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Shifting Climate Normals: A moving target for seasonal weather comparisons
By: Heather Wheeler, Summer 2014 RET - University of New Hampshire
Advisor: Mary Stampone, Department of Geography & New Hampshire State Climate Office - University of New Hampshire
Abstract
Results
The 2013/14 New England winter was considered by many to have been colder than a
typical New England Winter. In this study, climate division level temperature and
precipitation averaged for the 2013/14 meteorological winter (December, January and
February) across New England are compared to the average winter values for the 1981–2010
climate normal period. Results indicate that, despite complaints about the cold, winter
2013/14 for most of New England was statistically normal for both temperature and
precipitation. Further comparison of winter 2013/14 to preceding 20th century normal periods
illustrates the 2013/14 winter temperatures and precipitation to have been well within the
range of normal variability for the region. The common perception of New Englanders that
the 2013/14 winter was unusually cold suggests that the lens through which climate is viewed
may literally change with the weather. With a rising set of temperature norms over the 20th
century, winter 2013/14 may have seemed cold compared to recent warmer winters!
Compared to recent New
England winters (2005-2014),
winter 2013/14 was significantly
colder than the average for all of
New England (Table 1). It was the
coldest winter in just over 10 years
for most of northern New England
(ME, NH, VT), and the coldest
winter since 2011 for southern
New England (CT, MA, RI). There
were no significant differences in
total precipitation for winter
2013/14.
State Division
CT
MA
ME
NH
RI
VT
Introduction
The 2013/14 New England winter was widely remarked in the region’s television news
networks to have been unusually cold. However, it is well understood that weather is often
confused with climate; current high and low temperatures, precipitation, wind speeds, etc.,
cumulatively constitute weather, while long term averages, extremes, and trends in those
variables constitute climate. The degree to which a current weather variable conforms to, or
differs from, the pattern an individual accepts as ‘normal’ is intrinsically dependent upon the
conditions to which it is being compared. Differentiating between weather and climate is the
first necessary step in accurately placing current weather experience into a comparative
climate context for ‘normal’ weather and is crucial for understanding climate change.
A climate normal is
quantified as the 30 year
average of meteorological
conditions,
usually
air
temperature and precipitation,
for a specific location (Figure
1; Arguez, 2012). Climate
normals have been calculated
for 30 year periods beginning
every ten years from the
earliest meteorological data
on record up to the current
Figure 1: Schematic diagram of a normal temperature distribution.
climate normal period of
Image Source: IPCC Working Group II - http://ipcc-wg2.gov/SREX/
1981-2010. Comparing the
winter 2013/14 to normal maximum, minimum, and average temperatures and precipitation
across New England offers insight as to how the 2013/14 New England winter compares to
the winters of New England’s past.
1 - Northwest
2 - Central
3 - Coastal
1 - Western
2 - Central
3 - Coastal
1 - Northern
2 - Southern Interior
3 - Coastal
1 - Northern
2 - Southern
1 - All
1 - Northeastern
2 - Western
3 - Southeastern
Table 1: Seasonally averaged winter mean temperatures for 2013/14 compared the average
over the past ten years.
Data Source: NOAA-NCDC “Climate Divisional Dataset.”
Mean Maximum Winter temperature
Compared to the NOAA 1981–2010
climate normal period, the 2013/14
mean winter maximum temperatures
were significantly below normal
across ME and southern NH and
VT. Southern New England as well
as northern NH and VT were within
the range of normal variability (+/one standard deviation) and
considered near normal. All
2013/14 values were near normal
when compared to the three earlier
normal periods (Figure 3).
Discussion
It is interesting to note that comparison of the 2013/14 New England winter to historic
normal periods dating back to 1950 demonstrates the 2013/14 winter to have been quite normal
when the period of comparison predates 1990. Only in the context of comparison to the normal
temperatures for the most recent normal period (1980-2010) does the 2013/14 winter seem
unusual. The disparity from normal increases drastically when the winter 2013/14 is compared to
an even more recent period of comparison, that of the last decade (2005-2014).
Further comparison of the
2013/14 New England winter to
historic normal periods dating
back to the early-20th century
illustrates the 2013/14 New
England
mean
winter
temperature was well within the
range of normal for the climate
normal periods from the mid-20th
century and was warmer than
early-20th
century
winters.
