3.0 Affected Environment - Knik Arm Bridge and Toll Authority

Knik Arm Crossing DraftFinal EIS
Affected Environment
Table 3-12. Study area climate
Month
Average
maximum
temperature
(°F)
Average
minimum
temperature
(°F)
Average
total
precipitation
(inches)
Average
snowfall
(inches)
Average
snow
depth
(inches)
January 19.2 5.7 0.86 10.5 12
February 24.4 9.6 0.94 12.9 13
March 32.4 16.5 0.74 9.0 11
April 43.5 28.8 0.57 4.8 3
May 54.5 39.4 0.61 0.2 0
June 61.8 47.8 1.07 0.0 0
July 64.8 52.1 2.10 0.0 0
August 63.1 49.9 2.58 0.0 0
September 54.6 41.6 2.42 0.2 0
October 39.2 27.6 1.78 9.6 1
November 27.0 15.3 1.19 13.4 4
December 20.9 8.6 1.30 16.5 9
Annual 42.1 28.6 16.15 77.0 4
Source: Alaska Engineering Design Information System, CRREL-UAA, 2004
Winter in Anchorage typically extends from mid-October to mid-April, the period when
measurable snow is likely to occur. On the winter solstice, the sun is above the horizon for
only 5 hours and 28 minutes. Spring usually begins about mid-April, immediately following
winter “break-up.” This season is brief, but characterized by warm, pleasant days and chilly
nights; the mean temperature rises rapidly and there is generally little precipitation.
Summer is from June through early September and is, in reality, two seasons of about equal
length, the first of which is dry, the second wet. At summer solstice, possible sunshine
amounts to approximately 19.5 hours. About mid-July, average cloudiness increases
markedly, and the remainder of the summer usually accounts for about 40 percent of annual
precipitation.
Autumn is brief in Anchorage, beginning about mid-September and lasting until mid-
October. The frequency of cloudy and rainy days generally drops sharply in early October.
Measurable snowfalls are rare in September, but substantial snowfalls, sometimes in excess
of 12 inches, occasionally occur in mid-October.
Some of the stronger southerly winds, occasionally with damaging effects, occur in late
summer and fall, as a result of postfrontal winds following the movement of storms from the
southern Bering Sea or Bristol Bay, northeastward across Interior Alaska. Less frequent, but
often more damaging, are the southeasterly “Chugach” winds that are funneled down the
creek canyons on the NW slopes of the Chugach Mountains. With gusts of 80 to
100 miles/hour, these winds have occasionally caused substantial damage to roofs,
powerlines, and other structures in the Anchorage Bowl.
Page 3-94 12/18/07