Plotting a line graph of the annual average maximum temperature since 1900 gives this:
16
15
14
13
12
11
10
Year
Clearly, this has a lot of variability. Looking at the average for each decade produces the following.
1900-1909
1910-1919
1920-1929
1930-1939
1940-1949
1950-1959
1960-1969
1970-1979
1980-1989
1990-1999
2000-2009
Mean temperature
12.616
12.892
12.894
13.155
13.37
13.12
12.808
13.15
13.12
13.797
14.111
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16
15
14
13
12
11
10
18
17
16
15
21
20
19
24
23
22
This reduces the variability and may indicate an increasing temperature.
These graphs are for the annual average temperatures, but if we suspect that the summers are getting hotter and the winters are getting cooler, then it would make sense to consider them separately.
Summers and winters
Looking at the average value for summer each year – using June, July and August temperatures and the average value for winter each year – using December, January and February gives the following.
Mean Summer highs
Year
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1900-1909
1910-1919
1920-1929
1930-1939
1940-1949
1950-1959
1960-1969
1970-1979
1980-1989
1990-1999
2000-2009
Mean Winter lows
3
2
1
0
5
4
-1
-2
-3
-4
Year
Again, these graphs have a lot of variability by individual years.
Looking at decades may help give a clearer picture of trends over time… or not!
Summer high
19.17
19.39
19.11
20.02
20.09
19.68
19.30
19.86
19.77
20.50
20.56
Winter low
1.45
1.90
2.11
1.76
1.08
1.19
0.85
1.83
1.33
1.99
2.10
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2,50
2,00
1,50
1,00
0,50
0,00
21,00
20,50
20,00
19,50
19,00
18,50
18,00
Summer highs by decade
Decade
Winter lows by decade
Decade
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Exploring with Boxplots
Sample boxplots for mean annual temperatures for the decades (top to bottom):
1900-1909, 1910-1919 and then 1990-1999 and 2000-2009
Using the median value removes the effects of extreme values. This seems to give a clear indication of a general increase in temperature progressively over the decades shown.
Annual Precipitation by decade
Top to bottom: 1900-1909, 1910-1919 and then 1990-1999 and 2000-2009
There doesn’t seem to be any trend for precipitation shown when the data are analysed in this way.
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Extension work idea: using moving averages
Using 10 point moving averages gives a clearer indication that temperatures might be increasing.
Mean annual temperature
10 point moving averge
15
14,5
14
13,5
13
12,5
12
Year
2,5
2
1,5
1
0,5
0
Looking at the winter minimums and summer maximums:
Winter minimum
10 point moving average
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Summer maximum
10 point moving average
21
20,5
20
19,5
19
18,5
18
It seems as if the summers are getting warmer, but it’s not clear what’s happening with the winters.
1300
1200
1100
1000
900
800
700
600
Precipitation
Similarly, with precipitation it is difficult to ascertain whether it is static or not.
Annual Precipitation (mm)
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Using 10 and 100 point moving averages possibly show an increasing trend.
Annual precipitation
10 point moving average
960
940
920
900
880
860
840
Annual precipitation
100 point moving average
940
935
930
925
920
915
910
905
900
895
890
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