United States
                   Environmental Protection
                   Agency
Atmospheric Research and Exposure
Assessment Laboratory        N\ V4 '
Research Triangle Park IMC 27711 -^ "
                   Research and Development
EPA/600/S3-89/025 Aug. 1989
&ERA         Project  Summary
                  A  Climatology of
                  Temperature and  Precipitation
                  Variability  in the United States

                  Brian K. Eder, Lawrence E. Truppi, and Peter L. Finkelstein
                    This summary  examines the
                  seasonal and annual variance and
                  standardized range for temperature
                  and  the seasonal and  annual
                  coefficient of  variation  and
                  normalized standardized range for
                  precipitation on a climatic division
                  level for the contiguous United States
                  for the period 1895 to 1985.
                    Examination of the temperature
                  variance revealed a continentality
                  phenomenon in  which the  largest
                  variance occurs in the upper midwest
                  section of the  country, while the
                  smallest  variance is  generally found
                  in coastal regions along  the west
                  coast, the Gulf coast, and  south-
                  eastern states.  The winter  season
                  displayed roughly twice the  amount
                  of seasonal  variance as did spring
                  and roughly four times that of sum-
                  mer or autumn. Analysis of the stan-
                  dardized  temperature range supports
                  the continentality phenomenon; how-
                  ever, the transitional  seasons, spring
                  and autumn, displayed the  largest
                  amount of within-season variability,
                  with winter and summer displaying
                  the least amount.
                    Examination of the coefficient  of
                  variation  for precipitation depicted a
                  propensity for the largest seasonal
                  and annual variation to occur over
                  the southwestern states from Texas
                  to California.  Conversely, the smal-
                  lest coefficient of variations were
                  found over the northeastern sections
                  of the country from New England into
                  the mid-Atlantic and Great Lakes
states.  Analysis of the seasonal and
annual  standardized precipitation
range reveals that the pattern mimics
the coefficient of  variation patterns
but exhibits less of a  gradient,
resulting in a smoother pattern.
Areas  of greater  than normal
seasonal and annual precipitation
ranges  include the  southwestern
states from Texas to California, while
areas of less than normal ranges
include the  northeastern  and Ohio
River Valley states.
  This   Project  Summary  was
developed by EPA's Atmospheric
Research and Exposure  Assessment
Laboratory, Research  Triangle  Park,
NC, to announce key  findings of the
research project  that  Is  fully
documented in a separate  report of
the same title (see Project Report
ordering information at back).

Introduction
  Despite the  increasing interest shown
by the scientific community in climate
and its interactions with the evolution of
ecosystem structures, there continues to
be  a lack of a  consensus among
climatologists and ecologists concerning
the future of global climate and its pos-
sible impact upon ecosystems.   Policy
makers and planners need plausible des-
criptions  of possible  long-term changes
of such  ecologically important variables
as temperature, precipitation, evapora-
tion, and soil moisture conditions  on all
spatial and temporal scales.

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  Such descriptions may be found with
climatic scenarios, which  are sets  of
solutions either derived empirically from
observational data  (paleoclimatic  or
instrumental analogues), or from Global
Climate Models (GCMs),  often in  the
form  of seasonal maps  showing  the
range of conditions or possible variances
that may occur in the future.
  Although research has begun in EPA's
Atmospheric Research and  Exposure
Assessment  Laboratory,  the  develop-
ment of climatic scenarios  that have real
utility for  ecological impact assessment
is still  rudimentary.  Subsequently, this
development  must be supported  by  an
enhanced  understanding of the climatic
sensitivities  of   a  broad range  of
ecological activities and of the detailed
nature  of recent and past  climatic
patterns and their variability.  Two such
variables  which  should receive   a
concentration   of research efforts  are
temperature  and   precipitation.   From
these two  measured variables, numerous
derived parameters relevant to local eco-
systems,  such  as  surface   moisture
stress, duration of  rainless  periods, and
length  of growing  season,  can   be
calculated. The development and evolu-
tion of ecosystems  are as sensitive to the
ranges and variances of temperature and
precipitation  as   they  are  to  mean
conditions. Because of this, ecosystems
evolving in regions that have  exhibited
little variance  in temperature and pre-
cipitation over the years are likely to  be
more sensitive to climatic  changes that
those  ecosystems which evolved  in
regions exhibiting  larger variability.
Therefore,  a  need exists  to  not only
delineate  these  regions   of  differing
variance, but to also establish monitoring
networks within both  types of  regions,
which may provide an understanding  of
potential ecological  responses  toward
future climatic change.
  Though  the delineation of such regions
may seem to be  trivial,  little,  if any,
literature  concerning the subject  is
available.   This   summary  therefore
represents an initial  effort toward the
fulfillment  of the requirements mentioned
above through the delineation of areas of
the country which  experience differing
amounts of temperature and precipitation
variability.  This is accomplished through
the examination of  the variance and
standardized range of temperature data
and the coefficient  of variation and
standardized range of precipitation data
across  the contiguous United States,  on
a climatic  division level,  from the period
1895 through  1985.   Establishment  of
monitoring networks  within  these
delineated  regions  will  help provide a
new understanding of  key ecosystem
processes,  as well as their responses to
possible  climatic change, which should
therefore enhance their  treatment  in
GCM based  scenarios as well as  pave
their  way  for  their  representation  in
observationally based  scenarios.

