Download Changing air mass frequencies in Canada: potential links and

Int J Biometeorol
DOI 10.1007/s00484-013-0634-2
ICB 2011 - STUDENTS / NEW PROFESSIONALS
Changing air mass frequencies in Canada: potential links
and implications for human health
J. K. Vanos & S. Cakmak
Received: 26 July 2012 / Revised: 13 November 2012 / Accepted: 15 January 2013
# ISB 2013
Abstract Many individual variables have been studied to
understand climate change, yet an overall weather situation
involves the consideration of many meteorological variables
simultaneously at various times diurnally, seasonally, and
yearly. The current study identifies a full weather situation
as an air mass type using synoptic scale classification, in 30
population centres throughout Canada. Investigative analysis of long-term air mass frequency trends was completed,
drawing comparisons between seasons and climate zones.
We find that the changing air mass trends are highly dependent on the season and climate zone being studied, with an
overall increase of moderate (‘warm’) air masses and decrease of polar (‘cold’) air masses. In the summertime,
general increased moisture content is present throughout
Canada, consistent with the warming air masses. The moist
tropical air mass, containing the most hot and humid air, is
found to increase in a statistically significant fashion in the
summertime in 46 % of the areas studied, which encompass
six of Canada’s ten largest population centres. This emphasises the need for heat adaptation and acclimatisation for a
large proportion of the Canadian population. In addition,
strong and significant decreases of transition/frontal passage
days were found throughout Canada. This result is one of
the most remarkable transition frequency results published
to date due to its consistency in identifying declining trends,
coinciding with research completed in the United States
J. K. Vanos (*) : S. Cakmak
Health Canada Ottawa, Environmental Health Sciences Research
Bureau, Tunney’s Pasture,
Ottawa, ON K1A 0K9, Canada
e-mail: [email protected]
S. Cakmak
e-mail: [email protected]
(US). We discuss relative results and implications to similar
US air mass trend analyses, and draw upon research studies
involving large-scale upper-level air flow and vortex connections to air mass changes, to small-scale meteorological
and air pollution interactions. Further research is warranted
to better understand such connections, and how these air
masses relate to the overall and city-specific health of
Canadians.
Keywords Spatial synoptic classification . Canada .
Air mass . Human health . Heat stress . Climatology
Introduction
A great deal of attention in the climate change literature and
in the media has focussed on assessing and understanding
the changing of one variable, namely air temperature, over
time. The global temperature is projected to rise between 1.0
and 6.5 °C by the end of this century, which could result in
potentially dramatic effects on public health, overall planetary weather patterns, and ecosystems (Karl et al. 2009).
According to Coward and Weaver (2004), climate change
models suggest that Canadian cities and more northerly areas
will undergo amplified increases in temperature. Many studies
have displayed changing long-term trends of individual
weather variables, such as air temperature, moisture content,
surface pressure, and precipitation, throughout Canada
(Vincent and Mekis 2006; Vincent et al. 2007; Yagouti et al.
2008). Vincent et al. (2007) found increases in air moisture
content from 1953 to 2006 that were said to coincide with
warming across the country.
However, trend analyses based on an overall weather
situation (air mass), rather than a single variable, have
Int J Biometeorol
not been completed for Canada. One of the most comprehensive methods of air mass categorisation is the
spatial synoptic classification (SSC) system (Sheridan
2002; Sheridan and Dolney 2003). A combination of
weather variables, which synergistically affect human
health, are used to create an atmospheric situation that
is differentiated into weather type categories (Davis et
al. 2003; Greene et al. 2011).
Factors such as humidity and radiation can have large
impacts on the human–energy balance system (Vanos et al.
2010); hence, assessing all variables is useful in the study of
human biometeorology, and in understanding the short-term
physiological effects of weather. Development of Heat Health
Warning Systems (HHWS) and Cold Health Warning Systems, based on air mass classification, is supported by both the
World Health Organisation and the World Meteorological
Organisation. These organisations recognize the potential
health improvements associated with the use and development
of such systems, and have supported their implementation in
lesser studied areas, such as Korea (Kalkstein et al. 2008) and
China (Tan et al. 2004). However, within Canada, the use of
such warning systems are not based on SSC weather types.
The current study uses the SSC solely to understand synoptic
air mass-based trends in Canada.
SSC categories are able to account for seasonal variability
in a relative way (Sheridan and Kalkstein 2010), based on the
distribution expected for a specific time of year (Davis et al.
2010). This is important, as Canada experiences large yearly
ranges in weather extremes temporally and spatially, with
unique climate zones present throughout the country.
Epidemiological, public health, and climatological studies using the SSC have become prevalent in the United
States (US) literature. Many SSC and air mass based studies
have been completed in North America (e.g. (Cheng et al.
2007a, b; Hajat et al. 2010; Pope and Kalkstein 1996; Power
et al. 2006; Rainham et al. 2005; Sheridan 2002; Sheridan
and Dolney 2003; Sheridan and Kalkstein 2004)). The SSC
has been applied in the assessment of ozone variability
(Davis et al. 2010), general air pollution in Toronto, Canada
(Rainham et al. 2005), rainfall extremes in Ontario, Canada
(Cheng et al. 2010), urban heat island initiated rainfall
(Dixon and Mote 2003), climatology of US winter transition
days (Hondula and Davis 2011a), near-surface air temperature lapse rates (Blandford et al. 2008), and the development
and assessment of HHWSs (Kalkstein et al. 2011; Sheridan
and Kalkstein 2004; Tan et al. 2004).
Recently, Knight et al. (2008) found overall yearly statistically significant increases in warm and moist air masses,
at the expense of cold and dry air masses, from 1948 to
2005. They also found a decrease of transition days as
defined by the SSC (See Table 1), which indicates fewer
frontal passages (Hondula and Davis 2011a), but may also
indicate weaker and/or quicker cyclone movement, and
positive feedback mechanisms related to snow and ice cover
(Serreze et al. 2007). Research by Hondula and Davis
(2011b) addressed declining transition days in the US
by assessing transition (TR) variables for the winter
season (when TR days are most abundant). They found
many areas with a significant decrease in TR days,
particularly in the northwestern US. Yin (2005) cited
fewer frontal passages to be caused by decreases in
temperature variability, and hence temperature gradients
between latitudes. According to Hondula and Davis
(2011a), climate change could significantly affect, and
may already be impacting, the frequencies and characteristics of frontal passages in mid- and high-latitudes.
