SNOWFALL REGIME:
The amount of snow that falls, depends upon two principal climatological
factors:
1) the air temperature and 2) the amount of precipitation that falls when
the temperature is near or below 0 degrees Celsius.
Based upon what we have already discussed with respect to the P.D.O., it is
not surprising to find that during the months of February and March,
significantly more snow falls during the cold phase of the PDO than in the
warm phase. (graph #10). The south coast, as represented by Agassiz, and the
southern interior at Vernon, both show this same pattern. The first cold
stage (1892-1924) in particular had much more precipitation falling as snow:
15% at Agassiz and almost 80% for Vernon Coldstream. Thereafter, the
percentage falling as snow at both of these sites has been on the
decrease-through the first warm phase, the second cold period and the second
warm stage. During the present warm PDO, percentages have dropped to
approximately 40% at Vernon and to only 7% at Agassiz. The months November
to January, have not shown the same rate of decrease (graph #11).
Vernon's percentage of snow has decreased to some extent from just over
75% to 70% recently. Agassiz though has dropped a little more dramatically
from approximately 12% to 7% in recent times.

Turning our attention to snow transects on the south coast; at Wolf River
on northern Vancouver Island and the Trapping Creek transect in the
southern interior, both show decreasing snow amounts particularly at the
lower elevations (graphs# 12-14). After quite low snowpacks in the 1980's,
snowpacks have once again increased in the 1990's. However, at the lower
elevations snow amounts have not increased to the same extent. This is
particularly evident late in the season (the May 1st snowpack on the coast
and April 1st in the interior). Elevations below about 1000 m. on the coast
and 1400 m. in the southern interior reflect this trend to relatively less
snow. Decreasing snowpacks are also to be observed at the lower elevations
when many Environment Canada climate stations are examined (table #1). This
has been particularly true in the south part of the province and also close
to the coast, including the north coast (Terrace Airport). Prince George
Airport and Barkerville in B.C.'s central interior, in a colder region have
not shown the same steep decrease.
Interestingly, the months of November and December have not had a
decrease anywhere but at Agassiz on the south coast. These observations
reinforce earlier findings in this analysis that the late winter has warmed
much more than the months of November and December. Even more significant
decreases in late winter snowpacks have been reported in another study in
this part of the world(9).

Another much more extensive study based upon satellite imagery has shown
that the overall extent of annual snow cover has declined by approximately
ten percent in the Northern Hemisphere from the mid 1970's to the mid
1990's(10). The largest snow decrease has been in the months of April and
May which may in turn be largely responsible for warming in the spring of
the year(11). The whole radiative balance of the Northern Hemisphere is
likely being affected by decreases in snow cover. This particular study
attributes the decline to milder annual temperatures (12).