This paper represents the first comprehensive analysis of the in situ environmental conditions that occur high on Mount Everest and their impact on hypoxic and hypothermic stresses. This is accomplished as a result of the availability of high-frequency (i.e., less than 1 h) and long-term (i.e., over 7 months) barometric pressure, temperature and wind speed data from the South Col of Mount Everest. These data collected at an elevation of 7,986 m a.s.l, as well as estimates of the WCT and FFT that are derived from it, allow for the first characterization of variability due to large-scale weather systems and seasonality across the pre-monsoon, monsoon and post-monsoon periods. It also allows for a characterization of the daily cycle in these parameters for the same periods. It should be emphasized that the pre-monsoon period is not fully characterized as a result of the lack of data prior to the middle of May, when the station was installed during a climbing expedition. This is unfortunate as the pre-monsoon period is when most summit attempts occur.
As we have seen, seasonality plays an important role in the environmental, hypoxic and hypothermic conditions that climbers would experience on the mountain. Most noticeable is the impact that the progression in the seasons has on the conditions. During the spring climbing season, April is typically spent acclimatizing, and May is when most summit attempts occur. During the fall, the corresponding months are September and October. From the results presented, with the caveat noted above as conditions when we have no data during April and early May 2008, one can see that climbers in the spring are ascending into increasingly benign conditions, which are of course terminated by the onset of the monsoon in early June. The opposite is the case in the fall when climbers are ascending into increasingly severe conditions. In the fall, it is the cessation of the monsoon in early September that limits the date at which the process can begin.
The results presented in this paper confirm earlier work as to the extreme environment experienced by those attempting to summit Mount Everest and other high Himalayan mountains [16, 17, 20]. It has, however, the advantage of relying completely on in situ data and so is not subject to the caveats applied to earlier work that relied on global meteorological data or assumptions as to the character of some of the parameters. However, it should be noted that possible influence of local (complex) topography can affect meteorological observations (in particular the wind record), thus partially limiting the generalization of the obtained results to the entire region.
In addition, the results presented clearly show that the pre- and post-monsoon periods are characterized by an increase in variability on the weekly timescale that is undoubtedly the result of the passage of large-scale weather systems. During these months, large changes in barometric pressure, temperature and wind speed can occur on the weekly (or lesser) timescale. The magnitude of the barometric pressure changes during the pre- and post-monsoon periods are of sufficient magnitude to be of physiological importance [6, 7]. This variability is also reflected in large variability in the hypothermic parameters. Thus, during these periods, care must be taken in timing ascents so as to minimize the severity of the environmental conditions. In contrast, the monsoon period is one in which the impact of large-scale weather systems is minimized, and this is reflected in the more uniform conditions during this period. Although conditions are generally more benign and stable, there are other hazards, such as heavy snowfall and avalanches, that assume a greater importance during this period.
The daily cycle in the various parameters was also investigated. A semi-diurnal cycle in barometric pressure was evident with an amplitude that was higher in the post-monsoon period. This semi-diurnal variability is most likely related to atmospheric tidal effects and is surprising in that data over the Tibetan Plateau suggest that a diurnal cycle in barometric pressure, forced by solar heating, should dominate . This suggests that there may be some height-dependent effect with regard to the relative importance of the diurnal and semi-diurnal forcing of surface pressure variability.
A diurnal cycle in temperature was observed during both May and July. There was, however, a difference in that the warmest temperatures occurred earlier in the day during July. This is most likely the result of differences in the surface energy balance during the pre-monsoon and monsoon periods. During the pre-monsoon period, the South Col is snow-free, and hence, the high heat capacity of the surface would tend to delay the period of the warmest temperatures. In contrast, the South Col is usually snow covered in July, and so, the warmest temperatures would tend to occur around noon, following the daily cycle of solar radiation, as is observed. During October, there was evidence of a weak diurnal cycle in temperature that is in phase with that during May. Again, this is to be expected given the weaker solar forcing later in the year as well as a snow-free surface. The daily variability in temperature is, however, largest in October. This is most likely the result of increased large-scale weather activity whose timing of warm and cold advection is not tied to the solar forcing.
The wind speed shows a clear diurnal cycle in May, and it is most likely a signature of enhanced convective activity tied to solar forcing that is particularly evident in the development of strong thermal wind regimes along the Himalayan valleys located at lower altitudes . Such convective activity can result in severe weather and even thunderstorms on the mountain . The other months do not have as pronounced a diurnal cycle. During July, this is most likely the result of the generally weaker winds during this period, while in October, it is most likely the result of the aforementioned asynchronous timing of the large-scale forcing.
During May and October, the WCT and FFT are approximately constant throughout the day, indicating that the risk of hypothermia is essentially uniform. The reasons for this behavior are however different. During May, the wind speeds are highest during the day when the temperatures are warmest and vice versa. As a result, the wind speed and temperature effects cancel out. In contrast, during October, the wind speeds and temperatures are more uniform during the day. The daily variability in these parameters is highest in October, when it is possible anytime during the day to experience WCTs as low as −50°C and FFTs as short as 2 min. During July, the warmer temperatures and lower wind speeds result in a greatly reduced risk of hypothermia, with WCTs in the order of −20°C and FFTs in the order of 60 min. These results suggest that care must be taken throughout the summit day during the spring and fall climbing seasons, and even though temperatures may be warmer during the day, the higher wind speeds can nevertheless result in an elevated risk of hypothermia and frostbite.
Finally, the results presented provide one with a measure of the environmental risk factors at the South Col. However, wind speeds on the mountain typically increase with height as one moves closer to the core of the subtropical jet [11, 15], and temperatures tend to decrease . As a result, the approximate 1-km difference in height between the South Col and the summit can result in an elevated risk of hypothermia [17, 20]. As a consequence, the values presented in this paper should be taken as lower bounds for the risk of cold injury above the South Col.