A common perception of Canada is that a cold climate is widely prevalent, hence warm-hot conditions are
not always viewed as a serious threat (Smoyer-Tomic et
al. 2003), as reflected in the sparseness of studies
addressing these conditions. Lemmen et al. (2008) anticipate a growing heat-related public health risk, reporting a high probability of extreme heat events of
heightened intensity, duration and frequency. Relative
heat-related mortality predicted to double by 2050 in
select Canadian cities (Cheng et al. 2005). Such heat
in Canada has become more prevalent. For example, the
summer of 2010 was one of the hottest on record, and
2012 experienced extreme early-season heat waves with
a number of temperature records >30 °C set at the end
of May (Environment Canada 2012). Mortality has been
shown to increase dramatically during early-season heat
waves in more northerly US cities where acclimation to
heat is inadequate (Greene et al. 2011). This underlines
the importance of the temporal and spatial nature of climatehealth studies, where both climate change magnitude and
thresholds of human responses are highly variable (Gosling
et al. 2009; Sheridan and Kalkstein 2004); accordingly, we
would expect to find different results in SSC studies completed in Canada versus the US.
The goal of this study was to complete a large-scale
investigative analysis of long-term (56-year average)
trends in air mass frequencies across Canada for 30
stations. We interpret these results based on how the
prevalence of each air mass type has changed in the last
five-to-six decades, drawing comparisons between summer and winter seasons, as well as climate zones, to
enhance our understanding of temporal and geographical
influences. This is the most extensive regional comparison of air mass frequencies in Canada, and infers risks
and potential future climatological and health challenges based on the results. Emphasis is placed on
evaluating changing patterns of transition-type weather
(TR), as well as changes in five air masses: dry moderate (DM), moist moderate (MM), dry polar (DP),
moist polar (MP), and moist tropical (MT).
Int J Biometeorol
Methods
Selected SSC study sites
Canada currently has SSCs completed on a daily basis at
77 locations across the country (Sheridan 2012), with
records dating back over six decades for many stations.
Air masses are classified into one of seven SSC categories, listed in Table 1. The SSC, developed by Sheridan
(2002), uses ‘sliding seed days’ representing expected
and observed meteorological conditions at each location
throughout the year for each air mass type. Surface
weather observations of cloud cover, moisture, air temperature, air pressure, wind velocity, and air mass duration are used. To select seed days for each season and
location, typical characteristics are quantified, and
ranges specified to indicate threshold values of meteorological variables between weather types. Days that
satisfy these criteria are extracted, with confirmation
for representativeness completed thereafter. Thus, each
location has an individualized set of weather types,
which allows for a local human response and threshold
to stressful weather to be assessed and developed, respectively (Sheridan 2002). Therefore, the characteristics
of a DT air mass in Toronto and Halifax, for example,
would differ. Air and dew point temperatures for DT in
Toronto are on average 32 °C and 16 °C at 4.00 p.m.,
respectively, while the same air mass at 4 p.m. in
Halifax has respective temperatures of 27 °C and 19 °C.
The population centres used in the current study were
chosen based on the availability and completeness of SSC
data, relative location to account for the whole country,
differentiation in size/population, and relative numbers in
each of the Canadian climate zones (see Table 2 and Fig. 1).
Statistics Canada defines a population centre as ‘small’,
‘medium’, and ‘large’ with respective population sizes of
1,000–29,999; 30,000–99,999; and ≥ 100,000; hence, each
of the 30 stations fell into a select population centre
classification.
Climate zones are based on principal component analysis
(PCA) decomposition and k-means clustering of air masses
from Sheridan (2012), defined as follows:
&
&
&
&
3a: “Laurentian”—high variability, large seasonal fluctuations, limited tropical influence
3b: “Northern Rockies”—temperate zone experiencing
high variability, limited tropical influence, and high
terrain nearby
4: “Arctic”—coldest zone, with mainly polar, arctic, and
showing the highest frequencies of DP or MP weather
types
8: “Marine”—very moist air, with milder temperatures;
adjacent to ocean waters that moderate climate
Data analysis
At each station (population centre) listed in Table 2, a daily
air mass category was known based on SSC data from
Table 1 Description of six distinct air mass typesa and the transition (TR) category, applied to Canadian geography
Air mass type
Description
Dry polar (DP)
A cold, dry air mass with clear skies, similar to the traditional ‘continental polar’ (cP). More common in the winter season in
Canada. Associated with cold-core anticyclones and polar air advecting in from northern Canada or Alaska
A dry air mass with mild, near normal temperatures. Commonly found on the eastern side of the Northern Rockies after air
has warmed and dried with descent, and/or under high pressure situations
The hottest and driest air mass, with higher than average temperatures and sunny, clear skies. Has a very low frequency of
occurrence in Canada, predominantly occurring in summer. Present subsequent to, or during, anticyclone events with large
regional subsidence of air (e.g. Chinook) common in the Canadian Rockies, and/or advection of drier continental air into a
region
A cool air mass, with high moisture levels, thus having overcast skies and light precipitation. Can be associated with
advection of air over large bodies of water, such as oceans or the Great Lakes. Also arises from a frontal overrunning
Mild and humid, with more clouds than the MP air mass. Occurs when warmer air meets polar air from the north on the
eastward side of a low pressure system, and/or often south of an MP air mass
The warmest and most humid air mass. Primarily present in Canadian summers, yet little to no presence in the winter.
Sourced from tropical Atlantic or Pacific air on the coasts, and warm Gulf of Mexico air inland subsequent to the passing of
a warm front. Skies are partly cloudy in the summer, and cloudy in the winter. Can commonly produce convective cells
leading to precipitation and/or thunderstorms
A special category, defined based on different criteria than the remaining air masses. The focus turns to diurnal ranges of dew
point, pressure ranges, and wind shifts. If all of these parameters are 1.3 standard deviations greater than the period mean,
then the day is classified as TR. Such large shifts commonly occur with passing warm or cold fronts associated with midlatitude low pressure systems
Dry moderate
(DM)
Dry tropical (DT)
Moist polar (MP)
Moist moderate
(MM)
Moist tropical
(MT)
Transition (TR)
a
Davis et al. 2010; Knight et al. 2008; Sheridan 2002
Int J Biometeorol
Sheridan (2012). Analysis was divided into summer (JJA)
and winter (DJF) seasons over a long-term period up to
2010 (56-year average, with a maximum of 67 years). In
the event that greater than 10 days of data were missing
from a given season, the year was excluded from the analysis. The time series of the occurrence of each type of air
mass was constructed using standard ordinary least squares
(OLS) regression, and the changing air masses were evaluated based on the slope of the linear regression line, which
represents the relative change in number of air masses in the
given season per year. All dependent data was tested for
linearity, collinearity, and normality, and were found to have
a linear relationship with the independent variable of time
(years). Normality testing was completed using the ShapiroWilks test of normality.
