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The Impact of the Wind

Certainly all Mount Washington enthusiasts are familiar with the concept popularly referred to as "Wind Chill." Whenever the temperature drops and the wind speed rises, the observer on duty at the Observatory expects a spate of calls asking, "What is the wind chill factor?" While such a question may send an uncomfortable tingle up and down a meteorological technician's spine, it does touch upon one of the more pronounced effects of the wind on Mount Washington, but it also ignores other impacts of the wind that can be even more significant.

Wind Chill

photo - see caption below
Rock Cairn

The concept of "wind chill," as well entrenched as it is in the popular media and the popular psyche, is not without controversy among meteorological professionals. No one contests that, at least within certain bounds, accelerated air movement will lead to a greater rate of heat loss from a body until that body reaches the ambient temperature. Exposed flesh will feel colder at zero degrees with a fifty mile per hour wind than it does at zero degrees in calm conditions, other things being equal.

Edwin Kessler of the University of Oklahoma, writing in The Bulletin of the American Meteorological Society, noted several flaws in the use of the "wind chill index" at that time such use failed to allow for the difference between official wind speeds, normally measured at 10 meters above the ground, and ground level winds which individuals are actually exposed to; it also failed to allow for the warming effects of solar radiation. Kessler also noted that the original and limited data generated by Paul Siple dealt with the cooling of water in cylinders, not the cooling of human subjects. Furthermore, limited experimental data suggested that demonstrable errors of almost 20 degrees may be embedded in the usual wind chill charts.

Kessler was critical of the use of wind chill as it placed a degree of precision on a matter which is terribly imprecise. He summed up that "our use of the wind chill index entails all of attributions of substance to mere form, addiction to hyperbole, and passion for technological glitter." He also criticized "the contribution present procedures make to portrayal of nature as mankind's enemy." Some of Kessler's criticisms may have been addressed by the 2001 revision of the wind chill calculation and chart; but others, especially the last criticism regarding the characterization of the elements as man's enemy, have not been.

On Mount Washington, we may, perhaps, be forgiven for portraying nature as a potential enemy of the mountain visitor. When we use such terms as "a hostile environment", we speak from our experience. We know that, for those who venture into challenging conditions ill prepared -- and sometimes, for any one who ventures into conditions which are too challenging -- exposure to the mountain's weather, and especially its fierce winds, can be damaging or deadly.

Curiously, it is the "wind chill" -- that effect of the wind which is most ballyhooed -- that we find the least difficult to deal with in our day-to-day work at the Observatory. We follow the rule of "no exposed flesh" when our readings of thermometer and anemometer indicate frigid conditions. Pants are secured to boots with gaiters or taut cuffs, parka is well cinched around waist, and gauntlet cuffs of mittens extend well up our sleeves, keeping the teasing or tearing wind at bay. Most important, and most difficult, is preventing the cold from nipping us about our faces, while still maintaining the ability to see and breathe (talking is secondary, and eating and drinking, as important as they are, are best reserved for more sheltered locations - not a great concern for a weather observer who will return to the safety of a building in an hour or so, though a real consideration for a hiker or climber). Perhaps no observer has yet found the perfect combination of hat, goggles, scarf, face mask, and hood which provides easy breathing and maintains excellent visibility in cold, windy conditions with fog, rime ice, and blowing snow (especially with eyeglasses on). Most have learned, either from their own experience or from that of others, that in brutal conditions every square millimeter of flesh must be protected, or else the tell-tale blanch of frostbite will quickly appear.

In spite of the challenge of maintaining body temperature in frigid wind chill equivalent temperatures, we have learned that it is possible to remain comfortable -- well, at least relatively comfortable -- in such conditions. We know, too, that it is one thing to survive; it's another thing to do productive work in such conditions. Efficiency drops drastically when one is moving many pounds of extra clothing with every step, when digital dexterity is limited by wearing multiple layers of insulation on one's hands, and when only the briefest exposure of unprotected fingers can be budgeted, complemented by a lengthy period of re-warming in preparation for the next rapid uncovering.

By exercising due caution, though, we can still comply with the assertion that Observatory staff member Sal Pagliuca made in the first issue of the Observatory's news bulletin in 1937. On January 29, 1934, the temperature dipped to -46.5 degrees F., and the wind gusted to 100 miles per hour. "Under these conditions," wrote Sal, "some of the observers, equipped with windproof clothing, performed outdoor duties for nearly one hour without experiencing any physical discomfort."

"But how about when the wind chill is "off the chart?," you might ask. And indeed, one of our frustrations is that the normal wind chill chart stops at 60 miles per hour -- hardly sporting for Mount Washington in mid-winter, when that figure is only slightly above our average winds. When the wind chill equivalent is colder than 50 below zero Fahrenheit, it usually is because of very rapid winds, and then another impact of the wind is felt, one that clothing is no proof against.

