The Orographic Effect: When the snowstorm fails to come, it’s because mountains write their own weather forecasts

Well, this winter, a couple of times they got it right.

Although in both of the storms that hit Albany in late January and the first week of February the Weather Channel and other local channels were at first a bit uncertain about the amount of snow Albany was going to get, they all were calling for  “significant” amounts of snow well in advance of the first flakes.

That term is, of course, nicely ambiguous.  If you have to brush off your car, that’s “significant.” It’s also “significant” if you have to shovel a path to get to that car.  And, when schools start closing down and commuters are urged to get an early start and the forecasters start calling for “six to twelve inches or more of snow” — well, that is “significant” in anyone’s estimation.  It’s a term that covers every forecaster’s tail.

And, here in Albany, the fluffy stuff piled up generally in excess of 11 inches in both storms with a good bit more in the higher elevations.  All those kids who gambled on snow days by not doing their homework got extensions and all their teachers got to sleep in.

When I was teaching high school Earth Science, my snow-day mornings consisted of rising late, making French toast, and having that extra cup of coffee while gazing out at the falling snow and thinking of how great the skiing was going to be on the coming weekend and marveling that the forecasters had gotten it right.

But, of course, how many times have we Albanians seen the forecast go awry?  The weather maps show a menacing-looking front advancing from the west or a sinister mass of counter-clockwise swirling clouds lurking in the Gulf of Mexico and poised for a run at the Northeast like a cougar.

The alarms are sounded, people rush to supermarkets to load up on milk, bread, and toilet paper in anticipation of a recurrence of the Great Blizzard of 1888 (and apparently believing that the city snow-removal systems are also rooted firmly in the 19th Century) — and then the storm arrives and delivers a scant inch or two, or a dusting, or nothing at all.

And the folks who do the TV weather come on looking embarrassed and utter some variation on, “Well, folks, this is what we thought would happen but”—pause for a giggle—“darned if that old storm just didn’t deliver.”

Albany area weather

is tough to predict

But, at least in the Albany area, when the forecast for a snowstorm fizzles, we need to cut the meteorologists some slack, for scientists who study the weather will tell you that the Albany area is one of the most difficult places in the contiguous 48 states for which to forecast the weather.

And much of the blame can be laid upon our geography, resulting in a phenomenon known as the “Orographic Effect.”  The term — as with many scientific terms — comes to us from the Greek:  “oros” meaning “mountain,” and “graphein” meaning “to write.”

Without going into the evolution of the term, suffice it to say that it is meant to convey the concept that “mountains write their own weather forecasts.”

Anyone with a high school student’s understanding of science is aware that the temperature of the Earth’s atmosphere decreases with elevation above the surface.

This is why, on a summer day when the temperature at ground level may be in the 90s, the high, wispy cirrus clouds that frequently appear in the sky, heralding a change in the weather are made of ice crystals: They may form at elevations of five or six miles or higher where the temperature hovers at around 85 degrees below zero. This is also why visitors to the big island of Hawaii are astounded to hear of snowboarders racing down the slopes of the great volcano known as Mauna Kea with its summit approaching 14,000 feet above sea level.

Once you get more than a couple of miles up, it’s very, very cold.

This simple fact explains why a mountain may get snow when the surrounding countryside gets rain; it also explains why higher elevations get greater amounts of snow than the lower elevations during a storm: The colder temperatures form lighter, fluffier snow that tends to accumulate to greater depths.  So Albany gets 14 inches of snow and Berne gets 24. Q.E.D.

But, as it happens, the Orographic Effect is far more complicated.

To begin with, it is colder at high elevations than it is at lower ones because the air pressure on a plateau or a mountaintop is lower than it is at sea level, and it drops off dramatically with increasing elevation.  There are simply fewer air molecules to bump together and produce heat by friction.

Anyone who has ever experienced “altitude sickness” driving over the Rockies or skiing on them understands this: It is harder to breathe at that elevation because the lower density means lower amounts of oxygen with each breath, and, for some people, this can cause nausea and headaches.