Although several methods to determine the robustness
of a dataset and smoothing purposes are available, they
were not used here (e.g., bootstrapping and jackknifing).
These procedures are most useful when there is a suspected bias in the data, for example, they are widely
Table 2 Climate, population, and daily mean air temperature ( T a )
statistics for the 30 Canadian locations studied. T a are for summer
(JJA) and winter (DJF) seasons, obtained from airport historical
climate dataa. Cities are classified as large (≥ 100,000), medium
(30,000–99,999) and small (1,000–29,999) population centres (Statistics 2011). SSC Spatial synoptic classification
City, Province
Large population
Calgary, AB
Edmonton, AB
Halifax, NS
London, ON
Montreal, QC
Ottawa, ON
Quebec City, QC
Regina, SK
St. John Õs, NF
Toronto, ON
Vancouver, BC
Windsor, ON
Winnipeg, MB
Medium population
Charlottetown, PEI
Fredericton, NB
Fort McMurray, AB
Kamloops, BC
Prince Albert, SK
Sydney, NS
Populationb
Years of data
Distance to Wx (km)c
T a JJA
T a DJF
1,214,839
1,159,869
57
49
10.3
27.1
15.2
15.0
−7.4
−11.8
3b
3b
390,328
474,786
3,824,221
1,236,324
765,706
210,556
196,666
5,583,064
2,313,328
319,246
730,018
56
57
61
60
54
58
60
60
57
33
57
6.3
9.4
16.2
11.3
11.0
4.7
7.4
19.0
6.2
3.5
7.4
17.3
19.3
19.6
19.6
17.9
17.7
13.9
19.5
16.8
21.5
18.3
−4.8
−4.9
−8.3
−8.9
−11
−13.8
−4.1
−4.9
3.9
−3.1
−15.2
8
3a
3a
3a
3a
3b
8
3a
8
3a
3a
64,487
94,268
61,374
98,754
42,673
33,691
58
57
60
45
58
61
4.2
13.5
12.6
8.8
5.6
9.7
17.1
18.0
15.6
19.9
16.3
16.2
−6.6
−8.1
−16.3
−2.5
−14.5
−4.8
8
8
3b
3b
3b
8
1,081
1,283
250
8,509
6,744
5,336
6,719
12,839
1,861
26,028
19,234
58
54
58
61
54
58
67
44
49
67
57
8.4
4.9
4.6
3.7
9.7
2.4
1.6
6.3
2.5
1.7
3.6
16.8
15.7
15.1
15.8
15.0
17.2
14.8
14.2
12.1
12.8
14.8
−13.9
−23.3
−21.6
−23.3
−16.6
−16.0
−5.6
−22.4
−20.7
−15.4
−24.6
3a
4
4
3a
3b
3a
8
4
4
4
4
Small population
Bagotville, QC
Fort Simpson, NT
Fort Smith, NT
Kapuskasing, ON
Peace River, AB
Sioux Lookout, ON
Stephenville, NL
Thompson, MB
Wabush, NF
Whitehorse, YK
Yellowknife, NT
a
1971–2000 seasonal averages (Environment Canada)
b
Statistics Canada 2011 census
c
Straight-line horizontal distance from downtown city centre to airport where first order weather station is present
SSC climate zone
Int J Biometeorol
4
4
Ft. Simpson
Yellowknife
Whitehorse
Ft. Smith
Peace River
6
3b
Edmonton
Kamloops
8
Thompson 4
Ft. McMurray
Calgary
PrinceAlbert
3b
Regina
4
3a
Winnipeg
Wabush
Sioux Lookout
3a
Vancouver
Kapuskasing
310 620
Bagotville
Stephenville
8
Quebec City
Charlottetown
Montreal
Sydney
Fredericton
Ottawa
Toronto
8 Halifax
Windsor
0
3a
1,240
1,860
St. John's
London
km
2,480
Fig. 1 Map of Canada displaying locations of the 30 stations assessed
in the current study, as listed in Table 2. Five climate zones are present
in Canada, segregated using principal component analysis (PCA) and
synoptic scale classification (SSC): 3a Laurentian, 3b Rockies, 4
Arctic, 6 Boreal Coast, 8 Marine
skewed from a normal distribution (Chernick 2007). However, the data used for analysis generally passed the tests
of normality. Thus, we felt that such resampling procedures were unnecessary here, as data “oversmoothing”
may occur, which may remove important variation related
to time.
In Canada, DT and MT+ air masses are very infrequent or
non-existent, particularly in more northerly areas, and hence
were not analyzed in the current study. In cases where data
was insufficient (i.e. too few observations), such as the
winter MT air mass, and MT air in the Arctic, no trend
was calculated or included in the final analysis. In select
cases where sufficient data was present yet evidence of nonnormality occurred, a log10 transform was applied.
Pearson correlation analysis was completed to examine
how one air mass changed with another, and to relate transitional slope trends to latitude. Additionally, we determined
if the frequency of two consecutive transition days was
changing over the period of study. We summed these per
season and applied linear regression to determine long-term
trends. All regression trends and correlations were deemed
significant at P<0.05.
When combining results based on population centre size
or climate zone, a pooled random effects model was used
Table 3 Average trend analysis, displaying the regression slopes
(number of days air mass is present per year) divided by climate zone
(see Table 2) and weather type. Combined seasonal results are
presented, with summer and winter in the corresponding parentheses.
