Basic Alaska Area Knowledge
- General Information
- Earth Quakes
- Climate Regions
- Weather Hazards
- Weather Products
- Alaska Forecast Areas
1. General Information
Alaska is a big state. Contrary to weather maps and textbooks which often place Alaska in the little box in the corner of the nation's map, Alaska's size of over 586,412 square miles encompasses striking variations in terrain from the flat lowlands of the Yukon-Kuskokwim Delta to the highest mountain in North America, Mt. McKinley in the Alaskan Range. In acreage, Alaska equals the combined land area of the states of Wyoming, Montana, Washington, Oregon, Idaho, and Utah.
Alaska's landscape varies from a lush coastal forest to a treeless Arctic tundra, and it has complexes of mountains and ice fields that cover thousands of square miles. The climate ranges from brief cool summers and long dark winters in the Arctic to southern coastal areas where temperatures fall below zero only during the deepest winter months. Alaska has more square miles of land than people, and sixty percent of the population is located in the two urban areas of Anchorage and Fairbanks. Access to much of the State is by air or water, as the road network reaches only a small portion of the state.
Alaska is the westernmost extension of the North American Continent. Its east-west span covers a distance of over 2,000 miles and north-south a distance of over 1,100 miles. The state’s coastline is over 33,000 miles in length and is 50% longer than that of the conterminous U.S. In addition to the Aleutian Islands, hundreds of other islands, mostly underdeveloped, are found along the northern coast of the Gulf of Alaska, the Alaska Peninsula, and Bering Sea Coast.
Alaska contains 375 million acres of land and over three million lakes. There are twelve major rivers plus three major tributaries of the Yukon, all of which drain two thirds of the state's rivers. Four rivers, the Yukon, Alsek, Stikine, and Taku can be classed as major international rivers.
The two longest mountain ranges are the Brooks Range, which separates the Arctic region from the interior, and the Alaska and Aleutian Ranges, which extend westward along the Alaska Peninsula and the Aleutian Islands, and northward about 200 miles from the Alaska Peninsula and then eastward into Canada. Other shorter but important ranges are the Chugach Mountains, which form a rim to the central north Gulf of Alaska, and the Wrangell Mountains, which extend to the northeast of the Chugach Range and south of the Alaska Range. Both of these shorter ranges merge with the St. Elias Mountains, extending southeastward through Canada and across southeastern Alaska as the Coast Range. Numerous peaks in excess of 10,000 feet are found in all but the Brooks Range. The highest peak on the North American Continent, Mt. McKinley (20,320 feet above sea level), is also in Alaska. Several other mountains tower above 16,000 feet. Despite this, it is interesting to note that nearly all of the inhabited sections of the state are at 1,000 feet elevation or less.
Coastal waterways, a myriad of islands large and small, inlets, bays, and sounds make up Alaska's coastline. These, along with the Aleutian Islands, make Alaska a water-oriented state as evidenced by its fishing, tourist and cruise-ship industry.
Permafrost is a major factor in the geography of Alaska. It is defined as a layer of soil at variable depths beneath the surface of the earth in which the temperature has been below freezing continuously from a few to several thousands of years. It exists where summer heating fails to penetrate to the base of the layer of frozen ground. Permafrost covers most of the northern third of the state. Discontinuous or isolated patches also exist over the central portion in an overall area covering nearly a third of the state. No permafrost exists in the southcentral and southern coastal portions, including southeastern Alaska, the Alaska peninsula and the Aleutian Chain.
Earthquakes have long been a part of the earth's growing pains. However, most people are only dimly aware of the frequency and destructive power of an earthquake. As a matter of fact, the earth trembles sufficiently to produce a shock every thirty seconds on the average. Each year about a million quakes rock our planet. Of the earth's major earthquakes in just the past 75 years, the 1920 quake in Kansu, China, killed 180,000 people and 12 years later in 1932 at Kansu, another 70,000 lives were lost. More recently, the 1970 quake in Northern Peru killed 70,000. The Alaskan quake of March, 1964, severely damaged the cities of Anchorage, Seward, Valdez, Whittier, and Kodiak causing in excess of 500 million dollars in damages and loss of 115 lives.
