ber of days per month with "measurable" precipita-
6.2 Station climatologies
6.2.1 Anchorage International Airport
tion (0.025 cm or more) is approximately 13. A pre-
Anchorage International Airport, the most north-
dominantly clear day, in this application, is one in
ern weather station polled for this study, is located near
which the majority of that day's cloud-cover obser-
the northern end of Cook Inlet, where it branches into
vations are categorized as only 0-2 oktas (zero to 2/
Knik and Turnagain Arms. Nearly 3.2 km equidistant
8ths) overcast. Fifty-six percent of Anchorage's total
from the Inlet on three sides (north, west, and south),
annual precipitation occurs during July, August, Sep-
Anchorage is situated in open, gradually up-sloping
tember, and October (ACRC 1997a). The precipita-
terrain characterized by marshes, shallow depressions,
tion range shown in the figure represents the "near-
low knolls, and glacial moraine. Anchorage is consid-
normal" precipitation, that is, the middle 33% of the
ered to have a Transition-type climate but is the most
data for the 19611990 period of record. The data
Continental of the four stations used in this study. Ap-
points within the range are the median precipitation
proximately 16 km to the east, the broad valley en-
values. Anchorage's growing season averages 124 days
closing Cook Inlet Basin rises abruptly to the 1,500
in length, with the first frost of fall occurring on Sep-
3,000-m- (5,00010,000-ft-) high summits of the
tember 16 on average. The earliest frost was recorded
northeast-to-southwest-trending Chugach Mountains.
on August 16 (NOS 1994).
The range inhibits the inflow of moist, maritime air
Autumn is considered to last from early September
from the Gulf of Alaska, so that precipitation in the
to mid-October, when below-freezing air temperatures
Anchorage area is only 1015% that of the region on
and snowfall become more frequent. The mean
the Gulf of Alaska side of the mountains (NCDC
monthly air temperatures for September and October
are 10 and 2C, respectively. From 1973 through 1997
2000a). For example, the mean annual precipitation
for Anchorage is 40 cm, whereas it is 445 cm just 52
these two months experienced mean totals of 2 and 18
km to the southeast, at Whittier on Prince William
days, respectively, when the daily low temperature was
Sound. Approximately 160 km from Anchorage, the
below freezing (Fig. 22). On average the mean daily
Alaska Range encloses the Inlet's west, northwest, and
air temperature remains below freezing after October
north sides. During winter, these mountains help to
23 (NOS 1994). Northerly and southerly winds pre-
shelter the basin from the inflow of cold, polar air from
vail, with nearly equal occurrence in September, and
the north. Although the climate of the Anchorage area
total more than 40% of all observations (see wind roses
is moderated by the mountain ranges, the seasonal
in Appendix E). Some southerly wind events (post-
changes nevertheless are well defined. Prevailing
frontal winds that follow storms tracking northeast-
winds during the summer are from the south, with the
ward out of the Bering Sea or Bristol Bay) can achieve
months of April and September marking the periods
damaging proportions in the Anchorage area. Less-
of wind shift between north and south.
frequent "Chugach" winds, which arrive from the
The summer season is generally considered to last
southeast, accelerate through the Chugach Mountain
from late May through August. The warmest month is
passes, and funnel down canyon creeks, sometimes
normally July, with a daily high temperature mean of
create gusts in excess of 47 m/s (92 knots). These can
19C and a daily low of 12C (Fig. 22). The highest
cause considerable structural damage. September has
temperature recorded in each of June, July, and Au-
the highest total precipitation and the most days with
gust from 1973 through 1997 was 27C. Winds are
measurable precipitation. Rainfall then drops off dra-
normally light and southerly during the summer in
matically in October. Measurable snowfall is rare in
Anchorage. The mean monthly wind speed for each
September, but occasional snowfalls of 25 cm or more
month ranges between 3.0 and 3.9 m/s (5.9 and 7.6
begin occurring in mid-October. The average date of
knots), with May and June having the highest mean
first snowfall is October 15, but measurable snowfall
wind speeds (Fig. 22). Similarly, mean monthly maxi-
has arrived as early as September 20 (NOS 1994).
mum winds ranged from 5.3 to 8.5 m/s (12.3 to 16.6
Winter in Anchorage is generally considered to last
knots), with May and June again having the strongest
from mid-October to early April, when lakes and
winds and the lowest frequency occurrence for calm
streams are frozen over. It is a time of clear, cold
wind observations (Fig. 22). Interestingly May is also
weather alternating with mild, cloudy periods. Air tem-
the month with the least extreme wind speed on record
peratures decrease steadily into January, when day-
time highs average around 5C. The mean nighttime
(19.0 m/s; 37 knots). Since 1973 the highest recorded
lows are near 11C, but clear and cold winter nights
wind speed has been 31.9 m/s (62 knots) (March and
can approach 30C. The lowest temperature recorded
July). After May the occurrence of rain increases (Fig.
from 1973 to 1997 was 34C. Total annual freezing
22). During June through August, only about 5 days
degree-days average 1008C-days (1814F-days).
per month are predominantly clear, and the mean num-
61
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