Significance testing was completed using a pooled random effects
model
Air mass
Laurentian
Rockies
Marine
Arctic
DM
DP
MM
MP
MT
TR
0.055* (0.012, 0.098*)
−0.050* (−0.091*, −0.009)
0.066* (0.094*, 0.038*)
−0.065* (−0.039*, −0.090*)
0.056* (0.100*, 0.011)
−0.071* (−0.103*, −0.040*)
0.090* (0.042, 0.137*)
−0.097* (−0.121*, −0.074*)
0.065* (0.084*, 0.045*)
−0.019 (−0.007, −0.031)
0.040* (0.040*, NAa)
−0.070* (−0.042*, −0.098*)
0.052* (0.062*, 0.041*)
0.019 (−0.049*, 0.088*)
−0.018 (0.032, −0.067*)
−0.102* (−0.120*, −0.084*)
0.084* (0.084*, NA)
−0.026* (−0.040*, −0.012)
0.040* (0.062*, 0.019*)
−0.085* (−0.119*, −0.052)
0.086* (0.108*, 0.063*)
−0.009 (−0.051*, 0.032)
NA (NA, NA)
−0.074* (−0.081*, −0.067*)
*Statistical significance (P<0.001)
a
Not applicable
Int J Biometeorol
Fig. 2 Increasing or decreasing
air mass changes of five air
masses (dry moderate (DM),
moist moderate (MM), dry
polar (DP), moist polar (MP),
and moist tropical (MT))
separated into climate zones.
Each bar represents the number
of stations within each climate
zone that are increasing (above
x-axis) or decreasing (below xaxis) at a statistically significant
rate (P<0.05); white summer
frequencies, grey winter
frequencies
with summary statistics to determine overall values of significance at P<0.001. Statistical analyses were completed in
R version 2.14.1, 2012 (The R Foundation for Statistical
Computing, http://www.r-project.org/).
be declining consistently in both seasons in the Laurentian
and Marine climate zones, while the moderately moist
(MM) and dry (DM) weather types are becoming more
frequent in all climate zones. The implications of transition
day changes are presented in detail below (see section on
Transition days).
Results and discussion
Moderate and polar air masses
An initial comparison of the three population centre sizes was
carried out to rule out any urban heat influences on the airport
weather stations. There were no distinctive patterns or differences found between the population centre sizes in trends or
strength of these trends. Hence, the air mass changes reported
in the current study are most likely due to a general large scale
change in the air mass climatology, as opposed to an exacerbation of urban heat penetrating into more the rural areas
where airport weather stations are located.
Table 3 displays overall trends for seasons and climate
zones. Both seasons display strong and significant decreases
of TR air mass frequency across the country, as well as in
the coldest and driest weather type of dry polar (DP). The
presence of the moist polar (MP) weather type was found to
Fig. 3 Seasonal air mass
pairings of polar and moderate
air masses (DP with DM, MP
with MM), displaying
directional changes in slopes of
regression trend lines. Each bar
represents the total number of
stations where a moderate air
mass is increasing, or a polar air
mass is decreasing. The grey
section represents the
proportion of the total that is
changing significantly (P<
0.05) in the given direction
From the frequency trend analyses, Figure 2 was generated
to display how the moderate (MM, DM) and polar (DP, MP)
air masses have been changing in the last six decades, and
how frequently these trends are statistically significant.
Three of the four zones show relatively high frequencies
of significantly increasing ‘warm’ DM and MM air masses
(mean = 50 % and 43 %, respectively). The cold air masses
of DP and MP, as well as the TR category, are counteracting
this effect slightly, where average percentage of statistically
significant decreasing trends is 45 %, 28 % and 60 %,
respectively.
Figure 3 displays the number of trend changes from one
air mass to another, focussing on increasing DM and MM
Int J Biometeorol
Table 4 Select important findings with respect to climate zone and seasonal analyses in MP and DP weather types
Weather
type
Main findings
Overall DP
Greatest decreases occurring predominantly in the summer season, with 22 of the 30 stations (75 %) revealing significant decreases;
hence the slope is largely controlled by the summer season, where winter results give a weak and insignificant slope of −0.005. In
contrast, summer DP slope overall is −0.097 (P<0.001), or approximately 5.5 less DP days in the summer than 60 years ago
Greatest decrease of DP occurring in the Rockies, resulting in, on average, almost eight less days less per summer season today than
60 years ago (slope=−0.121, P<0.001)
This has the weakest, yet still significantly decreasing, trend of DP in the summertime (−0.049, P<0.001). Individually, four of the
seven Marine stations are significant in this change
Exhibits an overall negative trend for both seasons combined (−0.053, P<0.001), equal in both summer and winter
This is the strongest result found in the Marine climate zone, experiencing the greatest summertime MP decrease of all the climate
zones. Stations within the Marine climate zone experience seven less MP days per summer, on average, than 60 years ago. Overall
for both seasons, Marine MP gives a slope of −0.102 (P<0.001), or one less days per decade
Rockies DP
Marine DP
Overall MP
Marine MP
air masses, paired with decreasing DP and MP air masses
(DP and MP). A number of statistically significant changes
are also displayed, emphasising those that are changing to
the greatest extent and in opposite directions (i.e. a negative
trend balanced by a positive trend). From this we see that
decreases in MP and increases in DM are more prevalent in
the winter season.
The Marine climate, representing most coastal stations,
gives muted air mass trends (with the exception of MP). The
Marine zone has the highest frequency of transition days,
with the remaining air masses varying between the summer
and winter seasons, when water bodies are relatively cooler
and warmer than the land for the respective seasons. The
warmer waters increase MM prevalence, and cold waters
increase MP. This moderating effect results in less significant changes in this climate zone than the others. For example, only one station in the zone has experienced significant
increases in the MM air mass, yet two have decreased
significantly. A summary of important findings is displayed
in Table 4.
Correlation analysis results (Table 5) display which
trends are changing together, based on both temperature
(e.g. a cold air mass being replaced by a warm) and moisture
relationships (e.g. a moist air mass replacing a dry air mass).
