Scientific Research, Weather on Mount McKinley
SCIENTIFIC RESEARCH*
WEATHER ON MOUNT MCKINLEY
John G. Houghton, University of Nevada at Reno
In 1968 and 1969, weather observations were taken on the McKinley West Buttress Route by four expeditions. These included two climbing groups, the 1968 Nevada Expedition and the 1969 Watkins Expedition, plus the scientific parties of the Institute of Arctic Biology in both years. About half the research was financed by Project THEMIS (Institute of Arctic Biology, Peter Morrison, director). Many of the data were gathered by field observers Jerry Kreitner (IAB, 1968), Brian Bartlett (IAB, 1969), and Hank Noldan (Watkins, 1969). I am particularly indebted to Bradford Washburn for providing records from his 1951 West Buttress and 1947 Muldrow expeditions. All West Buttress observations were made in June or early July, while those on the Muldrow Route were taken in April, May, and June.
To compare the weather at different levels, the West Buttress Route was divided into four sections.
Kahiltna Glacier: levels between 6800 and 9500 feet, relatively sheltered and with mild temperatures due to the low altitude.
Lower West Buttress: the series of exposed ridges between the head of the Kahiltna Glacier at 10,000 feet and Windy Corner at 13,200 feet. Ice Bowl: the mild, sheltered, south-facing basin between 13,500 and
feet.
High West Buttress: the highly-exposed system of summit ridges above
feet.
Washburn’s Muldrow observations were divided into (1) the sheltered Lower Muldrow Glacier, with records between 5500 and 7500 feet, (2) the more exposed Upper Muldrow Glacier between 9000 and 11,000 feet, and (3) the highly-exposed Harper Glacier above 15,000 feet. From the observed data, characteristics of climbing weather were evaluated for each location.
Cold weather on McKinley may cause frostbite or death by exposure if climbers are caught unprepared. Of basic importance is the thermal stress (heat gain or loss) affecting the body. This can be expressed as the windchill index, which measures the heat loss from exposed skin in the shade for moderately-active climbers. The heat loss (in kilogram-calories per square meter per hour) can be computed from the air temperature and wind speed. Thermal stress can also be expressed in terms of equivalent temperature, that which the body actually feels under given conditions. With strong winds this sensible temperature may be more than 50°F lower than the air temperature, whereas with winds less than 3 mph the air feels warmer than the reading on the thermometer.
TABLE 1. - Range of Observed Temperature and Wind Effects.
Loca
tion1
Temperature °F
Wind Speed MPH2
Equiv. Temp.°F
Windchill3
Max.
Min.
Mean
Max.
Min.
Mean
Max.
Min.
Mean
Max.
Min.
Mean
KG
65
5
34
35
calm
7
65
- 31
29
1390
290
710
LWB
52
0
25
60
calm
13
55
- 42
8
1530
410
960
IB
61
- 9
18
40
calm
6
61
- 40
11
1500
350
910
HWB
26
-13
6
75
5
19
11
- 78
-32
1930
910
1420
LMG
34
-15
9
20
calm
6
34
- 51
0
1630
660
1030
UMG
33
-26
11
70
calm
14
44
- 53
- 5
1660
550
1100
HG
20
-29
- 5
80
calm
20
34
-101
-38
2200
660
1490
Ov
65
-29
12
80
calm
12
65
-101
- 9
2200
290
1150
KG = Kahiltna Glacier; LWB = Lower West Buttress; IB = Ice Bowl; HWB = High West Buttress (observations in June-early July); LMG = Lower Muldrow Glacier (April); UMG = Upper Muldrow Glacier (May); HG = Harper Glacier (late May-June); Ov = Overall Mount McKinley.
Sustained wind speed (1-minute average, not peak gust); based partly on estimated values.
Heat loss in kg-cal/m2/hr. Values less than 400 are very warm for moving climbers; 400-800 may be too warm on sunny days; 800-1400 are comfortable for moving but cold for sitting; above 1400 exposed flesh freezes.
