 2013), modified
hydrologic cycles (Grimmond et al. 1986), and thermal pollution of nearby streams(Sommers et
al. 2013). The impacts that cities have on the environment is the focus of many studies because
they show clear examples of human induced change, particularly when it comes to local
climates (Oke, 1997).
Urban weather patterns are often driven by large synoptic and meso-scale features,
however they are unique to the local characteristics of the urban setting (Meir et al., 2013). In a
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Mercedes L. Gomez Jacobo
study by Glanz (1990) it was recognized that cities possess several characteristics which make
them interesting” laboratories” or analogues in which research questions regarding the
mechanisms and impacts of global climate change can be studied. Since General Circulation
Models (GCM) cannot fully capture all of the complex interactions that happen in the
atmosphere regarding future projections, particularly regional variability, it is important to study
areas where climate is changing, such as cities (Glanz, 1990).
Differences in temperature in cities are related to land cover use/change as well as
structural differences between cities and rural areas (Stewart and Oke, 2012). This is of
importance since half of the world’s population live in cities and this figure continues to grow
(Grimm et al. 2008). Because of the UHI effect, cities are more vulnerable to heat waves or
extreme heat events. Although there is no single definitions that is agreed on for the term heat
wave, Souch and Grimmond (2004) report that ‘heat’ when referred to as a hazard goes largely
under recognized as an strong impact. There is a need to better educate the public about UHI’s
and their potential impacts.
Another element to consider when discussing UHI’s is the hydrologic cycle. It can be
modified by cities due to thermal storage capacity and impervious surfaces such as concrete or
asphalt. After a rain event some of these surfaces evaporate water a higher rate when compared
to a vegetated surface due to the high storage of heat collected before the event (Ramamurthy
and Bou-Zeid, 2014). Other studies have attempted to quantify the amount of energy that urban
regions hold, however urban heat storage is difficult to capture because of so much variability
present due to variety of materials used in construction as well as the 3-dimensionality of cities
(Roberts et al., 2006). Another factor that affects the atmosphere of cities is pollution;
atmospheric pollutants exist in higher concentrations in cities (Rotach et al. 2005). Souch and
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Mercedes L. Gomez Jacobo
Grimmond (2006) state that UHI’s remain as one of the most intensely studied climatic features
of cities. The height of buildings, amount of vegetation or lack thereof and impervious surfaces
all modify and affect the urban heat island.
Heat Waves in the US
The definition of a heat wave is not agreed on by the scientific community, and is also variable
by region. Generally, a heat wave is defined by as an event that exceeds average temperatures
for a minimum over a number of days, usually 2-3 (Peterson et al., 2013). Heat is the number
one weather killer in the US; heat related deaths averaged 237 per year during the 10 year
period of 1994-2003(National Weather Service, 2014). When compared to other weather related
deaths, the same study found that floods killed 84; lightning, 63; tornadoes, 58; and hurricanes,
18.
The heat wave of Chicago in 1995 claimed the lives of over 700 people and heat stress
was amplified by high dew point temperatures (Palecki et al., 2001; Meehl and Tebaldi, 2004;
Souch and Grimmond,2004). The Midwest was faced with another heat wave in 1999, this time
affecting people in 21 states with 309 heat-related deaths (Palecki et al. 2001). The states
hardest hit were Illinois and Missouri, this time Chicago had 114 heat related deaths and St.
Louis had 36. Many parts of the Midwest experienced temperatures of 32 ̊Celsius for 10 days or
more (Palecki et al. 2001).
On June 29, of 2012 a strong line of thunderstorms moved through Midwestern and
Eastern states accompanied with high winds the storms left many without power and the
following days temperatures exceeded 37̊Celsius (100̊F) (US Department of Health and
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Mercedes L. Gomez Jacobo
Services, 2013). This event was responsible for 32 heat –related deaths in Maryland (12), Ohio
(7), Virginia (12) and West Virginia (1) (Palecki et al., 2001).
During a heat wave event low winds coupled with higher temperatures offer no relief to
urban areas at night. Heat waves in cities can be longer lasting and extend to the rural
surroundings (Meir et al., 2013). Intensity and frequency of heat waves is expected to grow in
the coming years (Meehl and Tebaldi, 2004). Studies have found that there is an interaction
between UHI’s and Heat Waves (Li and Bou-Zeid, 2012).