The Brute Force of the Wind

Winds over a certain speed -- varying with the subject individual's experience and strength, but for a figure let's take 50 miles per hour -- bring with them such sheer power that their impact on a puny human is remarkable. Limited exposure in benign conditions can be exhilarating. Sudden exposure for the unprepared tyro can be frightening. Lengthy exposure can be tiring; too long an exposure can be deadly.

The sheer physical force of the wind is one which is often grossly underestimated. Summer visitors on a breezy day occasionally ask if our anemometer is broken -- since they are being pushed back and forth by the wind, surely it must be 80 or 100 miles per hour, and not the measly 50 that the anemometer reads? No, the anemometer is correct, and the inexperienced should learn thereby that a "mere" 50 miles per hour packs a wicked punch.

But if there's that much force in a 50 mile per hour wind, how strong is a 100 mile per hour wind? We might presume twice as strong, but here is where this invisible challenger packs a surprise. Recalling the basic formula from physics, F = ma , we realize that the force of the wind, as compared to its speed, roughly speaking, increases exponentially. We're oversimplifying it a bit, but for rough-and-ready field purposes, we can estimate that if one wind is two times the speed of another, then its force is two squared, or four times, as great as the force of the lesser wind. A wind ten times as fast as another is ten squared, or one hundred, times more forceful than the lesser wind. Thus a one hundred mile per hour wind is four times as forceful as a fifty mile per hour wind. A fifty mile per hour wind is enough to make headway difficult; a one hundred mile per hour wind will require a hiker to stop in his or her tracks, to struggle, aided by a ski pole or ice axe, to maintain balance and to stay in one place. Progress is virtually impossible.

When one is planning a winter visit to Mount Washington, the actual experienced winds will differ from the observed winds. The Observatory reports (and the National Weather Service forecasts) wind speed as measured atop our instrument tower. In all but exceptional circumstances these winds will be higher than any experienced elsewhere in the Presidential Range. Atop the 6,288 foot pinnacle of the Range, and away from most of the effects of surface friction, the winds will be very high. Lower on the mountain, and within five to seven feet of the ground, the winds will normally be significantly less. It is conceivable that a few areas above treeline could experience higher winds if the peculiarities of atmospheric air flow, the wind direction and local topographic features dictate. Such occurrences are probably rare; if a winter climber reports 125 mile per hour winds in Edmands Col and the Observatory records a peak gust of 48 miles per hour, then the climber's estimate is somewhat suspect.

Though winds, as actually experienced, will most likely be less than the forecasted and tower-observed winds, there is a factor which is usually not given sufficient notice, and that is the gustiness of high winds. On Mount Washington, one is tempted to make the unqualified statement that high winds are always gusty winds. Probably no observer has witnessed truly steady 80, or 90, or 100 mile per hour winds. Such winds vary -- 70, 75, 65, 60, 80, 85, 75, and so on sometimes for hours and hours, remaining high but varying in speed. One might imagine that one could become accustomed to a strong but steady wind, but the buffeting effects of typical high Mount Washington winds are annoying, tiring, aggravating, exhausting, maddening. There are the gusts which whip your zipper pulls in a staccato on your cold lips, there are the gusts which whip bits of frozen ice crust into your nostrils, there are the gusts which blow your breath back onto your goggles, leaving the blinding tracery of frostwork, there are the gusts which catch you in mid-step and leave you lying amidst the icy rocks, giving you but a moment before the next assault.

Such forceful gustiness can wear out the strongest hiker and send him or her retreating back down to treeline, even on a day when the air is crystal and the horizon seems to go on forever, when the temperature -- and even the wind-chill -- would seem to be within reasonable limits. Such gustiness wears out the body and the spirit. Even the fit and experienced are taxed physically by blow after blow; the prudent hiker is worried down, always asking, "Will the next gust get me off balance and send me sprawling? Will my crampon catch and will my ankle be broken? What if I broke my leg here in these conditions....?" The Wind's Blinding Fury

Though wind can chill, and wind can bully, surely modern technology can prove more than a match in some fashion? Though a hiker's abilities can be surpassed by the cold and force of the wind, can't the summit weather observers, in their snow-tractor trips up and down the mountain, travel comfortably in any conditions? The answer is no, for the wind has other tricks up its gossamer sleeve.

No matter how brutally low the wind-chill, no matter how potent the wind, we have found, in more than 60 years experience on Mount Washington, that if we exercise proper planning and due diligence, we can remain safe and secure at the Observatory. That safety and security can end as soon as we leave the door if the combined forces of wind, snow, and fog gang up on us. Safety in one place on the mountain is one matter. Safe travel is another, and that depends, perhaps more than on any other factor, on the ability to always see where we are and to always see where we are going. To travel in marginal visibility requires constant attentiveness and caution, and part of that caution is to recognize when the margin is passed and travel becomes, not merely challenging, but unacceptably risky.

