Jökull - 01.12.1969, Page 55

when ice is most frequent, we find bi = 0.15 aj..
In other words: A pressure difference of 10 mb
between Cape Tobin and Galtarviti in the last
month is as effective as every 1.5° C in the
JM-temperature in keeping the drift ice from
the Icelandic coasts in the coming month.
From this we conclude that b = 0.15a in Eq. 2.
The next step is to find the constant a. For
this purpose we use the data in Table 3, and
furthermore the long time normal value of E
for every month.
We can rewrite Eq. 2 as:
log ((1.1 - i)/i) = a (J + 0.15E) + c (Eq. 4)
Then we compute the JM-temperature corre-
sponding to every value of annual ice inc.id-
ence in Table 3. This may be done rewriting
Eq. 1:
J = (1/0.75) log ((12 - I)/I) - 0.1 (Eq. 5)
Thus we can for the month January find 6
pairs of the expressions log ((1.1 — i)/i) and
(J + 0.15E) to put in Eq. 4. A linear re-
gression then gives the constant a. It turns out
to be very nearly 1.0 for January as well as
other months, the values varying from 0.9 to
1.1 for all months. Then the constant b must
be 0.15.
The third step is to determine c, which is
different for different months. We then only
need the normal ice incidence for January,
February, etc., together with the corresponding
normal value of J. This has been obtained
from the ice records for the years 1781 to 1968.
Furthermore we use the normal value of E for
every month. We use the equation:
c = log((l.l —i)/i) — J — 0.15E (Eq.6)
It turns out from this computation that c is
the following:
TABLE5
Jan Feb Mar Apr May Jun
-1.0 - - 1.3 - 1.4 - 1.6 - 1.5 - - 1.2
Jul Aug Sep Oct Nov Dec
-0.7 - -0.1 1.0 1.4 0.5 - -0.2
These values of c can now be tested and
antended with the aid of data from the last
Fig. 6.
Weighted JM-temperature, Pressure difference Cape Tobin-Galtarviti
June—November. in last month, mb.
A graph for monthly ice forecasts (see text, page 52).
JÖKULL 19. ÁR 51