We reach the heart of Fall this month, starting with displays of orange and yellow sugar maples, yellows and purples of ash and birch, and scarlets from sumac and swamp maples during the first half of the month, transitioning to the snow-frosted mountain tops, and fading yellow-brown valleys through the last two weeks.
We may see our first snowflakes at some point, often just snow in the air, or a dusting in the valleys, while a light coating is not uncommon over the higher terrain. Average temperatures gradually decline, starting with highs in the 60s in the valleys, near 60 over the hills, while nights fall to the 40s, with 30s becoming more common, and frosts increase in frequency. By Halloween, a typical afternoon finds readings in the low 50s in the valleys, upper 40s across higher elevations, and nights, as often as not, reaching the freezing point.
A transition toward more and more clouds, one of the primary characteristics of October, does not indicate more storms necessarily. October averages slightly less precipitation than either September or November. The colder air tracking across the warmer waters of the Great Lakes, as well as the thousands of smaller bodies of water, transfer their moisture into the atmosphere, fertilizing the clouds so that they grow profusely, decreasing the already failing daylight hours.
Another feature of mid-Autumn weather shows up in the list of daily record high and low temperatures (See chart). If you look down the list, notice that there are several instances of records on consecutive days. In 1947, and again in 1972 and 1979, there are three-day stretches of these record extremes in temperatures. The warm or cold weather patterns that brought these events did not just come and go – they persisted. From October into mid-November, the weather exhibits a tendency to stagnate, particularly with regard to regions of high atmospheric pressure. Areas of higher pressure represent domes of air that acquire characteristics of the region over which they form, like the Arctic, the middle of the continent, or the tropics. The air tends to sink, which keeps skies mainly clear, while winds remain relatively light. During the day, this maximizes the amount of sunshine, and the light winds permit the warming process to continue as long as possible. If this air arrived from warmer locations to our south, then the temperature rise could reach their warmest potential, resulting in record warmth. Conversely, should the same clear skies and light winds settle over us during the night, the surface of the Earth radiates its warmth out through the atmosphere, resulting in a cooling process that lasts until sunrise. Should this air have origins to our north, the already chilled air could keep cooling to the point of setting a record minimum temperature.
Such persistent weather patterns don’t necessarily produce records. In fact, another element to notice on the list of records, there have been no record maximum or minimum temperatures since the mid-1990s. With a record as long as the Fairbanks Museum’s 125 years, the instances of record highs and lows decreases.
One factor that plays into the more frequent occurrence of stagnant weather patterns in October and November comes from the seasonal weather patterns over the Pacific Ocean, and the physical structure of the Earth’s surface in the northern hemisphere. (See Figure 2) Much like the Atlantic Basin, the northern Pacific Ocean experiences an annual period of tropical cyclone activity, concentrated from late summer through the middle of autumn, when ocean temperatures reach their maximum. If the Pacific season is particularly active, it can drive a considerable amount of warmer air north. In addition, should the Pacific Ocean temperatures west of Canada be warmer than average, it encourages a region of higher pressure to form over the eastern Pacific. This can be favorably connected with colder air developing over the higher, mountainous terrain of the western U.S. and western Canada. If a few to several storms establish an early snow-pack, this helps to maintain cooler air over the western US during the autumn. The atmosphere reacts, attempting to balance this out, by strengthening the semi-permanent high-pressure area over the sub-tropical North Atlantic, known as the Bermuda High, or the Bermuda-Azore High, to expand west into the east-central and southeast US. Such a position favors a relatively warm southwesterly airflow through the eastern two-thirds of the U.S., creating what is sometimes called “Indian Summer.” A map of the continental US shows the average air pressure to be higher through the southeastern US during October, (figure 3) the result of more frequent areas of high pressure there.
If these patterns come about from the physical surface of the Earth, and that surface doesn’t change, then wouldn’t each autumn be about the same, weather-wise? Obviously, that doesn’t happen, or we would not have the variety of weather patterns we see. If the Pacific tropical season produces fewer tropical cyclones, or the Pacific water temperatures are cooler, rather than warmer, it causes higher pressure over the eastern Pacific to weaken, or form in a different, more favorable locations. Atlantic hurricane activity can influence the Bermuda High, changing its influence over the eastern US. This natural variability ensures plenty of variety in our weather, not just in October, but throughout the year.