Table 1 shows the range of actual and equivalent temperature, wind speed, and windchill index observed at each location on McKinley. The air temperature follows the normal lapse rate of 3°F per 1000 feet elevation gain. On the Kahiltna Glacier it averages above freezing, with the hottest day reaching 65° in the shade, though nights drop below freezing and large daily ranges occur in clear weather. Warm days and large ranges occur upto 15,000 feet, where one day in the Ice Bowl ranged from-6° to +61°. In contrast, the High Buttress averaged only +6° and readings never rose above freezing.
Wind compounds the cooling with elevation, since winds are stronger at the high levels. On the West Buttress Route the equivalent temperature cools 6½°F per 1000 feet, averaging a bitter -32° on the High Buttress. However, striking variations occur with exposure. Winds are lightest in the Ice Bowl with an average speed of only 6 mph, which combined with southern exposure creates warmer sensible temperatures than on the Lower West Buttress, despite the higher altitude.
On the Muldrow Route the temperatures were colder, due to the earlier season (April and May at the lower levels) as well as the northern exposure. With no sheltered site at high levels, winds increase steadily with height and conditions are consistently severe above 14,000 feet, 2000 feet lower than on the West Buttress Route. The coldest windchill occurred here on June 15, 1947, when a 50 mph wind combined with-20° weather produced an effective temperature of-101°.
TABLE 2. - Mean Temperature and Wind Effects under Different Conditions on the West Buttress Route.
Loca
tion
Temperature °F
Wind Speed MPH
Equiv. Temp. °F
Windchill
Sun.
Day1
Cldy Night Day
Sun. Cldy Night Day Day
Sun. Cldy Night Day Day
Sunny
Day
Cdy Night Day
KG
43
35
28
6
9
5
37 27
25
620
740
760
LWB
29
25
15
8
19
11
20 -1
3
820
1050
1010
IB
26/57
23
9
6
6
6
23/55 14
0
790/420
880
1040
HWB
9
5
-2
17
22
18
-21 -38
-42
1290
1480
1530
Avg
27
22
12
9
14
10
17 3
- 2
860
1010
1080
1. Temperature in shade on sunny days, except Ice Bowl: values in shade and sun.
Table 2 shows how temperature and wind effects vary with sky conditions and time of day. A marked diurnal variation is present with warmest weather on sunny days and coldest weather at night. Darkness is absent at this latitude in summer, but the sun dips below the horizon for several hours causing a sharp temperature drop particularly in the Ice Bowl, which acts as a reflector oven by day and a cold air sink at night.
Temperatures in the sun average 30° above those in the shade on sunny days, and the windchill effect is decreased by 50%. Sometimes the effect of sunshine in a glacier basin is extreme due to reflection from the mountain walls. On July 4, 1968, the temperature on the Kahiltna rose to 136° in the sun, causing heat exhaustion in two expedition members. For this reason, many parties travel at night at the lower levels.
Wind speeds vary little between day and night, but are stronger on cloudy days since cloud and storm tend to go together. This increases the windchill, which coupled with lack of sun makes cloudy days feel much colder than clear days. On the other hand, cloudiness makes nights warmer since it inhibits heat loss from the atmosphere. The end result is a reduced temperature range in cloudy weather (Figure 1 ). Strong winds may do the same through air mixing, and observations at 17,200 feet (a windy location) thus show smaller variations than those at 14,200 feet. Figure 1 also shows short-term fluctuations during the day, associated with clouds passing over the sun which cause a temporary drop in temperature. If cloudiness and wind begin simultaneously, the change in heat stress may be enormous in only a few minutes.
TABLE 3. - Frequency of Windchill Conditions.
Windchill
West Buttress (June-July)
Muldrow (April-June)
Category
Heat Loss
Ka
hiltna
Gla
cier
Lower W. Buttress
Ice
Bow11
High
W.
