Atmospheric Pressure At Mount Everest

barometric pressures near the summit of Mt. Everest (distance 8,848 one thousand) are of keen interest to loftier-distance physiologists because the force per unit area is and so depression that the $P O_{two}$ is very near the limit for human survival. Until recently, simply one directly measurement of barometric pressure had been made on the Everest summit. This was done past Dr. Christopher Pizzo on Oct 24, 1981 during the American Medical Research Expedition to Everest (AMREE), and the value he obtained was 253 Torr (8). The measurement was made with a crystal sensor that was calibrated in the field against a mercury barometer.

Despite being merely one data point, the value of 253 Torr has been extensively used in high-altitude physiology. For example, it was the pressure selected for the "summit" in the long-term low-pressure chamber report Operation Everest II (1). In addition, the summit measurement and additional measurements only higher up the Southward Col at an distance of 8,050 thousand were used to define the relationship between barometric pressure and altitude on high mountains at latitudes virtually the equator with skilful predictive ability (five). Mt. Everest is especially suitable for determining the force per unit area-altitude relationship because not but is its top at 8,848 thou the highest betoken on Earth simply besides the S Col (distance vii,986 m) is a well-defined saddle, and therefore its distance has been accurately determined.

Clearly, it would be desirable to accept additional data on barometric pressures at farthermost altitudes on Mt. Everest, and in the last three yr two new studies accept been carried out. The nowadays paper describes the results, compares the new information with that already available, and discusses the physiological implications.

1997 NOVA EVEREST EXPEDITION

In the jump of 1997, the telly science programme NOVA organized an expedition to Mt. Everest, and five people reached the summit. The scientific program included measurements of neuropsychological function at farthermost altitude, and these were compared with the results from brain imaging before and later on the expedition. Cardiopulmonary studies were also made.

We were non enlightened of the expedition until the last minute simply were able to send a handheld barometer in the hope that it would be possible to obtain a measurement on the tiptop. The instrument was a Pretel Alti Plus K2 barometer (Groupe Pretel, Claix, France). Although we had used the barometer extensively at altitudes upward to ∼v,000 one thousand at normal ambient temperatures, there was not time before the expedition to calibrate information technology for the very low pressure expected on the summit and for the very low temperature.

A single measurement was made by David Breashears at 7:00 AM Nepalese time on May 23, 1997. The value was 346 mbar, which corresponds to 259.5 Torr. The barometer was carried in his haversack and so was exposed to the ambient temperature. This was not measured straight, but according to conditions balloon soundings fabricated at the same time (run across conditions airship soundings) the temperature was approximately −22°C.

When the barometer was returned to University of California San Diego, information technology was direct calibrated confronting a mercury cavalcade at barometer temperatures of 21, −6, and −20°C. The graph of barometer reading against mercury height showed first-class linearity. However, the barometer read 1–2 Torr depression at 760 Torr and 5.3 Torr loftier at barometric pressures between 232 and 283 Torr at a temperature of 21°C. When the measurements were made after the barometer had been left overnight in a −20°C common cold room, the fault increased to 7 Torr in the force per unit area range of 232–283 Torr. Therefore, the corrected pressure level was 259.five Torr minus 7 Torr, that is, 252.v Torr. This is in close agreement with the value of 253 Torr obtained by Pizzo on Oct 24, 1981.

WEATHER Airship SOUNDINGS

Additional information on barometric pressures at an altitude of 8,848 m in the vicinity of Mt. Everest at the fourth dimension of the straight measurement was obtained from atmospheric condition balloon soundings. These radiosondes are released from many locations all over the world at 0000 and 1200 UTC (Universal coordinated time) every mean solar day. Nosotros used data from radiosondes released from the xiv stations closest to Mt. Everest at 0000 UTC on May 23, 1997. This corresponds to five:40 AM local fourth dimension and so is shut to the time when the straight measurement was made. The breadth and longitude of the Everest summit are 27°59′North and 86°56′E, and the weather stations were all between 22 and 38°N and between 74 and 95°Due east. The data for each radiosonde were retrieved by Laurence One thousand. Riddle of the Scripps Institution of Oceanography, University of California San Diego.

