Upslope Snow “Orographic Lifting”

2016-10-28 19:02:00.000 – Taylor Regan, Summit Intern

With the ongoing storm forecasted to drop upwards of a foot of snow on the higher summits, and heavy rain across much of the region, one question you might have is, why does the heaviest predicted precipitation often seem to be concentrated to one side of a mountain range? One reason is a phenomenon called orographic lift.

In general, orographic lifting is a process that occurs when low level (surface) winds are driven into an obstruction, such as a mountain range, and forced to rise up and over that obstruction. In doing so, the air expands, due to the decreasing pressure as elevation is increased. The expanding air cools, sometimes until the air temperature is equal to the dew point. At this point, the air is considered saturated and any additional ascent will cause supersaturation. Cold air can’t hold as much moisture as warm air, and as a result, moisture in the supersaturated air mass condenses into precipitation.

So, that’s orographic lifting in a nutshell, but how does this contribute to one side of a mountain range seeing more precipitation than the other? As the moving mass of air hits the windward side of the mountain, the cooled air is essentially squeezed of much of its moisture. This forced precipitation is called upslope precipitation, and, when the temperatures are cold enough, upslope snow. As the cold and significantly drier air descends the leeward side of the mountain, it begins to warm, increasing its ability to hold moisture, while simultaneously containing less moisture.

Imagine that you are hiking over the top of a mountain with a drinking cup that can change size. At the base of the mountain, the cup is maybe half full (or half empty, your call). As you climb the mountain, the cup shrinks, until the water completely fills the cup, but you still have further to go! Eventually, the cup has shrunk so much that some water spills out of the cup. If the cup were a parcel of air, this spillover would be upslope precipitation. As you descend the other side of the mountain, the cup gradually returns to its original size, but, because you lost some water on the way up, you are left with much less in the cup.

The White Mountains are uniquely situated perpendicular to the regional prevailing westerly winds, and with little in the way to break up the moving air mass, there are lots of opportunities to see upslope precipitation on the western slopes. However, upslope patterns (specifically upslope snow) are driven by three ingredients: moisture, low-level (surface) winds, and below freezing temps (for snow), and therefore, upslope patterns can develop on other faces of the mountains as well, as long as the “recipe” is met.

The storm currently raging over the summit and much of the Northeast is a good example of the effects of orographic lifting, the resulting upslope precipitation effect, and subsequent rain shadow. A strong onshore flow, depicted in the graphic below, is pushing a relatively moist air mass (having formed over the ocean), over the low-lying coastal region, with little to impede its motion until it reaches the White Mountain Region. The prevailing southeasterly winds over the duration of the storm create favorable upsloping conditions on the eastern side of the range, with the result being heavy precipitation along this edge of the mountain range.

To the northwest of the Whites, as depicted in the above graphic, the precipitation is much lighter. The once moisture-rich air, having cooled and moved over the mountains, descends in a much drier state, as evidenced by the lack of precipitation being picked up on the radar. This “rain shadow” is a direct result of the forced upslope precipitation on the eastern edge of the Whites.

Taylor Regan, Summit Intern

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