But
tress
Low
er
Mul
drow
Up
per
Mul
drow
Har
per
Gla
cier
Hot
0-100
-
-
- (2)
_
-
-
-
Warm
100-200
-
-
- -
-
-
-
-
Pleasant
200-300
1
-
- (4)
-
-
-
-
Mild
300-400
1
-
2 (3)
-
-
-
-
Cool
400-600
10
8
13 (8)
-
-
1
-
Very Cool
600-800
19
15
18 (3)
-
10
12
4
Cold
800-1000
6
12
18 (1)
2
3
5
19
Very Cold
1000-1200
3
13
22 -
4
12
6
5
Bit. Cold
1200-1400
2
11
13 -
15
4
13
11
Frostbite
1400-2000
-
6
3 -
21
4
9
56
Gt. Danger
Over 2000
-
-
-
-
-
3
Number of Observations
42
64
89(21)
42
33
46
98
1. Ice Bowl values in parentheses: based on temperature in direct sunlight.
In Table 3, the windchill index is arranged in categories based on sensible heat loss. The top four classes represent conditions where light clothing can be worn comfortably in the shade. Moving climbers in the sun feel uncomfortably hot under these conditions, which occur occasionally at sheltered sites on calm summer days. At the Kahiltna, Lower Buttress, and Ice Bowl levels most windchill values ranged from cool to bitterly cold. With a cool or very cool wind effect, moving climbers may still feel hot in the sun since the windchill is several hundred calories less than in the shade.
On the other hand, cold, very cold, or bitterly cold wind effects may be comfortable for the moving climber but cold for inactive climbers exposed to the elements. Tired climbers forced to bivouac overnight may suffer from frostbite and exposure under these conditions. When the windchill exceeds 1400, moving climbers must normally cover the face to prevent frostbite. In midsummer this situation is uncommon below 15,000 feet, but above this level half the observations (60% on the Muldrow Route) were in the “flesh freezes” category. The Arctic Aeromedical Laboratory has proposed three danger zones for frostbite hazard:
Little danger: windchill less than 1400.
Increasing danger: 1400-2000 (flesh may freeze within one minute).
Great danger: over 2000 (flesh may freeze within 30 seconds).
Conditions of great danger are rare on McKinley in summer, though they were observed at Denali Pass in June 1947.
The strongest wind gust measured on the West Buttress Route was 87 mph at Windy Comer by Bradford Washburn in 1951. In 1968 the Nevada Expedition estimated gusts of 100 mph at the 19,000-foot level, while the 1947 Muldrow expedition reported 100 mph gusts as low as 11,000 feet. Such winds, however, are the exception under spring and summer conditions. The average speed for 400 summer observations was 12 mph, about double the average for a typical lowland station. Even at the high levels it was only 20 mph, enough to drastically increase the windchill but not enough by itself to impede travel.
Precipitation and Cloud Cover
Summer is the rainy season in the Alaskan interior, with August the wettest month. The sunniest, driest weather comes in spring so that the climbing season on McKinley begins in April, despite colder windchill conditions. However, early and midsummer weather is drier than that of late summer and early fall. Much midsummer precipitation falls as convective showers in the lowlands, while the high peaks often remain clear. On the other hand, cyclonic storms bring high winds which drop their moisture as they cross the mountains. Hence most precipitation on McKinley is caused by cyclonic storms, which are most common later in the summer.
The Alaska Range forms a climatic divide between the dry interior to the north and the milder, damper region to the south. Talkeetna on the south flank of the range has an annual rainfall of 29 inches, whereas McKinley Park on the north gets only 14. Correspondingly, the south side of McKinley facing the Pacific Ocean receives more snow than the north.
In 1969 the Institute of Arctic Biology measured about 12 feet of snow on the West Buttress over a 37-day period. This was nearly twice the total reported by Washburn on the Muldrow Glacier from April to June, 1947. Hans Gmoser on his descent of the West Buttress in 1963 estimated that 12 feet of snow fell in four days at 13,000 feet. Despite heavy snowfalls the route is relatively free of dangerous avalanches; the 1968 Nevada party observed only one large avalanche in 21 days on the mountain.