From the seasonal analysis, three notable results were found,
and are emphasised in Table 5. First, 23 of the 30 stations
were found to have a significant negative correlation between DP and DM air masses, with a mean correlation for
all cities in the summer of −0.40 (range: −0.62 to −0.10);
hence, the DP air masses are becoming less prevalent with
time. The DP weather type is present 18.3 % of the time
during the summer in the 30 cities (as opposed to 39.5 % in
the wintertime), and is the coldest and driest weather type in
both seasons. A reduction of dry and cold conditions,
replaced by moist and warmer conditions, is consistent with
patterns of both temperature [International Panel On Climate Change (IPPC) 2007], and moisture (Robinson 2000;
Vincent and Mekis 2006) increases observed in Canada and
the US. Six of the seven remaining insignificant results were
in the Marine climate zone, which agrees with the previous
Table 5 Relationships between specific air masses using Pearson correlation analysis. Columns display the number of statistically significant
relationships between given air masses, and average Pearson coefficient for all cities, divided into summer and winter seasons
Air massesa
Summer
Thermal relationships
No. stationsb
DM vs DP
MP vs MM
Moisture relationships
DM vs MM
DP vs MP
Winter
rc
No. stations
r
23
8
−0.40
−0.09
10
4
−0.14
−0.03
8
19
−0.17
0.26
6
24
0.13
−0.54
a
Correlation of one air mass with another, representing how they change simultaneously. For example, a negative relationship of DM vs DP (x,y)
denotes that a dry polar air mass is decreasing while the dry moderate air mass is increasing
b
Number of stations where statistically significant correlation coefficients were found between the two given air masses (P<0.05). The maximum
amount for each air mass within a season is 30
c
Average Pearson correlation coefficients for all 30 stations per season
Int J Biometeorol
weak Marine zone results. All four of the thermal relationships shown (two per season) are negative, with moderate
air masses increasing and polar air masses decreasing
overall.
Second, a strong negative winter correlation exists
between MP and DP air masses, with 24 of the 30
stations having a statistically significant negative correlation (mean = −0.54, range: −0.94 to 0.10). In the Arctic
zone winter, the MP-DP correlation is significantly strong
at −0.89, yet weak and insignificant in the Arctic summer (r=−0.07). Therefore, air is becoming more moist
(MP air is replacing DP air) in the Arctic zone’s winter.
The least change in polar air masses is seen in the
Rockies climate zone (r=−0.36). Last, we see that in
the summertime, the MP and DP air masses are changing
predominantly in the same fashion, with a positive relationship at 19 of the 30 stations (mean r=0.26, range: −0.10 to
0.57). Based on summertime results presented in Fig. 2, overall these two air masses are both decreasing. In the same
respect, the DM and MM have positive correlations, hence
increasing in similar fashion over the period of study.
These results, as well as those shown in Figs. 2 and 3,
show that overall, moist and mild air masses are increasing
across the country, with dry and cool air masses decreasing,
respectively. This was found by Knight et al. (2008) in the
US for full-year analyses. However, seasonal analysis shows
the summer to be dominated by the increase of predominantly DM air masses, and some MP, replacing DP air
masses, while changes in the winter are predominantly from
moist air replacing dry. This shows an overall increased
moisture content in the air throughout Canada in the summer, which coincides with increasing air temperatures
(Kalkstein et al. 1990; Vincent et al. 2007). Further, the
simultaneous increase of DM and MM in the Arctic winter,
with the negative DM to DP relationship, also agree with
Arctic trends found by Kalkstein et al. (1990). These latter
researchers also found enhanced increases of air temperatures when assessing within-category warming, as
Fig. 4 Sixty year frequency trends of the MT air mass in the summer
season displaying four of Canada’s largest population centres: London,
Quebec City and Toronto in the Laurentian climate zone, and Vancouver in the Marine zone. Slopes represent the change in MT air mass per
expected in the north based on positive climate feedback
mechanisms (e.g. snow-ice albedo feedback) causing amplified warming and redistributed heat (Groisman et al. 1994;
Serreze et al. 2007, 2009).
This enhanced moisture in the Arctic is expected based
on a warmer climate, which has a direct effect in horizontal
latent heat transport poleward (Held 1993), and eddy enhancement (McCabe et al. 2001). These have implications
on storm tracks and low pressure systems, which represent transition days, and are discussed in the section
on Transition days.
Summertime moist tropical weather type
The moist tropical (MT) air mass is rarely found in the
Marine, Rockies, and Arctic climate zones, with many stations displaying zero MT days. The greatest presence is
found in the Laurentian climate zone, averaging a 15.8 %
frequency in the summer over the last 60 years. On average,
this air mass has been increasing over the last six decades
with a slope of 0.100 (P<0.001) (range = −0.015 to 0.280),
thus yielding 6 more MT days per summer, on average, than
in the 1950s. Considering that MT days are present on
average 14.5 days per summer in this climate zone, 6 more
days represents a 41 % increase.
Figure 4 displays the 60-year summertime frequency
trends of the MT air mass for four of Canada’s largest
population centres: London, Quebec City and Toronto in
the Laurentian climate zone, and Vancouver in the Marine
zone. This displays how rarely Vancouver experiences such
oppressive conditions in comparison to the other three,
which all display significantly positive slopes (P<0.05).
Six of the Canada’s top ten population centres are in
the Laurentian zone, hence the MT trend increases represent the greatest threat to human health due to the
sheer high numbers of people affected. In a comprehensive study by Smoyer-Tomic et al. (2003), the Prairies,
Southern Ontario, and the St. Lawrence Valley region of
year in the summer season; hence the slope of 0.120 in London
represents an average increase of 7.2 MT air masses per summer in
the 60-year period. * Statistical significance (P<0.05)
Int J Biometeorol
Quebec, contained predominantly in the Laurentian
zone, had the highest incidence of heat waves. Additionally, these researchers found that the Pacific and
Atlantic coasts (Marine zone) and the north areas
(Arctic) experienced minimal heat waves. The southernmost population centre is Windsor, ON, which has the
highest frequency of MT air masses in the summer
(21.3 %). Here, we also found the largest slope (0.280,
P<0.05), which represents almost 3 more MT days per
decade (10 days more for the 33-year record for
Windsor). A recent report by the Union of Concerned
Scientists (Perera et al. 2012) also observed that the overall frequency of summertime oppressively hot air masses
was increasing, while cool air masses were decreasing
over the last six decades, in ten Midwestern US cities.
Cities within the Laurentian zone have wide yearly temperature ranges; hence, there is a greater need for thermal
acclimatisation to withstand early season heat. For example,
Winnipeg ranges from a mean daily minimum temperature
of −20.2 °C in the winter to a mean daily maximum of
24.7 °C in the summer. In contrast, Vancouver’s respective
range is 0.9 °C to 20.9 °C, giving a 24.9 °C difference in
absolute mean range between the two cities. Hence, individuals living in Winnipeg must undergo much greater acclimatisation to the heat, yet with prevalence of hot air masses
increasing, people may not be prepared for the first and/or
early heat events, which have greater impact on human
mortality (Greene et al. 2011).