Of course, for mountain climbers, the region above 24,000 feet on a mountain such as Everest is known as “the death zone” because no one can long survive on the pitiful amounts of oxygen that remain at that altitude.  Our ears pop when we ride an elevator or drive rapidly through changing elevations as the air within our heads adjusts to the change in ambient pressure around us.

Now another concept becomes important in understanding the Orographic Effect, and this is the relationship between what is called the dew point and the temperature of the air mass around it.

The dew point is the temperature at which a given mass of air would become saturated — that is, have a relative humidity of 100 percent — allowing condensation and perhaps precipitation to begin.

It is determined by a number of factors, chief among them the amount of water vapor a mass of air is carrying. The closer the air temperature is to the dew point, the greater the likelihood of condensation followed by precipitation; when they are equal, these results are all but certain.

Now envision a mass of air with a temperature of 28 degrees F and a dew point of 20 degrees F that is moving toward a mountain or mountain range.  The mountain or the range represents an obstacle to that movement: A single mountain will cause a portion of that air to rise and, as it does, both its temperature and dew point will drop with increasing elevation, but the temperature drops faster than the dew point.

The two numbers soon coincide, and voila!  The mountaintop experiences precipitation — which, given these temperatures, will likely be in the form of snow.  At temperatures above 32F, the mountaintop may be capped in fog or experience rainfall.

The accompanying photograph was taken from a high slope of Cadillac Mountain in Acadia National Park in Maine on a humid summer morning.  It shows the islands of Frenchman Bay capped in morning fog while the area around the islands remains clear.

The wind is carrying the moisture-laden air just high enough so that the dew point and air temperature have met and the air has become saturated on both the wind-facing or “windward” slope of the islands and their tops.  Fog forms.

Adiabatic warming

But air that has been forced upward by increasing elevation continues its forward movement and eventually begins to descend.  Now another phenomenon known as “adiabatic heating” comes into play for, as the air moves into elevations of decreasing altitude, both the ambient air pressure and temperature increase, moving the air temperature and the dew point farther apart. The result is a drier, warmer air mass on what is known as the “leeward” slope of a mountain.

Now — take a look at a topographic map of eastern central New York State: the city of  Albany is surrounded by mountains and plateaus.  To the east rise the Berkshires and to the west looms the Appalachian Plateau, locally called “the Helderbergs.”

South of Albany are the heights known as “the Catskills,” which, to just about everyone, sure look like mountains, but are described by geologists as the steep eroded remnants of an ancient plateau.  No matter:  the Catskills are high, exceeding 4,000 feet in a couple of places.  (Of course, north of us are the great Adirondacks, but storms seldom if ever approach the Albany area from due north.)

However — storms at any season commonly approach us from the south, southwest, or east and to reach us they must rise to great heights as they pass over mountain range or plateau — and then dive into the Hudson Valley — a textbook demonstration of the Orographic Effect.

And so, a winter storm approaches from the southwest and Rensselaerville gets 26 inches of snow while Albany gets 10;  a snowstorm moving toward us from the south brings a foot of snow to the ski areas of the Catskills but two inches falls in Albany; a huge “Nor’easter” roars up the coast, dropping a foot and a half of snow on Worcester and Pittsfield, both on the windward side of the Berkshires — and Albany on the leeward side gets two inches of wet snow, or one inch, or rain.

And sometimes the adiabatic warming effect can be sufficient to cause Albany to get nothing at all.

All of this is, of course, a very simplistic explanation of the factors that affect the weather in Albany, and there are many other variables.

But the Orographic Effect neatly explains why the cities of Denver and Colorado Springs have desert climates while a few scant miles to their west the high, thickly forested slopes and valleys of the Rockies may lie under many feet of snow; it accounts for the fact that Seattle is notorious for rain while to its east —  beyond the down-slope of the Cascade Mountains — Spokane’s climate is arid.  And it is why Keene Valley and the western shore of Lake Champlain may have little or no snow when Lake Placid and the High Peaks of the Adirondacks may be buried in the snows of deep winter borne by winds from the west.

And it also may be the reason for the discomfort of your local meteorologist — forehead perspiration easily visible in HD — who opens a weather forecast with a nervous smile and begins, “Well, everybody, here’s what those computer models said was going to happen….”

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