Concerning precipitation equivalent, the 1960 Milton Expedition took snow samples on the Eldridge Glacier southeast of McKinley, and found an annual mean water content of 20 inches at the 6500-foot level. On the West Buttress in 1968-69, the heaviest snowfall occurred between 10,000 and 13,000 feet. Below this level the snow is wet and may be mixed with rain, whereas above it the snowfall decreases with the overlying thickness of cloud. The uppermost levels may actually be drier than the interior lowlands, though strong winds at these heights make precipitation measurements impractical.
To determine the characteristics of cloud cover, observations were made several times a day including the amount of sky covered by cloud, and the type and estimated height of each visible layer. Figure 2 shows the percentage frequency of cloud tops and bases observed at different elevations. Notably there is a concentration of cloud layers at three levels.
The first and deepest layer consists of stratocumulus and/or altocumulus on the lower two-thirds of the mountain, either as a single deep layer or as two or three shallow layers. The cloud base may occur anywhere below 14,000 feet, with 10,000 the average height. In contrast, cloud tops are infrequent at the lower levels but very common near the 14,000-foot level. Between the base and top of this layer the Lower West Buttress is often “socked in” with heavy fog, while above it the Ice Bowl may have sunny weather. The presence of this layer explains why precipitation is heaviest at the lower levels.
The second layer is the cap cloud, a localized lenticular cloud caused by high winds forced to rise over the summit. The cap cloud may float above the peak, but more often envelops the summit and may descend as low as 140 feet. The average base is at 19,000 feet with the top above 20,500 feet. This cloud is treacherous, often forming suddenly and wrapping the summit in dense fog, sometimes causing a blizzard while the lower levels are clear. The summit route should be wanded even in fair weather, owing to the unpredictable cap cloud. The uppermost layer consists of cirrus clouds, which often portend an approaching storm. Tall forms such as cumulonimbus may also extend above 20,000 feet, adding to the frequency of tops at these heights.
West Buttress Route, June-July 1968-69
Muldrow Route, April-June 1947
Location
Obs.
Sky
Cover
Location
Obs.
Sky
Cover
Kahiltna Glacier
39
6.5
Lower Muldrow Glacier
32
5.7
Lower West Buttress
66
7.4
Upper Muldrow Glacier
47
5.6
Ice Bowl
97
5.8
Harper Glacier
91
5.2
High West Buttress
41
7.1
All Levels
243
6.5
All Levels
170
5.4
4 Lowland Stations 1968-69
7.0
Fairbanks Normal, April-June
6.8
The mean sky cover (Table 4) follows the pattern of cloud layers, particularly on the West Buttress Route. The cloudiest levels occupy favored zones for two main cloud layers: stratocumulus-altocumulus on the Lower Buttress and the cap cloud on the High Buttress. In contrast the Ice Bowl, often above the stratocumulus layer, has the clearest weather on the whole route. The north side of the mountain has less cloudiness than the south at all levels, due to (l) the leeward position away from the Pacific Ocean, and (2) the earlier season of observation with its drier weather.
The mountain as a whole has less cloud cover than the surrounding lowlands. In fair weather, stratocumulus or cumulus often develop over the lowlands without reaching the heights of McKinley, and weak storms may also miss the upper part of the mountain. On July 4, 1969, Talkeetna reported rain and strong winds while the weather was clear at 14,200 feet. Of course the opposite may also occur as the mountain breeds its own weather. In the blizzard of June 3-12, 1969, wind gusts reached 70 mph and 6½ feet of snow fell at the 10,000-foot level. Simultaneously, drought prevailed in the lowlands with only a single day of light showers at Fairbanks.
Table 5 shows the number of fair, partly cloudy, and cloudy days observed on the mountain. Only one-fifth of the days were really fair, with less than 3/10 cloud cover, whereas cloudy days occurred more than 40% of the time on the West Buttress and 30% on the Muldrow. In 1947, Washburn reported only five perfectly clear days in three months spent on the mountain.