Implications for climate, health, and society
The large increase in MT air masses in the summer is a
critical finding, particularly in large population centres.
The general increase in warmer air masses in both
seasons can be viewed in various ways depending on
the area and season being studied. For example, increasing warmer air masses in the winter season may decrease short-term and seasonal mortality burden, yet
specific epidemiological studies must be completed to
associate air mass type with specific human health outcomes. Martin et al. (2012) predicted overall warmer temperatures to slightly decrease the burden of cold-related mortality
in 11 of Canada’s largest cities, and to increase the burden of
heat-related mortality in 4 cities.
Increases in oppressive weather and heat waves result in
increased strain on the health care system, and a higher number of emergency medical services calls observed (Dolney and
Sheridan 2006; Vanos et al. 2012). Heat-related mortality has
been related to the occurrence the hot DT and MT+ air masses
(Sheridan et al. 2009; Sheridan and Kalkstein 2004), with
health effects present in the short-term (0–3 days). Common
effects include the exacerbation of cerebrovascular conditions, and distress in older individuals (>65 years)
(Díaz et al. 2002). Additionally, heat syncope, cramps,
exhaustion and stroke may result (McGeehin and Mirabelli 2001).
Furthermore, synoptic conditions with characteristics
of clear skies and high temperatures (commonly dry air
masses) are associated with higher photo reactive air
pollution types dangerous to human health, such as
ozone (Greene et al. 1999; Hanna et al. 2011). Hence,
synoptic weather predictions can aid in the prediction of
both extreme heat events and air pollution events, as in
Greene et al. (1999). A further avenue of research would
include associating of air mass, air temperature, and air
pollution interactions with human health outcomes during
extreme events.
Canadian projections by Health Canada (Séguin and
Berry 2008) show that the severity and duration of air
pollution episodes will increase as a result of a warmer
climate. This is due to the warmer temperatures increasing
the rates of chemical reactions, enhancing natural emissions
(e.g. nitrogen oxides from soil, volatile organic compounds
from trees), increasing emissions from human sources (e.g.
air conditioning use), prevalence of forest fires, and more
frequent stagnation of hot air (altering the types of air
pollutants persisting in the urban boundary layer) (Séguin
and Berry 2008). The report states that ozone levels will
increase the most in Windsor, Montreal, Toronto, Vancouver, Calgary, Edmonton and Winnipeg. As some of Canada’s largest cities, this is worrisome for human health. The
current study has found increases in the frequency of MT air
masses in six of the seven listed cities (Vancouver rarely
experiences MT air).
Transition days
Transition days represent the gradual meteorological
changes that occur between two distinct air mass types. In
the 30 Canadian cities studied, transition days represent, on
average, 11.1 % of days in the summer and 14.0 % in the
winter. Overall, the number of TR days across Canada have
been decreasing in the last six decades (Table 6), with 95 %
of the regression slopes showing decline, and 60 % being
statistically significant.
The trends were stronger in summer (−0.073) than in
winter (−0.050) (P<0.001), with 4.4 and 3.1 less TR days
per respective season today than 60 years ago. Knight et al.
(2008) also found large decreases in TR days across the US
(full-year analysis), driven largely by sharp winter declines,
as confirmed by Hondula and Davis (2011b). We found the
Marine climate zone (coastal areas) to have the weakest
average decrease for both seasons combined (−0.026, P<
0.001), or 1.6 fewer days. Coastal cities in the US, although
not in the Marine zone, were also found by Hondula and
Davis (2011a) to have the weakest TR decreasing trends.
Int J Biometeorol
The first potential reason was studied by Hondula
and Davis (2011a), who looked at wintertime transition
climatology across the US, creating four transition subtypes. Weak dew point temperature changes and small
24-h pressure changes resulted in the variable thresholds
for TR classification not being reached; hence, weak
frontal systems led to fewer transition days. This is
enhanced in the Arctic, which may be partially explained
by higher within-category temperature rises (Kalkstein et
al. 1990), as well as increased horizontal transfer of latent
heat (McCabe et al. 2001). In the current study, we found
an overall increase in moisture content in Canada. Weakened temperature gradients in the north, as a result of
warming trends, give rise to fewer mid-latitude cyclones
(Held 1993; Yin 2005), and have caused weaker weather
changes (that may not meet the criteria for a TR day).
Transition day climatology and within-category characteristic modelling has not been completed in the current
study or to our knowledge within Canada, hence leading
to a future avenue for novel and impactful research.
The second potential reason associates decreasing trends
of TR days with fewer mid-latitude cyclones (frontal passages), which has been hypothesised to be due to
contraction of the northern circumpolar vortex, as found in
the northwest of North America by Frauenfeld and Davis
(2003); Hondula and Davis (2011b). As the jet stream
follows the southernmost edge of the circumpolar vortex,
the relative intensity and movement of the two are highly
linked. This contraction of the vortex has been observed
to augment a poleward shift of storm tracks (McCabe et
al. 2001; Yin 2005). We investigated the relationship
between station latitude and the frequency changes in
transition days. We found that the slope of decrease
becomes greater with latitude for both seasons together
(r=−0.10); however, the result is strong and statistically
significant in the winter season (r=−0.51, P<0.05), and
insignificant and weakly positive in the summer (r=0.22).
This wintertime result signifies that the declines of TR
days in the north are more dramatic, decreasing to a greater
extent with increasing latitude. This corresponds to the issue
of the contraction of the circumpolar vortex, which shifts
storm tracks polewards (Angell 2007; Davis and Benkovic
1994; Fu et al. 2006; McCabe et al. 2001; Yin 2005). The
lower temperatures gradients in the north as a result of
warming trends are hypothesized to have resulted in this
vortex contraction, and hence the storm tracks may not
expand as far south in the winter (McCabe et al. 2001; Yin
2005). Therefore, the total area experiencing frontal activity
is decreased in the winter, and less frontal activity may be
occurring in and around the midlatitude stations tested in the
current study. The mean location of the jet stream in the
winter is 38 °N; hence, all stations in the current study are
found at a higher latitude than the mean position. However,
decreases in cyclone activity in the mid-latitudes (30° N to
60° N—in which all but two of stations in the current study
are found), have been observed in the last several decades
by McCabe et al. (2001) from 1959 to 1999, Serreze et al.
