General Circulation Patterns
It would be impossible to understand Alaska’s weather patterns without some understanding of global weather patterns on the whole.  Masses of air do not flow around the earth in a completely random fashion.  There is quite a bit of order to the system and on the large scale, at least, it is not too complicated.  Heat from the sun is the principal cause of air currents, so it is natural to begin this discussion at the location where the Earth receives more heat than anywhere else, the equatorial regions.

The Equatorial Lows
Contrary to popular belief, the air is not heated by the sun directly, but rather by infrared radiation emanating from the surface of the planet .  As the sun beats down relentlessly upon the equator, it heats the earth.  The earth releases infrared radiation, which heats the air closest to the surface. Because warm air can hold more moisture than cool air, moisture is evaporated as the air warms.  Warmer air masses also have less mass than cooler air, so the warm, moisture-laden air begins to rise.   As the warm air rises, it begins to cool.  Because cool air can’t contain the same amount of moisture as warm air, some of the water vapor is released.  If a smaller amount of moisture is released, clouds form.  If a larger amount is released, it falls as rain or snow.

The other phenomena caused by the rising air is that lower pressure is created at the planet’s surface.  Normally, air is pulled towards the Earth by gravity, creating an average air pressure of 15 pound per square inch.  However, if air is rising instead of being pulled down, the air pressure over that area is decreased.  Because there is almost constant solar heating at the equator, a region of pressure known as the Equatorial Low bands the entire planet.  Because low pressure areas are rainy due to the rising and subsequent cooling of moist air, we find the world’s great tropical rain forests of South America, Africa and Indonesia underneath the Equatorial Low.

The Subtropical Highs
The equatorial air has now lost its moisture and has risen high into the atmosphere.  Eventually is becomes cool enough and dense enough to begin to sink back to Earth, usually at about 30? north and south latitudes.  As this dry air sinks, it gets closer to the Earth’s surface and begins to warm.  Because warmer air can hold more moisture than cool air, this sinking air mass begins to evaporate any atmospheric moisture it encounters.  For this reason, we tend to find deserts around the globe at approximately 30? north and south, under the Subtropical Highs. Most of the great deserts of the world, such as the Sonoran, Sahara and Kalahari, fall within this region.

Once the warm, dry air reaches the surface it can flow either polewards or equatorwards.  The air that flows equatorwards is being sucked towards the Equatorial Low as if by a worldwide vacuum cleaner.  This creates the trade winds, which blow from the east to the west towards the equator in the northern and southern hemispheres.

The Subpolar Lows
The air that flows polewards slowly warms up due to its proximity to the surface and rises. A pressure system known as the Subpolar Lows that is centered between 45? and 60? latitude.  This region is characterized by moderate rainfall and contains some of the best agricultural land in the world, such as the American Midwest, Northern Europe and Eastern China.

The Subtropical High air that is pulled polewards by the Subpolar Lows creates the westerly winds, which tend to blow from the west to the east.

The Polar Highs
Once the Subpolar Low air has risen and lost its moisture, it either flows back toward the Subtropical Highs or flows northwards, only to descend over the poles as very cold, very dry air.  The Polar High pressure regions expand and contract toward or away from the poles, depending on the season, but the extreme north and south latitudes are polar deserts, that receive very little precipitation.  The only reason there is snow and ice at these latitudes is because on the rare occasion when it does snow, the temperature is too cold for there to be any melting.

The Polar air flows towards the Subpolar Lows from the east to the west, creating the polar easterly winds.

The reason the trade winds, westerlies and polar easterlies don’t blow from the north to the south, but instead from the east to west or vice versa, is because of a phenomena known as the Coriolis Effect.  This is an effect created by the Earth’s rotation that causes moving object in the northern hemisphere to appear to turn to the right and moving objects in the southern hemisphere to appear to move to the left.

The Highs and Lows of Southcentral Alaska
Alaska, like any other place on the planet, is affected by the continual movement of the high and low pressure zones relative to each other.  It is the constant interplay between warm air and cool air, and dry air and moist air that brings us our weather.  The two primary zones that affect Alaska are the Subpolar Lows and the Polar Highs.

In the North Pacific, near the Aleutian Islands, warm water from East Asia arrives and comes into conflict with cold water coming from the Arctic Ocean.  This warm water heats the air above it, which rises above the cooler air that arrives with the cold Arctic water.  This rising air is known as the Aleutian Low, a semi permanent low pressure system that brings near constant drizzle to the Aleutian Islands.  The westerlies push the Aleutian Low to the east, bringing major storm systems to southern Alaska as well as the rest of North America.

High pressure dominates the interiors of Alaska and Canada well into the springtime.  This is due to the fact that drier air over the continental interiors heats and cools much more rapidly than moist air near the ocean. For this reason, coastal areas have more moderate summers and winters while inland areas experience very hot summers and very cold winters. In the winter, the Polar Highs often reach as far south as southcentral Alaska, bringing cold, clear weather.  But the high pressure system often retreats, allowing the slightly warmer, snowy weather, that is associated with the Aleutian Low into the region.   Still, the driest months in Southcentral Alaska are April and May, due to the fact that 1) the high pressure region has grown over the winter and forces the lower pressure air masses to the south, away from most of the state and 2) the ocean waters are coolest this time of the year, therefore little water is evaporated from which precipitation can develop.

While the Aleutian Low is present most of the year, it tends to dissipate during the late summer months.  It is replaced by the northern edge of the Subtropical High that dominates the North Pacific, the Hawaiian High.  While this can bring beautiful, hot days to southcentral Alaska, the second half of summer tends to be rainier because 1) the high pressure has completely disappeared from the interior, allowing moist ocean air to flow eastwards across the state and 2) the ocean water itself is warmest in the early fall, and is easily evaporated, allowing more moisture in the air for precipitation.