A report by the US Department of Health and Human Services (2013) points out that
most vulnerable demographic is the elderly especially people who live alone, in the heat event
of 2012 over 69% of the victims lacked air conditioning. Some other factors that the study
mentioned were that even with government response, many people do not use the cooling
centers, due to stigmas, lack of transportation, restriction of pets and lack of dangers that
extreme heat poses. A study in Alabama used different heat indices to compare heat waves in
urban vs. rural areas, they found that having different heat index definitions resulted in different
association estimates when studying extreme heat events and the heat effects on humans (Kent
et al., 2004). The researchers also emphasized the need to develop heat wave response systems
that addressed both cities and rural areas. Zhao (2014) suggested that UHI’s will increase heat
wave stress on humans, particularly in wet climates where high humidity is coupled with high
temperatures (Zhao et al., 2014).
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Mercedes L. Gomez Jacobo
Equivalent temperature (𝑇𝐸 )
𝑇𝐸 is the temperature that an air parcel would have if all associated water vapor were
condensed and the resulting latent heat is used to increase the temperature of the parcel (Schoof
et al., 2014). Equivalent Temperature uses observed air temperature and moist enthalpy.
𝑇𝐸 = T + 𝐿𝑣 q/𝐶𝑝
T is the observed air temperature in °C. 𝐿𝑣 is the latent heat of vaporization in Joules per
kilogram (J kg-1). Specific humidity is given by q and Cp is the specific heat of air at constant
pressure in units of Joules per kilogram per degree Kelvin. The components on the right hand
side of the plus sign in the equation make up the moist enthalpy term whose subcomponents are
𝐿𝑣 q and 𝐶𝑝 .This thermodynamic metric can show us the joint behavior of temperature and
humidity (Pielke, 2005,Schoof et al., 2014).
𝑇𝐸 trends in the US have been found to be increasing in recent studies (Fall,2010, Schoof
et al, 2014). Gaffen and Ross (1999) found that specific humidity trends in the US had increased
over the period from 1961-1995, trends for humidity aligned with trends in apparent
temperature (Ta). Pielke (2004) suggested that in order to properly measure the effects of
“global warming” studying and analyzing temperature trends alone did not suffice. The
atmosphere has interactions with water and responds differently under high and low pressure
systems. Equivalent temperature lets us look at surface heat content; therefore it is a more
comprehensive way to analyze global climate trends (Pielke, 2004).
Fall (2010) found that 𝑇𝐸 showed a strong relationship to vegetation cover and areas with
higher transpiration and evaporation rates. In addition the moisture in the atmosphere increases
mostly from late spring to early fall, the warmest time of the year in the northern hemisphere.
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Mercedes L. Gomez Jacobo
The study found that temperature contributed more the magnitude of Te than specific humidity
did, this can also help us understand the impacts of humidity on vegetation (Fall et al., 2010).
Davey (2006) observed 𝑇𝐸 trends in four cities in the Eastern US, overall 𝑇𝐸 trends were
relatively warmer than temperature trends in the US. The study also highlights that moisture
trends can have an influence on precipitation trends with time, both in precipitation intensity
and spatial distribution(Davey et al. 2006). One other important element that this study points
out is that a natural response to increased temperature more evaporation occurs near surface
bodies of water, this will have an effect on near-surface moisture and could influence 𝑇𝐸 (Davey
et al., 2006). This will be one of the goals of this study, to calculate the magnitude of difference
if any between temperature and 𝑇𝐸 trends in cities with different urban characteristics.
Problem/purpose Statement
Elevated urban temperatures are a known health risk for urban populations. Despite
knowledge that humidity also plays a role, there has been relatively little attention paid to the
role of humidity in studies of urban climate hazards. The goal of the proposed research is to
address this shortcoming by investigating urban equivalent temperature trends in large cities as
they relate to urban characteristics.
Research Questions
1) How do temperature and equivalent temperature trends differ in urban areas?
2) How do urban equivalent temperature trends vary in relation to urban characteristics?