(1997) from 1966 to 1993, and Paciorek et al. (2002) from
Table 6 Average trend results for the transition air mass, divided into
climate zones and seasons. Mean slope of regression lines are displayed, with the average decrease over the 60-year time period in
parentheses (Δ60), and percentage of statistically significant decreasesa
of TR weather type days. Statistical testing was completed using a
pooled random effects modelb
The question arises as to what could be causing fewer
days to meet the requirements of transitional day classification. Based on current results, as well as past and present
literature review, we hypothesize three potential reasons:
(1) Weaker frontal systems, below thresholds needed to be
classified as TR;
(2) Fewer frontal systems are moving across the country;
(3) Changes in latitudinal storm track location and/or
speeds of midlatitude jet stream causing faster moving
fronts.
Climate zone
nc
freq%
Summer
Slope (Δ60)
Winter
% Decreased
Slope (Δ60)
% Decrease
Laurentian
Rockies
Marine
10
7
6
12.0
11.5
14.7
−0.103* (−6.2)
−0.042* (−2.5)
−0.040* (−2.4)
100.0
71.4
57.1
−0.040* (−2.4)
−0.098* (−5.9)
−0.012 (−0.7)
20.0
57.1
0.0
Arctic
All
6
30
13.0
12.6
−0.081* (−4.9)
−0.073
83.3
80.0
−0.067* (−4.0)
−0.050
83.3
40.0
a
Statistically significant increases were not present in any situation
b
Asterisk indicates statistical significance (P<0.001)
c
Number of cities studied within the defined climate zone
d
Percentage found to be decreasing in a statistically significant fashion (P<0.05)
Int J Biometeorol
1949 to 1999. Thus we have plausible physical suggestions
that can help explain the deceasing TR trends in the current
study. Additionally, we found stations within 10° N latitude
of the average winter jet stream position to be declining less
than stations >10° N of this point, experiencing 2.8 less TR
days per winter season versus 4.6 less days at the higher
latitudes. Hence, the more southern stations are experiencing less of a decline of frontal passages, as the position of
the winter jet stream is more frequent at these latitudes.
The larger summertime declines are not related to latitude. The NH storm track in the summer is weaker than that
in the winter, yet a slight a poleward shift for 9 of the 15
global climate models tested by Yin (2005) was found;
hence consensus among the models remains an issue. These
seasonal fluctuations reveal deepening (strength) in winter,
but become more shallow (weaker) in summer (Burnett
1993); yet, they change with surface-air temperature contrasts and, therefore, weaker cyclones and anticyclones are
found in the summer (Serreze et al. 1993). A study by
Lambert (1995) also found a significant reduction, yet increased intensity, in the total number storm tracks in the NH.
However, they found little-to-no change in storm track
position, as observed in studies by Frauenfeld and Davis
(2003), Fu et al. (2006), McCabe et al. (2001) and Yin
(2005). Contrasts in such observational studies reveal the
difficultly in drawing conclusions for the current study’s
findings, and understanding the mechanisms of poleward
shifts of storm tracks is under continuous investigation.
There has been high variability in the contraction and expansion of the vortex throughout the years and seasonally
(Frauenfeld and Davis 2003), with some research observing
no long-term temporal trends of contraction (Rohli et al.
2005), as observed in above cited studies.
Hondula and Davis (2011a) found that the vortex has
not contracted to the same magnitude over all of North
America—the northwest region displays the greatest shift,
and hence a greater decrease of TR days. Common cyclogenic regions in Canada are to the east of the Canadian
Rockies, with separate track convergence over the Great
Lakes (Serreze et al. 1993). In the current study, we found
the greatest average winter decrease of TR days in the
Rockies climate zone of approximately 5.9 days in the last
six decades (slope = −0.098, P<0.001) in which all stations are in the northwest of North America. However, this
was not the case in the summer, when ridges and troughs
follow a different pattern and the vortex is more expanded.
Accessing vortex data to understand these trends is a potential
area of future research for Canada. This can aid in better
understanding the influences of a north-shifted vortex across
the whole of North America (Hondula and Davis 2011b).
Last, we investigated if changes in latitudinal storm track
location and/or speeds of frontal passages are associated
with fewer TR days. This was completed with consecutive
day analysis of two or more TR days in a row passing over
one station. Overall, 88 % of the stations showed decreasing
consecutive 2 day frequencies. On average, an insignificant
and decreasing slope of −0.019 was found for all stations,
with summer decreasing greater than winter (−0.020
and −0.013, respectively). The Laurentian climate zone displayed the greatest decline (JJA=−0.030, DJF=−0.016) representing 2.0 and 1.0 less consecutive two-day TR events in
the last 60 years per season, on average. The Marine climate
zone was the weakest (JJA= −0.008, DFJ= −0.006). Although insignificant, these results feature small yet important temporal changes in the past 60 years.
We cannot assume from this that frontal movement is
quicker today than it was six decades ago; this would be a
very strong hypothesis to state, particularly with the low
numbers of times throughout a season that two consecutive
TR days occur. The faster moving cyclonic systems and
storm intensities found by McCabe et al. (2001) aids in the
plausibility of this reasoning. However, further answers may
lie in the more northerly position of storm tracks also found
by McCabe et al. (2001). If a mid-latitude cyclone passes
further north of a given station, a greater distance (and hence
time) between the warm and cold fronts would exist. Therefore, the probability of a cold front and warm front passing
on two consecutive days would decrease.
Effects of transition days on human health
The current results of decreasing transition days can have an
influence on the physical and emotional health, and quality
of life, of many Canadians. Health effects from transitional
days, which bring large shifts in weather, are often overshadowed by temperature effects on human health (as discussed in the section on Implications for climate, health, and
society). Many health conditions are weather sensitive, with
some ailments (e.g. migraines, muscle pain, asthma, blood
pressure) being aggravated more by a full synoptic condition, rather than merely air temperature. Specifically, the
frontal system frequencies (or transition days) that have
been examined in the current study bring large fluctuations in air temperature, air pressure, humidity, and wind
velocities, that can negatively impact health. The inclement weather associated with the transitional category
involves large pressure fluctuations, linked to a higher
incidence of serious health impacts such as ischaemic
heart disease death (McGregor 1999), as well as myocardial infarction and stroke (Feigin et al. 2000). Changing
weather patterns have also been linked to further health
issues such as migraines (Mukamal et al. 2009), blood
pressure (Hoppe 1982), myocardial infarction (Kveton
1991), in addition to overall well-being and mortality
(Jendritzky and Bucher 1992). For example, drops in
barometric air pressure have been associated with
Int J Biometeorol
increased emergency department visits for headaches 48–
72 h later (Mukamal et al. 2009).
