Differences In Air Temperature
In locations such as Fairbanks, where protecting hills still winter winds, dramatic differences in air temperature develop at various ground elevations. Furthermore, at a single location, in midwinter, the passage of a few hours time can result in a substantial temperature change even though there is little diurnal variation in solar heating.
The cause of these differences and temporal changes is the development and decay of inversions in air temperature. About half the time, there is no inversion in a locality such as Fairbanks. Then, if there is no wind or change in weather, the vertical profile of air temperature shows a fall with increasing altitude at a rate of approximately 0.6°C per 100 meters (0.3°F per 100 feet). This is the so-called adiabatic lapse rate. An air column with this temperature distribution is stable, that is, there will be no tendency for air in the column to move up or down. Consequently, this is the temperature distribution that would pertain if there never were disturbing influences such as solar heating or wind. Under such conditions a house on a hill 500 feet above the valley floor where Fairbanks sits will have an outside air temperature a few cooler than exists in the city. Hillside residents around Fairbanks have, at certain times, seen such differences.
But when winter skies are clear and when there is no wind, a condition develops that upsets the 'normal' temperature profile. If there is no cloud cover to intervene, the ground surface is able to radiate heat energy directly to space, just as does a person's bare face when exposed to the sky. The ground cools, and the air in contact with it also cools, not by radiation but by conduction to the ground.
The consequence of this transfer of heat energy -- from the air to the ground by conduction and thence from the ground to space by radiation -- is a cold layer of air near the ground, air that is colder than the air above. Thus the 'normal' trend of decreasing temperature with increasing altitude is inverted, hence the name inversion.
As time goes on, the thickness of the inversion layer grows because the cold layer of air near the ground cools the air just above, again by conduction. Within the near-ground inversion layer, very steep gradients in temperature can develop, as much as 50°F per 100 feet (30°C per 100 m) although gradients near 15°F per 100 feet are more common.
Now, the person who built a house up on the hillside has the advantage. Instead of being colder at those times when the inversion was lacking, the air temperature becomes noticeably warmer as the inversion develops. One person who lives on a hillside, 1500 feet above Fairbanks reports that the air outside his house typically is about 30°F warmer than reported in the city, after the inversion has lasted several days. Those living at intermediate altitudes on hillsides surrounding northern cities which experience inversions observe lesser differences, depending upon how high they are above the valley floor and how long the inversion has lasted.
Minor air currents sometimes make the cold lower air slosh back and forth across the valley floor, much like water in a tipped basin. Then, hillside residents may notice rather rapid oscillations in air temperature, perhaps as much as 5 or 10°F in a few hours.