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Mercedes L. Gomez Jacobo
Preliminary Methodology
Weather station data will be gathered for the 20 most populated cities in the Eastern US
ranked by population (Table 1). The data gathered will consist of hourly values for dew point
(Td), station pressure (P) and temperature (T) from the National Oceanic and Atmospheric
Administration (NOAA) Integrated Surface Database (ISD) which is available from the
National Climatic Data Center (NCDC) along with any available station metadata for the period
for 1948 to 2012. Homogeneity of the data will be an important part of the process since
weather stations are often moved or the instruments change over time, another factor that can
affect the data is urbanization and land use change around the stations (Schoof et al. 2014;
Peterson et al., 2013). Traditional rounding principles will be applied for variations in time
records, for example if a station records every hour versus every 3 hours. The data will have to
be cleaned and organized using MATLAB before equivalent temperature can be calculated.
The area of cities and population density will also be recorded in order to be able to say
something about trends in more vs. less densely populated cities in either coastal or continental
environments. See Table 1.
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Table 1
Continental and Coastal cities
NYC, NY*
Chicago, IL
Houston, TX
Philadelphia, PA
Jacksonville, FL*
Indianapolis, IN
Columbus, OH
Charlotte, NC
Detroit, MI
Memphis, TN
Baltimore, MD*
Boston, MA*
Washington, DC
Nashville, TN
Louisville, KY
Milwaukee, WI
Kansas City, MO
Virginia Beach, VA*
Atlanta, GA
Raleigh, NC
Miami, FL*
Total Population
Census (2010)
Land Area per Square mile
8,175,133
2,695,598
2,099,451
1,526,006
824,784
820,445
787,033
731,424
713,777
646,889
620,961
617,594
601,723
601,222
597,337
594,833
459,787
437,994
420,003
403,892
399,457
302.64
227.63
599.59
134.1
747
361.43
217.17
297.68
138.75
315.06
80.94
48.28
61.05
475.13
325.25
96.12
314.95
249.02
133.15
142.9
35.87
Population Density
27,012.50
11,841.80
3,501.50
11,379.50
1,100.10
2,270.00
3,624.10
2,457.10
5,144.30
2,053.30
7,671.50
12,792.70
9,856.60
1,265.40
1,836.60
6,188.30
1,459.90
1,758.90
3,154.30
2,826.30
11,135.90
24,775,343
Major Eastern US cities ranked by population, land per square miles and population density.
*Coastal City
Source: US CENSUS BURAEU, 2010
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Mercedes L. Gomez Jacobo
Conclusion
The cities chosen for this study are home to over 24 million people collectively. Heat
waves claim many lives annually and are predicted to become more intense and longer lasting.
Studies suggest that heat related illness and death will continue to increase in future years. The
proposed study will assess trends in temperature and equivalent temperature in order to create a
better understanding of the role of humidity in urban heat waves.
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Equivalent Temperature and Temperature Trends for the Eastern United States
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Fall, S., N.S. Diffenbaugh, D. Niyogi, R. A. Pielke, and G. Rochon. 2010. “Temperature and
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Grimmond, C. S. B., T. R. Oke, and D. G. Steyn. 1986. “Urban Water Balance: 1 A Model for
Daily Totals.” Water Resources Research 22 (10): 1397.
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Waves: The Impact in Cities Is Larger Than the Sum of Its Parts*.” Journal of Applied
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Meir, T., P. M. Orton, J. Pullen, T. Holt, W. T. Thompson, and M. F. Arend. 2013. “Forecasting
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Meteorological Society 94 (6): 821-834. doi:10.1175/BAMS-D-12-00066.1.
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Pielke, R.A. 2005. “What Does Moist Enthalpy Tell Us?.” Climate Science Roger Pielke Sr.
Roger Pielke Sr. July 18. http://pielkeclimatesci.wordpress.com/2005/07/18/whatdoes-moist-enthalpy-tell-us/.
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Ramamurthy, P., and E. Bou-Zeid. 2014. “Contribution of Impervious Surfaces to Urban
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Souch, C., and S.Grimmond. 2006. “Applied Climatology: Urban Climate.” Progress in Physical
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