Altitudes for barometric pressures of 150, 200, 250, 300, 400, 500, and 700 mbar were obtained for each radiosonde (from cognition of its ascent rate) and plotted as log barometric pressure vs. altitude. Figure 1 shows an example for the station at Gorakhpur, which is nearly Everest at 26.75°Due north, 83.37°East (Fig.two). The data points lay almost on a straight line but an fifty-fifty ameliorate fit was obtained by using a parabola, and the pressure for distance viii,848 g was then obtained by interpolation. For Gorakhpur, this was 338.ii mbar. These pressures in millibars were converted to Torr past multiplying by 0.7500, and the results are shown in Fig. two. Other data are as well available from the radiosondes, including temperature, humidity, wind direction, and wind velocity. The temperature data showed that, at an altitude of 8,848 chiliad, the temperature was about −22°C, and therefore this was used to calibrate the barometer as indicated under 1997 nova everest expedition.

Fig. 1. — **Fig. ane.**Derivation of barometric force per unit area at 8,848-k altitude from radiosonde data. Data points are from balloon that was released from Gorakhpur 26.75°N, 83.37°East at 0000 UTC (Universal coordinated time), May 23, 1997. Altitudes for barometric pressures of 700, 500, 400, 300, 250, 200, and 150 mbar were three,166, v,880, 7,600, 9,720, 11,020, 12,540, and xiv,390 m, respectively. Data points were fitted with a parabola (R ^two > 0.9999), and force per unit area for viii,848 m was interpolated to be 338.xvi mbar. This is equivalent to 253.six Torr.

**Fig. 2.**Barometric pressures at an distance of 8,848 grand in vicinity of Mt. Everest at 0000 UTC on May 23, 1997 every bit adamant from radiosonde data. At that place are fourteen nearby stations indicated by crosses, but 1 did non transmit information to a higher place v,850 thou. Gorakhpur is the station southwest of Everest where the pressure level at viii,848-m altitude was 253.half dozen Torr. At Gauhati station, southeast of Everest, pressure at eight,848 yard altitude was 252.7 Torr. Annotation the loftier-force per unit area system extending from just w of Everest in a southeasterly direction.

Figure 2 shows that the barometric force per unit area at an altitude of viii,848 m to a higher place Gorakhpur just to the s and west of Everest was 254 Torr, whereas above Gauhati just to the s and east of Everest the force per unit area was 253 Torr. In that location is another shut station at Patna at 25.60°N, 85.10°E, simply unfortunately this radiosonde failed and no data were reported in a higher place 5,850 m. The understanding between the radiosonde pressures and the directly measurement is close.

An interesting characteristic of the radiosonde data is that a loftier-pressure organization was located well-nigh the meridian of Mt. Everest at the time of the straight measurement. This is credible from Fig. 2 but is more than hands seen in plots of the distance for a barometric pressure of 300 mbar (225 Torr), which are available from the radiosonde information. Figure3 shows that, at the coordinates of the Everest acme, the distance of the 300-mbar pressure level was 9,715 m, which corresponds to virtually the highest altitude on the chart.

**Fig. 3.**Altitudes for a pressure of 300 mbar (225 Torr) in vicinity of Mt. Everest at 0000 UTC on May 23, 1997 every bit adamant from radiosondes. Coastline of India, Pakistan, and southeast Asia is shown past thick line. Note that Everest was well-nigh the center of a high-pressure organisation. L, depression; H, loftier; mb, mbar; WXP, weather processor; OZ, 0000 UTC.

BAROMETRIC PRESSURES ON THE SOUTH COL OF EVEREST (Distance 7,986 M) OBTAINED IN THE Summer OF 1998

An Everest study organized by the Media Laboratory of the Massachusetts Establish of Technology placed a weather station on the Due south Col (altitude 7,986 m) (3) in early May 1998. Barometric pressure was measured with a Motorola sensor (MPX 5100AP, Motorola, Schaumburg, IL), which has a pressure range of 0–760 Torr and can operate at temperatures down to −40°C. The data were transmitted upward to one of three polar orbiting National Oceanic and Atmospheric Administration satellites and and so to a ground station. These data were posted on the Web site http://janson.media.mit.edu/weather/14836, and up to 30 measurements of force per unit area and fourth dimension of twenty-four hour period were available every solar day from May 4 to August 26. The probe also transmitted temperature, wind speed, and degree of lightness or darkness.