Data
  The  monthly temperature and precip-
itation  data employed m this  analysis
were obtained from  the National Climatic
Data Center (NCDC) located in Asheville,
NC. These data, which  cover the period
1895 to 1985, are collected on a climatic
division  basis,  where  each  climatic
division is designed to represent regions
within  a state  that are  climatically
homogeneous or consistent.  Within  the
contiguous  United States, there are 344
such  divisions,  the  areal  coverage  of
which,  can vary tremendously, with  the
largest divisions generally  found in  the
western states and the smallest found in
the east.
  Stations   used  in  calculating  the
divisional monthly averages of tempera-
ture (measured to the nearest tenths in
degrees  F) and the  monthly totals  of
precipitation  (measured to the  nearest
hundredths in inches) include  all  first
order  stations and  those  cooperative
stations  which  have  maintained
consistent records.

Methodology
  The  seasonal and annual variability of
both temperature and precipitation  were
examined in  order to better understand
the variability of  climate within  the
contiguous  United States.   For tempera-
ture data this consisted of examining  the
variance  from season  to season, and by
examining the standardized range within
seasons over  the  United  States.
Standardization of the  temperature range
allows  for  direct comparison between
individual  climatic  divisions and  the
country as a whole.
Due to the  tremendous  range  in normal
precipitation  exhibited over the United
States,  a  different  approach  was
necessary  for the seasonal and annual
precipitation analysis.   Rather than take
the variance, which  would be  biased
towards areas of high precipitation,  the
coefficient  of variation  was examined
which  "normalizes"  the  variance.
Similarly, calculation of the standardized
range  also considered this extreme
variability  in precipitation and  was
therefore normalized.
Results
  Examination of the annual temperatun
variance  revealed  several interesting
features.  Most notable of these feature
was the tendency for the largest vanano
to occur in the upper midwest portions c
the country,  especially  in North am
South  Dakota and  eastern  Montana
where  the annual temperature  varianci
exceeds 3 F. A trend toward decreasm<
annual variance is exhibited as chmatii
divisions approach coastal regions.  Thr
pattern was depicted  especially  wel
along  the  west coast  from Washmgtoi
and Oregon to California, and again alone
the Gulf coast and southeastern states
where  the annual temperature  variance
reached a  minimum of less than 0.5  01
the southern Florida peninsula.
  The  standardized  temperature  rang<
maps exhibited, in a somewhat differen
manner the same contmentality as  seei
with the variance figures  Consistent wit!
the annual  variance  map,  the  larges
annual standardized  ranges occurred ir
the upper midwest,  especially in th<
states  of  North and South  Dakota anc
Minnesota.   A trend toward  decreasing
ranges was found near the coastal areas
especially along the  Pacific Coast state:
and the Gulf Coast states.
  Unlike the  seasonal  variance  map:
which  depicted  winter  as  the seasor
having the most variance, the seasona
standardized  range maps  depict the
transitional seasons,  spring and autumr
as  exhibiting the most variability  withir
their seasons.  This  phenomenon  is no
unexpected since the  range of monthly
temperature would be greater during the
transitional seasons than  during winter 01
summer.
  Examination of the annual precipitatior
coefficient of variation (%) also revealec
several interesting features.  Unlike the
temperature analysis, which  indicated 
north-south gradient,  the precipitatior
analysis  depicts  somewhat  of ar
eastwest gradient. This is supported  by
the propensity for the  largest coefficient
of variation to occur  over the south-
western states  from Texas to  California
where  the  values exceeds 25.9%,  while
the smallest variation  generally  occurs
over the eastern  sections of the country
from the  mid-Atlantic  and  Great  Lake
States  into New  England, where values
are  less   than  14.0%.   The maps
depicting the coefficient of variation for
seasonal precipitation are, with only a few
exceptions,  similar to  the  annual  map
Most notable of these  exceptions is the
extension  or  shift  of  high  variations