TABLE 5. - Overall Number of Fair, Partly Cloudy, and Cloudy Days.
Category
Sky Cover (Tenths)
West Buttress 1968-69
Muldrow Route 1947
Days
Percent of Total
Days
Percent of Total
Fair
Under 3
12
20%
17
22%
Partly Cloudy
3-7
23
38%
36
46%
Cloudy
Over 7
25
42%
25
32%
Expedition Movement
Wind, cloudiness and precipitation often restrict travel on McKinley. Cold weather alone is not a limiting factor in the summer, but when combined with storm conditions it can make travel difficult or dangerous owing to poor visibility and the threat of frostbite or exposure. Table 6 lists seven types of storm conditions and their occurrence at different locations.
For the mountain as a whole, some inclement weather occurs more than half the time. Fog shows a preference for the Lower West Buttress in the stratocumulus layer, and for the High Buttress and Harper Glacier due to the cap cloud. If the fog is dense or combined with snow or high winds, it may produce a whiteout which makes it impossible to follow an unmarked route. Whiteouts occur nearly one-sixth of the time during the climbing season. Snow is most frequent at the lower levels below the top of the stratocumulus layer, but blizzard conditions are most common at the upper levels where strong winds (over 20 mph) occur half the time. Lightning is not a hazard on McKinley. On the whole the best climbing weather is found below 10,000 feet on the Kahiltna and Muldrow Glaciers, and in the sheltered Ice Bowl.
TABLE 6. - Percentage of Observations with Inclement Weather.
Location
Weather Type
Observations with Bad Weather
Fog
White
out
Rain
Snow
Strong
Winds
Bliz
zard
Thunder
storm
Kahiltna Gl.
21%
12%
4%
41%
18%
11%
0%
50%
Low W. But.
36
23
2
35
30
21
0
54
Ice Bowl
25
11
0
30
13
8
3
49
High W. But.
36
26
0
30
55
34
2
66
Low Muldrow
12
3
0
36
6
0
0
42
Up. Muldrow
14
6
0
20
27
6
0
51
Harper Gl.
27
20
0
31
45
29
0
57
Overall
McKinley
26%
15%
1%
31%
29%
17%
1%
53%
In June 1969, one storm kept expeditions tentbound for ten continuous days on the upper part of the mountain, but the average length of good and bad weather spells was only four days for the climbing season as a whole. Light fog, snow, or strong winds alone may pose no problem, and often the weather is bad for only part of a day. Table 7 shows the effects of weather on daily expedition movements in 1968-69.
TABLE 7. — Percentage of Days with Given Travel Conditions on the West Buttress Route.
Location
No Inclement Weather
Slight Inclement Weather
Travel made Difficult
Safe Travel Impossible
Days with Restricted Travel
Kahiltna G1.
41%
26%
22%
11%
33%
Low. W. But.
25
28
32
15
47
Ice Bowl
38
32
26
4
30
High W. But.
18
20
40
22
62
All Levels
31%
27%
29%
13%
42%
At the Kahiltna Glacier and Ice Bowl levels, 40% of the days had no bad weather and expeditions moved unimpeded on two days out of three. In contrast, travel was restricted nearly half the days on the Lower West Buttress and nearly two-thirds of the days on the High Buttress. Climbers can often move in difficult conditions anywhere below the high camp at
feet, but a summit attempt may be unfeasible in doubtful weather, due to (l) the length of the summit day (3000 feet of climbing at high altitude) and (2) exceptionally strong winds above 18,000 feet. On June 29, 1968, clear skies and 15 mph winds occurred at High Camp simultaneously with hurricane winds and a cap cloud above Denali Pass, 1000 feet higher on the mountain. For the West Buttress as a whole in 1968-69, expeditions averaged four days a week without limiting weather, two days of difficult travel, and one day tentbound.