According to Bart and Bourque (1995), many physicians
are well-versed in areas of respiratory problems caused by
air pollution, and skin cancer caused by radiation; however,
they fail to consider atmospheric influences on migraines,
blood pressure, arthritis, mood, behaviour, and heart disease. Consistency has been found with low barometric pressure from frontal systems being associated with suicide and
hospital psychiatric admissions (Sanborn et al. 1970) and
depression diagnoses (Briere et al. 1983). Some inconsistencies in weather and emotional health studies were underlined by Driscoll and Stillman (2002), drawing upon many
observational studies attempting to relate emotional states
with frontal activity and weather variables. More recent
research into asthma and weather patterns has aided in the
development of “pollen reports” for seasonal allergies. This
is advantageous to warning allergy sufferers and children,
who were found by Cakmak et al. (2002) to have more
frequent emergency room visits for conjunctivitis and rhinitis when high ambient levels of pollen and fungal spores
were present in eastern Ontario, Canada.
The ability to track weather patterns based on individual
location, and to associate them with health ailments such as
arthritis, asthma, diabetes, heart disease and migraine can be
extremely useful (Bart and Bourque 2012). Since this type
of weather data is readily available, such warnings can be
more prevalent and headed by those individuals, particularly
the elderly and isolated.
Our findings of decreasing TR days, and thus frontal
activity, nationwide may be viewed as a positive change
for human health. Studies rarely address fluctuating weather
associated with transitional fronts; however, through better
understanding of one’s physiological reaction to transitional
weather, and the long-term trends of this weather category,
we can better prepare for the effects of weather related
ailments.
further the research literature and potentially provide more
answers and more robust results for better comparison purposes in weather, health, and air pollution studies. Cheng et
al. (2007a, b) used an automated synoptic weather-typing
system, finding it useful to predict air pollution concentrations in urban areas, as well as future air pollution concentrations in select cities. However, this weather-typing system
is limited spatially to four Canadian stations (Montreal,
Toronto, Windsor and Ottawa) (versus 77 for the SSC),
and hence a robust orthogonal stepwise regression analysis
would need to be recreated for each new location (Cheng et
al. 2007a).
The use of epidemiological time-series methods is also an
alternative to using synoptic based methods to assess
weather-health associations. Many time-series methods use
general additive, linear, or non-linear models based on single meteorological variables to predict health outcomes.
Although this negates the impact of a full weather situation
on human health, the outputs provide simple threshold
guidelines to follow and implement. Many such methods
use temperature-metrics, such as mean air temperature,
which provide more well-recognized and interpreted output
values for the public, and to inform health policy and
decision-making (Anderson and Bell 2009). When designing heat/cold warning systems, a variety of different synoptic methods should be included to determine the most
accurate predictions.
Last, many areas of North America and around the world
lack measurements of all variables needed for synoptic
classification, and hence calculation is not possible. In the
computation of SSC categories, there is also the added
complexity in new locations due to the need to make subjective decisions to define seed days and air mass characteristics (Sheridan 2002).
Limitations and alternative methods
The current study investigated changing long-term synoptic
climatology throughout Canada. Trend analyses of air mass
frequencies in summer and winter seasons, plus climate
zone division, showed an overall increase of moist tropical
air masses in all climate zones, excluding the Marine zone,
as well as increases in moderate air masses in all areas at the
expense of polar air masses. Associations of specific air
masses with human health outcomes were discussed, accentuating the need to better understand the impacts of specific
air masses, temporally and spatially, on human health in
Canada.
The large decline of transitional weather type days is
among the most notable results of SSC trend analyses published to date, and is supported by past SSC research studies. Hypotheses based on past and current literature were
There are a multitude of approaches currently in use to
evaluate changing weather applied to epidemiological
health studies. Synoptic studies based on similar principles,
yet different methodology, to identify air mass types have
been completed and used successfully in research studies
(e.g. Alpert et al. 2004; Bergeron 1930; Berman et al. 1995;
Cheng et al. 2007a; McGregor 1999), with the Bergeron
system being the most common air mass classification system. Applications of many widely accepted synoptic classification systems have become more prevalent and critical to
research in the last two decades (Yarnal et al. 2002). Studies
similar to the current one can be completed with synoptic
methods to derive long-term air mass trends. This would
Conclusions
Int J Biometeorol
provided, drawing upon surface weather observations to
upper-level air flow in connection with the observed TR
declines. We addressed how the circumpolar vortex, temperature changes, and poleward shifting storm tracks may be
affecting the frequency of the TR weather type. This serves
as an avenue for future research for examining large-scale
climate fluctuations throughout the country. Further examination into the specific climatology of air masses in Canada,
plus a US study on summertime transition days, would serve
as beneficial studies for understanding the synoptic trends.
Based on historical data, we have shown that enhanced
climatological challenges exist in Canada. When combined
with projected health outcomes, demographics, lack of seasonal acclimation, and further climate-related risks, this can
increase the future vulnerability of Canadians in the absence
of effective adaptations (Séguin and Berry 2008). A direct
air mass trend comparison with that of the US would be
beneficial. Hence, we can determine relative risks and future
climatological challenges for Canada, and if there is a greater
need for further mitigation to climate change and research in
the climate-health sector. Enhanced multidisciplinary research is needed to gain understanding and build a knowledge
base to adequately address such information gaps in climatology and health.
Acknowledgments The authors would like to thank the Climate
Change and Health Office at Health Canada, as well as Dr. Laurence
Kalkstein at the University of Miami, for their thorough reviews and
helpful comments. Funding for completion of this project was generously provided to Dr. Jennifer Vanos by a National Sciences and
Engineering Research Council (NSERC) Postdoctoral Fellowship from
the Government of Canada.
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