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If you're freezing, hope for a downslope wind

For the 33-plus years I’ve been in Buffalo, I’ve been talking about about downslope winds, maybe even to the extent of ad nauseum.

We currently are at the start of a March cold snap. For those of you who hold the cold in disfavor, downslope is precisely the kind of wind you need – but won’t get – for the next week or so. A downslope wind is one in which air flowing down a slope from a higher elevation to a lower elevation accelerates, compresses, dries out and heats up as it descends.

We don’t have a special name for our downslope winds – coming down the north slopes of Southern Tier hills to the lake plain – but there are many names for the more powerful downslope winds that occur elsewhere where topography has more variation. In North America, the best-known downslope winds are the Chinook winds that often race down the east slopes of the Rockies out into the high plains of eastern Colorado, Wyoming, western Nebraska, Kansas and the Dakotas. One common scenario (not the only one for downsloping): Moist Pacific air ascends the windward/west side of the Cascades and the Rockies, where as its rising motion cools it, the moisture condenses and is mostly wrung out before it begins to accelerate downward on the leeward/east side of the slopes. The cool, dried air on the ridges is of greater density. The force of gravity on these steeper slopes allows this heavy air to speed up as it rolls down the slopes to the east, sometimes to damaging velocities.

Here is a brief example from the University Corporation for Atmospheric Research (a leading source for continuing education for meteorologists) of an often-targeted city, Boulder: http://www.ucar.edu/communications/factsheets/winds.html

The heating that is produced from more powerful Chinook winds can be mind-blowing (no pun intended). Chinooks are even more prevalent east of the Canadian Rockies, in Alberta. On Nov. 19, 1962, a 106-mph Chinook took the temperature in Pincher Creek, Alberta, from minus-2 to 72 in one hour. So much for our slogan “If you don’t like Western New York weather, wait five minutes.” That slogan, by the way, is said over the majority of the country, each region’s people thinking THEY originated it. (I’ve heard it in Bangor, Wichita and Detroit.) But it’s more deserved in the high plains than in Western New York.

Buffalo’s hottest days are nearly always accompanied by a downslope flow from the southeast or south, with its heating and drying. Our models often underestimate downslope flow and its heating. If those winds become strong, we can really bust high temperature forecasts on the lake plain when we get this “runaway” downslope effect. We are much more unlikely to experience any of our rare days in the upper 90s with a lake breeze. Moist air may feel more uncomfortable, but its actual temperature is modified by the marine layer. That is the reason Buffalo and, yes, Tampa and Miami Beach have never reached 100 degrees. Our only chance to ever do that is with a downslope flow, at least in the near future. A warming climate may someday change that, but since it’s never reached 100 in Tampa, I’d say that’s fairly unlikely.

There are variations within the types of downslope winds studied by meteorologists. One type is called katabatic. The air at a high-elevation ridge may radiationally cool at night, with its density increasing, and then be carried by gravity as it drains down the slopes. Santa Ana winds in Southern California are generally hot and dry by the time they descend to lower elevations, and can make terrible contributions to the wildfire threat during dry periods. The most common locations for katabatic winds are off the elevated ice fields of interior Greenland and Antarctica. Unlike the hot Santa Ana winds, the katabatic winds off this icy topography remain intensely cold.

Speaking of cold, a recent article by Dr. Marshall Shepherd, chairman of Atmospheric Sciences at the University of Georgia and past president of the American Meterological Society, has discussed the downslope wind in the Antarctic region. The annual mean temperature at the South Pole is minus-76 degrees. Because this vast landmass has major topographical features, there are numerous places in which a downslope wind can develop and produce amazing-for-Antarctica temperature variations. The highest temperature for the continent occurred at the tip of the Antarctic Peninsula, at an Argentine research station. A sharp downslope wind (similar to a Chinook wind) brought a reading of 63.5  degrees in March 2015 at this low elevation location.

Image courtesy of UCAR/Comet Modules

As a measure of the power of downsloping, the warmest temperature on the Antarctic continent at altitudes above 2500 feet and largely out of reach of downsloping is 19.4 degrees. That’s quite a comparison.

Around here, a strong southerly downslope component can also delay the onset of precipitation with an approaching area of low pressure by creating a dry layer of heated air that has been compressed. It takes longer for the lower atmosphere to become saturated in a downslope drying wind. That drying can also lessen the total amount of precipitation falling during the passage of a storm system due to that delay. Even with our very unspectacular topography compared to western North America, the downslope wind on a southeast trajectory can sometimes bring damaging gusts to the Chautauqua County shoreline after descending the highlands known as the Chautauqua Ridge.

At the point of origin for our downslope winds, the Southern Tier, there is little impact from these winds until they have left the Southern Tier and flowed down to the lower elevations. Sometimes such a wind brings a high of 60 to Buffalo while readings remain in the 40s on the hills and in the valleys to the south.

So, if you like warmth, “heading downhill” is not a negative in the case of the downslope wind.

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