In general, the data were internally very consistent with small-scale daily and monthly SDs. However, there were 23 rogue measurements that were clearly outliers. For example, on May 7 all merely ane of the measurements were betwixt 10.7 and eleven.27 in. Hg, whereas one at 0246 UTC was 3.xi in. Hg. Once again on July ane, all the measurements were between ten.93 and xi.27 in. Hg except for 1 of 25.88 in. Hg, which was clearly an outlier. In addition, the measurements at the commencement of the data manual on May 4 and early on May 5 were erroneously high because the barometer was nonetheless being carried up at this time. If these outliers are rejected, the total number of measurements between May half-dozen and August 26 was 2,572.

The hateful and SD pressures for May, June, July, and August, respectively, were as follows: xi.17 ± 0.10, xi.24 ± 0.05, xi.27 ± 0.04, and 11.30 ± 0.03 in. Hg. Note that the monthly SDs were <0.5% of the means. When these these pressures were converted to Torr, the monthly means for May, June, July, and Baronial were 283.vii, 285.5, 286.3, and 286.9 Torr. The gradual increase in barometric pressure from May to July is to be expected because of the well-known seasonal variation (eastward.g., see Fig. 3 in Ref. viii).

Comparing WITH PREVIOUS DATA AND PHYSIOLOGICAL SIGNIFICANCE

Direct measurement on the Everest summit by the 1997 NOVA Everest Expedition.

The agreement within ∼1 Torr of this measurement with that made past Pizzo in October 1981 is gratifying. There is a substantial seasonal variation in barometric pressure at an altitude of 8,848 yard (as shown in Fig. three of Ref. 8). The highest pressures are seen in July and August and the lowest pressures in January. May and October are at approximately the same height on the descending limbs of the plot of pressure level against month. Therefore, we can await the pressures in these 2 mo to be similar.

The fact that the pressure was as high every bit 253 Torr does not mean that this is the hateful pressure level at the summit during May or October. Climbers tend to choose loftier pressure days for their summit bid, and, for case, October 24, 1981 was unusually warm with the temperature directly measured on the peak being −9°C (8). In the case of Breashears' measurement on May 23, 1997, Fig. 3 shows that Everest was nearly the center of a high-force per unit area system. Radiosonde measurements fabricated from New Delhi evidence that the mean monthly barometric force per unit area for an altitude of 8,848 thou at breadth 28°Due north is ∼251 Torr for both May and October (8). In July and Baronial the hateful monthly pressure is betwixt 254 and 255 Torr, whereas in January information technology falls to as low as 243 Torr.

Radiosonde measurements for 0000 UTC on May 23, 1997.

Again, the agreement between the radiosonde information and the direct measurement is close. The isobars for the force per unit area of 300 mbar (Fig. 3) testify that a loftier-pressure zone extended from the conditions stations at Gorakhpur and Gauhati to include Everest. The aforementioned pattern is seen in the altitudes for the 500-mbar pressure level, which is not reproduced here. Therefore, we can conclude that the radiosonde value at the Everest summit was shut to 253 Torr.

Barometric pressures on the Everest South Col, May to August 1998.

The South Col of Mt. Everest is ∼860 m below the summit, and it might therefore be argued that barometric pressure level data from that location are non valuable in determining the summit pressure. Notwithstanding, the altitude of the Due south Col is accurately known, and therefore the barometric pressure data tin exist used to locate the line relating pressure to distance.

The weather probe measurements can be compared with those fabricated at Military camp 5 during the AMREE in 1981. The altitude of this camp was ∼lx 1000 above the S Col at 8,050 m, and six barometric pressure measurements were made between October 12 and October 25, the range of values beingness 281.five to 285.ane Torr. The hateful and SD was 283.6 ± 1.v Torr (8). The mean pressure level recorded from the Massachusetts Establish of Engineering science (MIT) weather condition probe for May 1998 (when the seasonal variation is comparable with October) was 283.seven Torr so agreement between the ii studies was close. At this altitude, an increase in altitude of 60 one thousand corresponds to a autumn in pressure of ∼two Torr.

Relationship between barometric pressure and altitude on Mt. Everest.

Table one summarizes the barometric pressure level data available from the 1997 NOVA Everest Expedition, the 1998 Everest study organized past MIT, and the 1981 AMREE. Later the 1981 expedition, a line relating barometric pressure to altitude was derived on the footing of the measurements made at 8,848, eight,050, and five,400 thousand every bit shown in Table 1 (Fig. 2 of Ref. 8). The line used the logarithm of barometric pressure considering the information and so almost fall on a straight line. Subsequently, the Model Temper equation was derived that fitted these data together with barometric pressures measured at many high-altitude stations, the altitudes of which are accurately known (five). Many of these measurements were from locations inside thirty° of the equator, and most were in the summer. These locations and time of yr were chosen because the barometric pressures are lower at higher latitudes and in the winter months. The Model Atmosphere equation is Pb = exp (six.63268 − 0.1112 h − 0.00149 h²) where Pb is barometric pressure in Torr and h is altitude in kilometers. It was shown that barometric pressures of most locations were predicted within 1%.