One of the two major earthquake belts to girdle the earth is the ring of Fire, extending from Japan through the Aleutian Islands, the Alaska Range, and the western mountains of North America and South America. Earthquakes are not casual accidents but are part of the forces shaping the earth's features. Earthquakes are caused by the shifting of large plates of the Earth's crust, which are floating on the mantle of hot liquid rock which surrounds the hotter molten core of the Earth's center. As these plates move and shift together, they create great pressures on each other. As these pressures build, sooner or later, one or both of the plates let go, moves away from, or overrides the other plate. As the crust is squeezed together, and/or pulled apart, great tears, upheavals, bulges, etc. are created on the Earth's surface, along with a terrific shaking of the ground. Destruction can be great from an earthquake, to include the collapse of structures, bridges, freeways and roads, downed power lines, fire, flooding, uprooted trees, interruption of water services, runway and NAVAID destruction, etc.
A volcanic explosion is caused by exploding steam and gas and can be a most fearful natural outburst. Generally, these explosions are caused when fissures are opened by an impending eruption and ground water runs down and contacts the molten lava. The resulting steam can set off powerful explosions. Gas pressure up to six hundred times atmospheric pressure (9,000 PSI) hurls "bombs" out of volcanic craters at more than five hundred miles an hour. Many eruptions are preceded by numerous earthquakes. Heat is obviously involved in a volcano with hot gases, steam and molten rock which are all products of eruptions. The temperature of the earth increases with depth, and far below the surface there are sometimes great amounts of melted rock, or “magma,” at great temperatures and pressure.
Pressure tends to equalize, and magma follows the path of least resistance. If successful, it bursts forth as lava. Magma may also contain gas, as well as water or steam, in various quantities. The varying content of magma gives volcanoes their individual characteristics. In some cases, lava emerges from a volcano with little fuss and moves slowly over the surrounding country. Sometimes it bursts forth in an explosion, spewing rock, pumice, and ash high into the air. Great superheated clouds of gas may be ejected at tremendous pressure.
The most hazardous product of a volcano to aviation, other than the initial eruption, is the volcanic ash, called tephra. Tephra still in the air, or on the ground, causes major problems for aircraft. It is most abrasive in nature and can cause extensive damage to aircraft engines, pitting windscreens, sand-blasting aircraft surfaces, harming radio parts, landing gear parts, and air conditioning systems.
Active volcanoes in the Kenai area are Augustine, Mt. Redoubt, Mt. Iliamna, and Mt. Spur. Mount Redoubt ended a 24-year dormant period in December 1989 when it erupted violently, spewing ash over the Kenai Peninsula and as far away as Washington. A B747 approaching Anchorage from a polar route lost all four engines when it entered a cloud of ash, which the pilots saw but thought to be a common cloud. The engines were restarted but the aircraft incurred damage above $50 million(US).
Tsunamis do not just happen — they are caused. A tsunami is a traveling ocean wave that usually has its source in fairly deep water. These waves are believed to originate as vertically displaced columns of sea water, with submarine avalanches, displacement of the sea floor, and vibrations from volcanic and earthquake activity as the most likely causes. Their speed is controlled by water depth. As the waves enter shallower water speed diminishes but the wave height increases. Most often it is an earthquake that creates a tsunami by raising or lowering the ocean floor or by causing gigantic landslides beneath the sea. As the ocean floor drops away or rises up, great waves are generated which move for long distances until all the energy imparted to the water by the earthquake has been expended. Scientists are not positive about the exact mechanism of the generation of tsunamis, since they are produced miles below the surface of the sea, where it is difficult or impossible to know just what is happening. However, we do know much about what the waves do after they are generated. For one thing, they race at unbelievable speeds across the open sea. In such deep water a tsunami can travel six hundred miles an hour. Once formed, the waves behave much like those caused by a stone tossed into a lake, moving outward in concentric circles until they strike a shoreline or die out across a sufficiently large body of water. On the open sea a tsunami may measure fifty miles from wave crest to wave crest and perhaps less than two feet in height, so that it is generally not noticed by passing ships. Depending on the forces that create them, tsunamis have periods ranging from five to sixty minutes and longer. There are generally from three to five major oscillations, or waves, coming within an hour or two. The waves then taper off, although it may be several days before normal sea conditions prevail. Earthquake ground motion sometimes generates waves at inland bodies of water such as lakes.