Comparing historical climate
normal periods to others
illustrates consistently rising
temperatures
during
meteorological
winters
in
successive normal periods. This
interrelationship between normal
periods illuminates a shift in
climate (Figure 5).
The winter of 1917/18 is
the coldest on record for much of
Figure 5: State-averaged mean winter temperature for New England by year (black
New England with the exception
dashed line) and for all 20th century climate normal period (colored soled lines). Annual
of Rhode Island and Cape Cod.
variability is shown with respect to the average over the period of record (1895-2014).
Data Source: NOAA-NCDC “Climate Divisional Dataset.”
Given the historical comparison,
the 2013/14 New England winter was more similar to mid-20th century winters than to recent and
early-20th century winters in the region.
Summary
Figure 3: Mean winter maximum daily temperature for New England climate divisions
averaged for late 20th century climate normal period compared to the winter 2013/14 value.
Data Source: NOAA-NCDC “Climate Divisional Dataset.”
Mean Minimum Winter temperature
Mean minimum temperatures were
significantly below the 1981-2010
normal in only western and coastal
CT,
with
otherwise
normal
temperatures
and
precipitation
across the region (Figure 4).
Data & Methods
Spatially averaged, mean winter temperature
(minimum, maximum, and mean) and total liquid
water equivalent precipitation for New England
climate divisions (Figure 2; NCDC, 2014) were
used to calculate the climatological average (mean)
and variability (one standard deviation) for winter
weather over the four most recent climate normal
periods. Seasonally averaged winter temperatures
for 2013/14 were compared to the normal values
for temperature and precipitation over the latter
half of the 20th century as well as the 20th century
(1901-2000), period of record (1895-2014), and
recent (2005–2014) ten year averages.
Mean Winter Temperature
2005-2014
2014
26.9 +/- 2.6
23.8
29.5 +/- 2.5
26.6
31.9 +/- 2.5
29.2
24.9 +/- 2.5
21.9
27.7 +/- 2.6
24.6
31.9 +/- 2.2
29.5
15.8 +/- 2.5
12.4
21.7 +/- 2.4
18.5
24.8 +/- 2.2
21.8
18.6 +/-2.3
15.9
23.8 +/- 2.5
20.5
31.3 +/- 2.3
28.7
19.1 +/- 2.4
16.1
21.8 +/- 2.6
18.6
22.4 +/- 2.6
18.9
The 2013/14 New England winter has been regarded in the region as unusually cold.
Compared to winters in the region of the last decade, it was indeed! Compared to winters from
the most recent climate normal period (1981-2010), it was slightly unusual, and compared to
historic winters, it was quite normal. The period of time to which the 2013/14 winter is
compared is the critical variable in determining the level of typicality of the 2013/14 winter.
Given the differences in the periods of comparison, the shift in what constitutes normal becomes
apparent: the normal winter climate in New England includes rising winter temperature norms
over successive climate normal periods.
Literature Cited
•Arguez, A. et al., 2012: NOAA’s 1981-2010 U.S. Climate Normals An Overview. Bulletin of
the American Meteorology Society, Volume 93, Number11, 1687-1697.
•NOAA National Climate Center (NCDC), 2014: nClimDiv Dataset ftp://ftp.ncdc.noaa.gov/pub/data/cirs/climdiv/
Acknowledgements
Figure 4: Mean winter minimum daily temperature for New England climate divisions
averaged for late 20th century climate normal period compared to the winter 2013/14 value.
Data Source: NOAA-NCDC “Climate Divisional Dataset.”
Figure 2: NOAA-NCDC Climate Divisions for New
England.
Image and Data Source:
http://www.ncdc.noaa.gov/monitoringreferences/maps/us-climate-divisions.php
Comparison of the 2013/14 New England winter to historic normal periods illustrates that last winter
was well within the range of normal for the mid-20th century climate normal periods with average
winter temperatures and precipitation most similar to the 1951-1980 and 1961-1990 climate normals.
This research was supported with funding
from the National Science Foundation’s grant to NH
EPSCoR (IIA-1101245). I would also like to thank
Stephen Hale from the UNH Joan and James Leitzel
Center for Mathematics, Science, and Engineering
Education and NH EPSCoR for assistance with
Excel for data analysis.