**Table 1. Barometric pressures on Mt. Everest**
Distance, m	AMREE	NOVA	MIT	Model Atmosphere Equation
8,848 (Elevation)	253	253		253
8,050	284			282
7,986			284	284
5,400	400			399

Figure iv shows the line for the Model Atmosphere equation for altitudes over 4,000 m. The circles bear witness data points from AMREE, and the crosses show the new data. At the highest altitude of 8,848 m, the cantankerous indicating the NOVA measurement is superimposed on the AMREE information bespeak. At the altitude of ∼eight,000 one thousand, the cross indicating the mean of the MIT measurements during May is at a slightly lower distance than is the AMREE betoken and at the same barometric pressure. However, agreement is clearly very close.

**Fig. 4.**Barometric pressure-distance relationship. Circles, data from 1981 American Medical Research Expedition to Everest; cross at summit altitude (8,848 m), data point from 1997 NOVA expedition; cross at altitude of 7,986 m, from 1998 Massachusetts Institute of Technology trek. SDs are also small to show on the graph. Line corresponds to Model Atmosphere equation: Pb = exp (6.63268 − 0.1112 h − 0.00149 h²), where Pb is barometric pressure in Torr and h is altitude in km.

Physiological significance of the new data.

The new information greatly increment our confidence in the barometric force per unit area-altitude human relationship at altitudes of 8,000 k and in a higher place on Mt. Everest where in that location was previously and then piddling information. The new measurements confirm that the inspired $P O_{ii}$ is only 42–43 Torr on the summit and justify the apply of this value in determining the maximal oxygen consumption (5˙o _{2 max}) on the summit (four, 7). The measurements further emphasize the extreme sensitivity ofV˙o _{ii max} to barometric pressure at these great altitudes.

Measurements during AMREE on very well-acclimatized subjects at inspired $P O_{2}$ values of 48.5 and 42.5 Torr, respectively, showed that theV˙o _{two max} declined from 1,450 to 1,070 ml/min (see Tabular array 6 in Ref. 7). This means that the slope of the line relatingV˙o _{two max} to inspired $P O_{2}$ was ∼63 ml ⋅ min⁻ⁱ ⋅ Torr^−ane. A very similar result was constitute in Operation Everest Ii (4; see Fig. xi.17 in Ref. half-dozen).

The extreme steepness of the line relatingV˙o _{2 max} to inspired $P O_{ii}$ has some interesting physiological implications. For case, if the barometric pressure on the summit were 236 Torr, as predicted from the International Civil Aviation Organization Standard Atmosphere (2), the inspired $P O_{2}$ on the summit would be just 39.5 Torr. Therefore, the reduction in inspired $P O_{2}$ from the value of 43 Torr, which would exist for a summit pressure of 253 Torr, would be 3.5 Torr. The reduction of oxygen consumption would therefore exist ∼222 to a value of ∼848 ml/min, or a reduction of ∼21%. It seems very unlikely that the mountain could exist climbed under these weather.

In fact, even the reduction of barometric pressure level of only 10 Torr, such as occurs between summer and winter on the Everest elevation, is predicted to reduceV˙o _{2 max} by ∼133 ml/min. In other words,5˙o _{two max} on the summit would fall from 1,070 to ∼940 ml/min, that is, by ∼12%. This is presumably one reason why the mountain has not nonetheless been climbed in midwinter without supplementary oxygen.

In summary, the new direct measurement of barometric pressure fabricated in May 1997 and the very extensive series of measurements of barometric pressure level on the South Col in 1998 greatly increase our confidence in the barometric pressure-distance relationship at altitudes of 8,000 m and above on Mt. Everest. They provide additional evidence that humans at these altitudes are very shut to the limit of tolerance to hypoxia.

I am indebted to Liesl Clark and David Breashears of the 1997 NOVA Everest expedition, Michael Hawley and others at the Massachusetts Institute of Technology for assist with the information from the 1998 study, and Laurence K. Riddle of the Scripps Establishment of Oceanography, University of California San Diego for aid with the radiosonde information.

FOOTNOTES

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