The climate of Alaska is as varied as its landscape. Temperatures can drop to -70 degrees Fahrenheit during the long winter, and rise into the 90's during the warm interior summer. Annual precipitation can range from less than 10 inches to over 200 inches. Geographic and climatic influences combine to produce wide variations in Alaska’s weather. Alaska can be divided into six climatic/topographic regions for aviation weather purposes:
- Arctic Coast
North of the Brooks Range, it is characterized by cool summers and cold, dry winters with occasional prolonged periods of strong gusty winds. IFR conditions due to fog and stratus frequent the coast in the summer and early fall, with blowing snow in the winter.
- Interior Alaska
Located between the Brooks Range and the Alaska Range, where it is dry with cold winters and warm summers. Weather is generally VFR with light winds.
- Southeast Alaska and the North Gulf Coast
Here we find wet, cool summers and wet, mild winters with periods of moderate to strong wind. Winter snowfall can be locally very heavy. Frequent MVFR weather conditions occur.
- Cook Inlet and Susitna Valley
The home for a large number of Alaskans, which is encircled by some of the highest mountains in North America. Summers are mild and winters are cool with moderate snowfall. Wind is generally light to moderate. Frequent VFR weather conditions occur.
- Aleutian Islands and Alaska Peninsula
Cool, wet summers and winters often accompanied by high wind. Very strong storms move through this area. Occasional prolonged periods of IFR conditions occur both summer and winter. Frequent MVFR weather conditions occur.
- Bering Sea Coast and Southwestern Alaska
Characterized by cool summers and moderately cold winters with generally light precipitation. Occasional prolonged periods of strong, gusty wind occur during winter. weather conditions are generally VFR or MVFR.
7. Weather Hazards
Icing can occur anywhere in Alaska during any season. It is most common and severe along the higher terrain bordering the Gulf of Alaska, over the Alaskan Peninsula and in southeast Alaska. The relatively warm water of the North Pacific is a readily available source of moisture and the mountains provide a lifting mechanism necessary to produce preci- pitation and icing. Icing should always be considered a threat when flying in clouds or above the freezing level in visible moisture.
Mechanical turbulence, generated by strong low-level winds blowing across rugged terrain, occurs frequently in Alaska although mostly during the winter months. The Alaska-Aleutian Range and the mountains bordering the Gulf of Alaska are particularly prone to this type of turbulence; however, any range of hills or mountains which may seem insignificant on a terrain map can cause severe low-level turbulence under strong low-level wind conditions. During the warmer months, turbulence is usually limited to that associated with scattered thunderstorms, particularly in the interior of Alaska.
Much myth exists on whiteouts. The most popular is that a whiteout exists during a blizzard condition. This is totally incorrect. Strictly speaking, the whiteout is an optical effect rather than a meteorological phenomenon and it can not be forecast. Whiteouts can and do occur in any area of Alaska and numerous aircraft accidents have been attributed to them. The necessary conditions for occurrence of a whiteout are snow or ice covered terrain and a cloud layer of uniform thickness covering most of the sky. As the parallel rays of sun pass through the cloud layer, they are broken up and diffused striking the snow surfaces from many angles. This diffused light is reflected back to the uniform cloud base and then to the surface again. This process continues many times until all shadows are destroyed. Neither clouds nor horizon can be seen and sense of depth is lost. The pilot seems to be engulfed in a uniformly white glow. The whiteout is a phenomenon of polar regions and most often occurs when the sun is at an angle of about 20 degrees above the horizon. As an aircraft flies into an area where the necessary conditions exist, the onset of a whiteout is generally quite sudden. An immediate transition from visual to instrument flight must be made. Because of the lack of warning, the pilot flying over snow covered terrain under dull gray skies should be extremely alert to the possible occurrence of a whiteout. Precautions should be taken to recognize and avoid Alaska's special weather hazards.