West Buttress expeditions also depend on air transport to reach Base Camp. Landing a light plane on a glacier between mountain walls requires good visibility to find a location free of open crevasses and away from the cliffs. Furthermore, strong winds may preclude a safe landing if the surface is uneven with drifted snow. As a rule, favorable landing conditions exist with visibility five miles or more and wind less than 15 mph. Visibilities less than one mile, or winds over 25 mph normally create unsatisfactory conditions, while intermediate conditions are denoted as marginal. Table 8 shows the observed frequency of these situations at four landing sites.
TABLE 8. — Frequency of Aircraft Landing Conditions at Four West Buttress Sites.
Category
Conditions (Visibility and Wind)
7,300'
10,000'
14,200'
17,200'
Favorable
Marginal
Inadequate
V. over 5 mi., and W. under 15 mph V. 1-5 mi., or W. 15-25 mph V. under 1 mi., or W. over 25 mph
29%
32
39
10%
33
57
22%
49
29
6%
29
65
The 7300-foot site on the Southeast Fork of the Kahiltna Glacier is the starting point for climbing parties. It has favorable conditions less than one-third of the time, and some expeditions have waited two weeks to be flown onto the mountain. The 10,000-foot Kahiltna Pass and 14,200-foot Ice Bowl sites have been used for rescue, but pose greater problems of weather and topography. The 17,200-foot site is accessible only by helicopter and rarely usable; a rescue attempt here failed in 1969. Parties in need of help should be prepared to wait or move to a lower level, as expeditions can move on many days when aircraft cannot.
Figure 3 shows the pattern of wind direction at different locations. On the West Buttress the topography favors north or south winds, whereas the
Muldrow Route favors winds with a west or easterly component. As a rule, south winds bring stormy weather with Pacific moisture, while north winds from the continent tend to bring fair weather. Diurnal control is also present in the Ice Bowl, where upslope south winds by day alternate with downslope north winds at night.
Observations of air pressure in 1969 showed lower values than those at equivalent elevations in the middle latitudes. In July the air at 18,400 feet is only half as dense as that at sea level, and this — combined with cold weather — makes altitude a greater problem than it is at the same height in the Andes or Himalaya. Many climbers have reached 17,000 feet or higher on McKinley, only to be stopped by altitude on the summit ridges.
The climbing season on McKinley brings weather ranging from wilting heat to severe arctic cold. While storms may be violent anywhere on the mountain, severe conditions dominate only above 15,000 feet. Exposure is also important: the West Buttress Route has warmer weather with heavier snowfall than the Muldrow Route. The midsummer period from mid-June to mid-July has the best weather, with frequent fine days and lower windchill than that of the spring.
REFERENCES
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Carter, H. Adams and David L. Atherton. “Milton Mount McKinley Range Expedition,” American Alpine Journal, 1961, 12:2, pp: 291-6.
Davidson, Art. Minus 148°: The Winter Ascent of Mount McKinley. New York: W. W. Norton, 1969.
Eagan, Charles J. “Effect of Air Movement on Atmospheric Cooling Power,” Arctic Aeromedical Laboratory, Technical Report Series, no. 64-28. Fort Wain- wright, Alaska: 1964.
Falconer, Raymond. “Windchill, a Useful Wintertime Weather Variable,” Weather- wise. Vol. 21 (1968), pp. 227-29 and 255 Gates, David M. Energy Exchange in the Biosphere. Harper & Row Biological Monographs, ed. Allan H. Brown. New York: 1962.
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U. S. Environmental Science Services Administration. Climatological Data, Alaska, June 1968 et. al. Asheville, N. C.: Environmental Data Service, 1968 and 1969.
Washburn, Bradford. “Mount McKinley from the North and West,” American Alpine Journal 1947, 6:3, pp. 283-93.
Washburn, Bradford. “Mount McKinley Weather Experiences,” Weatherwise, Vol. 6 (1952), pp. 3-7.
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*Research supported by Gilkey Research Fund of the American Alpine Club.