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into the lower midwestern states during
the winter season, and  into the Pacific
coast states during the summer season.
  Examination of the annual standardized
precipitation  range,  revealed patterns
similar  to those of the  precipitation
coefficient  of  variation.    The
southwestern  states from New Mexico to
California  tend to have larger  annual
ranges when compared to the rest of the
country.  Another area exhibiting annual
ranges which are greater than "normal"
is found in the upper midwest from North
and South Dakota into Montana.  Areas
exhibiting  smaller than "normal" annual
ranges  include the  New  England and
Appalachian Mountain  states.   The
standardized seasonal ranges of  precip-
itation again somewhat mimic the annual
map;  the patterns, however, tend to be
somewhat flatter, indicating  less  within
seasonal variability.   Areas of greater
than  "normal"  precipitation  ranges
include  the  southwestern  states  from
Texas to  California,  while  the eastern
states, especially those in  New England
and the  Ohio River Valley, tend to exhibit
less than "normal" ranges.
Conclusions
  Because there continues  to  be  no
consensus among  climatologists and
ecologists concerning climate change
and  its  possible  impact   upon
ecosystems, the development of climatic
scenarios  will be  necessary in order to
assist scientists  in  evaluating possible
adverse effects of climatic change on the
ecology. Unfortunately, the development
of such scenarios as a utility in assessing
this impact  is  still  somewhat  in  a
rudimentary  stage,  and therefore  must
be  supported by an  enhanced  under-
standing of recent  and  past climatic
patterns and their variability.  In an  initial
attempt  to assist  in this  understanding,
this summary has examined the seasonal
and  annual variance  and  standardized
range for temperature and the seasonal
and  annual coefficient of  variation and
normalized standardized range for pre-
cipitation, on a climatic division level  for
the  contiguous United  States for the per-
iod  1895 to 1985.
  Examination  of  the   temperature
variance and  standardized range re-
vealed a contmentality phenomenon  in
which the largest variance occurred in the
upper  midwest section  of  the country,
while  the  smallest  variance  were
generally found in coastal regions along
the  west coast,  the  Gulf coast and
southeastern states.
  Examination of  the coefficient  of
variation and standardized  range  for
precipitation depicted a propensity for the
largest seasonal and annual variation to
occur over the southwestern states from
Texas  to California. Conversely,  the
smallest  variation  was  found  over  the
northeastern sections of the country from
New England  into the  mid-Atlantic and
Great Lakes states.
  Successful  climate scenarios,  whether
derived from climate  models or analogue
techniques, should duplicate the  patterns
produced in this summary as well as the
simple  mean  patterns.   Present models
are, for the most  part, unable to do this.
The design of ecological monitoring net-
works,  both for base line stations,  which
require some  climatic  stability,  and for
stations where a range  of climatic condi-
tions is required should  also be cognizant
of the information developed in this and
similar  studies.

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The EPA authors Brian K. Eder, Lawrence E. Truppl, and Peter L Finkelstein are
 with the Atmospheric Research and Exposure Assessment Laboratory, Research
 Triangle Park, NC 27711.
Brian K. Eder is the EPA Project Officer (see below).
The  complete report entitled  "A Climatology  of Temperature and Precipitation
 Variability In The United States,"  (Order  No.  PS 89-765  9301 AS; cost: $13.95,
 subject to change) will be available only from:
    National Technical Information Service
    5285 Port Royal Road
    Springfield, VA 22161
    Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
    Atmospheric Research and Exposure Assessment Laboratory
    U.S. Environmental Protection Agency
    Research Triangle Park, NC 27711
United States                   Center for Environmental Research
Environmental Protection         Information
Agency                         Cincinnati OH 45268
Official Business
Penalty for Private Use $300

EPA/600/S3-89/025

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