8. Weather Products
- Aviation Weather Observations (METAR and SPECI)
Aviation weather observations are taken at a variety of locations and by various types of individuals. Observations a NWS and FAA stations are taken by certified Federal Government employees. Contract observations are taken by private individuals certified to take weather observations and paid by the National Weather Service on a per observation basis. Observations at SAWRS are taken by certified observers but are normally taken irregularly to serve the needs of an airline, air taxi operations, etc. At locations where special observations (SPECI) are not taken, the remark NOSPECI is appended to the report. AMOS observations often exclude certain parameters important to aviation, such as ceiling and visibility. Many automated observing locations (AWOS, ASOS, and AWSS) have been established around the state. Ceilings directly above an automated observation station and visibility, minus obscuring phenomena, in a single direction are included in AWOS observations. The same is true with ASOS and AWSS observations except obscuring phenomena are also reported. Military observations are taken by qualified military observers either certified by their own service or by the NWS.
- Terminal Aviation Forecasts (TAF)
A terminal forecast is simply a description of the aviation weather conditions expected to occur at an airport. Terminal forecast are issued four times daily for a 24-hour period. Terminal forecasts are issued only when routine aviation surface observations are available so some of the terminals may not be issued four times daily. They are amended whenever significant changes in the weather at the terminal occur, unless otherwise noted on the terminal.
- Winds and Temperatures Aloft Forecasts (FD)
Computer produced winds and temperatures aloft forecasts are prepared twice daily for the Alaska locations. Forecasts are issued for the following levels above mean sea level: 3, 6, 9, 12, 18, 24, 30, 34, and 39 thousand feet. The only exception to this is that no wind forecasts are issued for levels within 1500 feet of a station's elevation and no temperature forecasts are issued for the 3000 foot level within 2500 feet of the station's elevation.
- Area Forecasts (FA)
Aviation area forecasts are routinely issued for the entire state of Alaska, including coastal waters up to 100 miles off shore. Responsibility for issuance of the FA is with the Alaska Aviation Weather Unit (AAWU) in Anchorage. The FA is a 12-hour forecast in which details on expected large scale weather conditions are given. In addition, the FA includes an outlook in categorical terms for the 18 hour period following the valid time of the forecast. It is emphasized that the FA is a large scale forecast and therefore, may not include localized situations which may affect aircraft operations. This is particularly true in areas where weather observations are lacking. The FA is primarily intended as briefing guidance for NWS and FAA personnel, and to serve longer distance briefing requirements. The following table lists the areas covered by each area forecast.
Alaska Forecast Areas
- Cook Inlet & Susitna Valley
- Copper River Basin
- Central Gulf Coast
- Kodiak Island
- Kuskokwim Valley
- Yukon-Kuskokwim Delta
- Bristol Bay
- Alaska Peninsula(Port Heiden-Unimak Pass)
- Aleutian Islands from Unimak Pass to Adak
- Aleutian Islands from Adak to Attu Island
- Pribilof Islands & Southeast Bering Sea
- Arctic Slope Coastal
- Brooks Range
- Upper Yukon Valley
- Tanana Valley
- Koyukuk and Upper Kobuk Valley
- Lower Yukon Valley
- Northern Seward Peninsula & Lower Kobuk Valley
- St. Lawrence Island & Western Norton Sound
- Southern Seward Peninsula & Eastern Norton Sound
- Coastal Waters South of Cape Decision
- Southern Southeast Alaska
- Coastal Waters North of Cape Decision
- Central Southeast Alaska
- Lynn Canal-Glacier Bay
- Eastern Gulf Coast
Questions or Comments Welcome