Geoscience Reference
In-Depth Information
Table 13.1
Monthly total precipitation for July at Musoma, Tanzania from
1931 to 1960 (Data from East African Meteorological Department, 1966).
Rainfall
(mm)
Rainfall
(mm)
Rainfall
(mm)
Year
Year
Year
1931
45
1941
0
1951
3
1932
23
1942
0
1952
3
1933
4
1943
0
1953
29
1934
2
1944
13
1954
145
1935
0
1945
18
1955
37
1936
13
1946
9
1956
103
1937
2
1947
38
1957
0
1938
2
1948
1
1958
20
1939
0
1949
11
1959
1
1940
31
1950
181
1960
3
Table 13.2
Mean, standard deviation, and coefficient of variation over 10-year
periods of the monthly total precipitation for July at Musoma, Tanzania from 1931 to
1960 derived from the data given in Table 13.1.
Standard
deviation (mm)
Coefficient of
variation (%)
Period
Mean (mm)
1931-1940
12.2
15.7
129
1941-1950
27.1
55.3
204
1951-1960
34.4
49.9
145
respectively, where
N
is the number of years, in this case 10. These values suggest
that there may be some form of trend in these data.
Running means
One commonly used way to smooth out variations in a time series and make
trends in the data more obvious is to use a running mean. The mean is taken over
an odd number of years centered on the year of interest, i.e., over (2
n
+
1) years
where
n
=
0, 1, 2 etc. Thus, a derived series of values,
P
i
′
, is calculated from the
initial series,
P
i
, using the equation:
(2)
j
=+
i n
1
∑
P
¢
=
P
(13.4)
i
j
(2
n
+
1)
j
=−
(2)
i n