What causes poor visibility? One factor is darkness. In clear -- confidently, stably clear -- conditions, travel at night can be quite reasonable. A hiker by headlamp, or a snow tractor driver by headlights, can readily travel up or down the mountain. Inject some doubt into that clearness -- a passing cloud, or a transient snow squall -- and a straightforward trip could be dangerously lengthened, or could even become a struggle for survival.

Darkness is often such a limiting factor in typical Mount Washington weather that a regular rule of thumb is to be below treeline by dark. The typical conditions -- with fog, snow, and most of all significant winds -- make travel above treeline in darkness questionable at best, and foolhardy at worst.

Fog, on Mount Washington, is present about 60% of the time. Not all fogs are equal. A dense summer fog may limit visibility to less than 20 feet, making even summer travel on the unmistakable Auto Road a slow and painstaking process. A less dense winter fog may allow 200 feet of visibility, which by our standards is not too bad. In some respects a dark daylight fog is better; if a thin cloudcap shrouds the summit, allowing substantial sunlight to penetrate, the combined effect of fog and light can remove contrast and, in extreme conditions, can be practically blinding.

Visibility is also regularly diminished on the mountain by falling snow. We don't commonly have snowfall from clouds which remain above us; usually those clouds engulf us, too, so fog and snow often combine forces. Paradoxically, though, the fall of new snow sometimes seems to improve visibility, though only marginally, as if the snow crystals effectively scour some of the fog droplets from the air.

The experience of being on Mount Washington in falling snow and calm air is a rare and enjoyable one. To walk about the summit as the snow falls in a heavy blanket of uniform depth, to trudge through snow everywhere on the summit, to see the snow fall vertically, is something of a novelty. It is a novelty, too, that can bear with it a sort of hidden dread, as a voice inside says, "Watch out -- this won't last for long."

Such conditions don't last for long, for almost inevitably that calm air gives way to gale force or even hurricane force winds. The snow which seemed so innocent is now whipped into a frenzy by the wind. Areas exposed to the prevailing wind may be scoured down to more resistant old snow, or even to bare ground; leeward areas grow ever deeper drifts. While some briefly hidden terrain features are exposed, others are buried deep by the deposited snow. The air becomes thick with blowing snow, reducing visibility to near zero. The change from calm to gale can come in a matter of moments; the transition from reasonable traveling to impossible traveling can come as quickly.

Even after a lengthy period with strong winds, blowing snow can remain a hidden hazard. Though a wind from one direction may have done its work to the utmost, sweeping off all snow within its reach, a change in the wind's direction can move snow that was once in sheltered areas back into the air.

For the observers' travels up and down the Auto Road, neither cold, nor wind alone, is normally the limiting factor. However, if the upper reaches of the mountain are covered with fog, if there have been recent snows, and if the wind is doing its usual job, limited visibility may make it impossible to safely navigate the Auto Road. Safe travel on the Road demands staying on the Road, as, in many locations on the Road's upper half, only a few inches separate the Road from steep drops into the Great Gulf. Maintaining that close margin can be exceptionally demanding at times when most terrain features are covered with snow, ice, or rime, making surface contrast minimal, and when dense fog, falling snow, and wind-whipped blowing snow fill the air.

In such conditions, some of the advantages of the snow tractor show their shortcomings. The windshield protects the driver from bitter blasts; it also can ice up, limiting a clear view of the already very limited landscape. The plow blade, which serves so well in carving a platform through deep wind-sculpted drifts, liberates more snow to blow into the air, sometimes sending volleys of snow across the windshield itself. There are times when a tractor trip is made at the limit of possibility, and when a passenger is asked to walk a few feet ahead of the tractor to mark the edge of the road for the operator. That few feet may be the absolute limit of visibility, and the participants in the enterprise may have the uneasy feeling that even that minimal distance could be deprived them if true "white out" conditions were to develop.

Hikers and climbers, of course, have essentially the same limitations of their visibility, with the added complication - and danger - of being far more exposed to the force and chill of the environment than are travelers in a snow tractor. Cairn-to-cairn travel in typically savage winter conditions demands physical fitness, excellent preparation, warm and windproof clothing, knowledge of the terrain, excellent map and compass skills, good teamwork, and a recognition that sometimes the most prudent course is back down to treeline.

In sum, the "wind chill" on Mount Washington -- as impressive as the low numbers may be, as significant a hazard as the rapid rate of heat loss truly is -- is only one of the impacts of the wind, and perhaps the one least difficult to deal with. Visitors to the mountain also need to be aware of the epic force of the wind, and of the havoc wind can wreak on visibility. Even the most experienced, vigorous, and well prepared visitor - hikers, climbers, and even those who travel in snow tractors -- need to be aware of these impacts, and to recognize that the wind's force and blinding fury demand respect, wariness, and, on many occasions, a willingness to retreat to more forgiving realms.

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