Jökull - 01.12.1986, Blaðsíða 61

scribed. Lack of space prevents further discussion of
this issue here; suffice it to say that every effort has
been made to ensure that only reliable information
has been included in the analyses presented. For dis-
cussion explaining the need for source analysis, and
detailing some of the techniques employed, see Vil-
mundarson (1969), Bell and Ogilvie (1978), Ogilvie
(1981) and Ingram et al., (1981).
After ensuring that the sources to be used were
reliable, data on the individual years and seasons were
extracted, compared and evaluated. In order to quan-
tify the data, information was then summarized, with
seasons categorized as “mild”, “severe”, “cold”,
“average” etc. (see Tables 1 and 2). The terms used
were, as far as possible, those of the original observers.
The seasons were divided into: winter (mid October of
one year to mid April of the next); spring (mid April to
early June); summer (June to August); and autumn
(September to mid October). These seasonal divisions
correspond most closely to the real seasons in Iceland,
and also reflect the perception of contemporary ob-
servers who divided the year into two main seasons:
summer (c. mid April to c. mid October), and winter
(the rest of the year). Winters are dated according to
the January, and all dates are New Style.
In order to compare regional differences in climatic
variability, the data were grouped into four main
areas: north, south, east, west. While it could be
argued that certain regions, the Western Fjords, for
example, have distinct weather patterns which should
be analysed separately, it was felt that the geographi-
cal divisions used here (see Figure 1) give an adequate
picture of regional variations in climate at a time
when data are limited.
By summarizing the data they were greatly simpli-
fied, and interesting detail was lost, but it is always
possible to refer back to the original sources if neces-
sary. The great advantage of summarizing the data
was that it was then possible to quantify them further
and present them in tables and figures. By adding up
the number of cold, mild and average seasons in a
decade, it is possible to get an idea of which decades
and years were colder, and which milder, during the
period 1701 to 1784. The two coldest decades during
these years are the 1740s and 1750s. The main
characteristics of these seasons are shown in Tables 1
and 2.
The data were then quantified further by construct-
>ng winter-spring thermal indices for the whole of Ice-
land, and also for the north, south and west of Ice-
land, based on the summarized data described above.
(Insufficient data made it impossible to do this for the
Fig. 2. Decadal winter/spring thermal index for Ice-
land 1601 to 1780. From Ogilvie (1981); see also
Ogilvie (1984). — 2. mynd. Mœlikvarði á hitastig vet-
urs og vors á íslandi frá 1601 til 1780.
east.) These indices (see Figures 2 and 3) are the
number of mild seasons per decade minus the number
of cold seasons (each weighted by the number of
regions reporting mildness or severity), divided by the
sum of these two weighted numbers. As stated, these
indices only cover the winter and spring seasons.
Fewer data were available for the summers and
autumns and the indices were , therefore, not drawn
up for these seasons.
As the indices are based only on qualitative infor-
mation, they cannot be regarded as giving the same
degree of accuracy as would be expected from quanti-
tative observations, but, as these are lacking, the fig-
ures give a reasonable idea of variations in climate in
Iceland as seen by contemporary observers.
A decadal sea-ice index was also constructed (Fig-
ure 4). This represents the number of seasons (winter,
spring and summer) with sea ice present off the coast
of Iceland per decade. As the ice only rarely appears
in autumn, this season was not included in the index.
Each number is weighted by the number of regions
(north, south, east, west) which report ice. By compar-
ing Figures 2 and 4, it may be seen that the relation-
ship between sea-ice incidence and temperature is
relatively strong. The correlation coefficient between
the ice and temperature data is -0.57 (18 decades).
This is similar to the annual correlation coefficients
given by Bergþórsson (1969) of -0.68 for 1845 to
1919, and -0.39 for 1920 to 1969. Figures 2 to 4
coverthe years 1601 to 1780.
THE CLIMATE OF ICELAND 1701 TO 1780
The most striking aspect of Figures 2 and 3 is the
dominance of cold decades over mild ones. Although
59