High Pressure and Air Quality

2011-07-31 17:25:01.000 – Rick Giard,  Observer / Educational Specialist

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Generally speaking, a ridge of high pressure building into the region is a harbinger of fair weather. True to form, today we have absolutely gorgeous conditions for alpine activity – mostly sunny, light winds, low humidity and good visibility. Compared with the previous two days of fog, rain showers and gusty winds above tree line, this is Hiker’s Paradise.

However, frequently when high pressure is in control for a period of time, the air quality in terms of pollutants can actually become worse than that typically seen during low pressure and stormy weather. A trained meteorologist stationed atop a prominent peak more than a mile above sea level can observe details that reveal subtle clues about the state of the atmosphere. These are data that no automated weather station can interpret.

Looking at this picture of today’s first morning Cog Railway train chugging to the summit, there are several significant visual observations one can make. First, notice that the coal smoke plume is not rising vertically, but rather moving horizontally and staying intact. This indicates sinking air and slow wind speeds, neither of which will dissipate pollutants effectively. Second, the region is nearly cloudless, for when air sinks it warms and dries. Third, scanning the horizon we see a distinct straight line separating hazy air below from clearer air above. This demonstrates a ‘capping inversion’, a layer of relatively warm air aloft where rising air tends to halt. This further reduces the available air that would otherwise help to dilute pollutant concentrations.

Fortunately, northern New England is situated where several major storm tracks cross. So, these calm conditions usually do not persist very long before the next weather disturbance stirs things up again.

Rick Giard,  Observer / Educational Specialist