John Tyman's Cultures in Context Series NEPAL ENVIRONMENT |
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John Tyman's Cultures in Context Series NEPAL ENVIRONMENT |
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| 011. Nepal’s latitude is subtropical -- between 27 and 30 degrees north of the Equator -- but its continental position (far from the sea) and its landforms are the main determinants of its climate. (Rice fields alongside the Marsyangdi River) |
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| 012. Similarly, while the range in altitude is such that temperature regimes range from tropical to tundra, the east-to-west orientation of the mountains is almost as significant as their height. (View north from near Pokhara) |
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| 013. Their orientation is important because of wind direction. In summer the winds blow from the south, in what is referred to as the “summer monsoon”, bringing moisture from the Bay of Bengal. As a result south-facing slopes are well watered and those on the leeward side of each range are correspondingly drier. Rising winds are cooled, bringing rain: descending air currents are warmed and soak up moisture instead. (Diagram based on one in Toni Hagen’s “Nepal”, published by Kummerley and Frey in 1980) |
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| 014. The winter winds from the north are cold and dry, and south-facing slopes are again favoured, being sheltered from the worst of these icy blasts. (Near Bhulbhule, north of Dumre) |
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| 015. The summer monsoon reaches the Terai at the end of May and the Midlands in June, and it lasts till the end of September. Its arrival signifies the beginning of the agricultural year, but it plays havoc with communications in rural areas. Annual totals range from less than 250 mm in the Mustang area adjoining Tibet, to more than 4000 mm at Lumle near Pokhara. |
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| 016. Temperatures, of course, decrease as altitude increases -- by 6 degrees C for every thousand metres -- and this has a dramatic impact on land use. Mountain slopes exhibit a sequence of land-use types -- from year-round cultivation of sub-tropical crops in valley bottoms, to a single cool temperate crop in summer on higher slopes, to seasonal grazing on windswept alpine pastures, then permanent snow. (Valley of Marsyangdi River) |
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| 017. The fact that it becomes colder as you go up a mountain is largely because air pressure decreases with increasing altitude, and air which is less dense can hold less heat. However, the sun’s rays are more intense at high altitudes, and one burns easily. Trekkers cover any exposed skin with sun-tan lotions, and they wear polaroid sunglasses to protect their eyes. (Porters crossing Thorung La pass) |
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| 018. Shadows are correspondingly intense at high altitudes, and the contrast in temperature between shaded and sunlit areas is extreme. This woman climbed on to her roof for warmth. (Near Tatopani) |
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| 019. The decrease in air pressure is also of physiological significance. At high altitudes with thinner air, though the oxygen percentage is the same (that is 21%) there are fewer molecules of everything, including oxygen. With a chest infection courtesy of another member of our group, I was forced to rest briefly but recovered overnight. (At Thorung Phedi at the base of the pass ) |
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| 020. I was fortunate. A lungful of air at 2000 metres contains 20% less oxygen than at sea level; and 45% less at 5000 metres. Visitors trekking at elevations in excess of 3000 metres often suffer from “altitude sickness” in one form or another. This young doctor from Belgium died crossing the pass at 5400 metres. |
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| 021. In contrast, the bodies of those who live at high altitudes are adapted to thin air. They typically have barrel-shaped chests, larger lungs and stronger hearts, a greater volume of blood, and more red blood cells to absorb the oxygen, plus shorter, more compact bodies so their hearts don’t have to pump their blood as far. (Porters resting near Pisang) |
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| 022. The reduced air pressure at high altitudes also has implications for cooking -- because the boiling point of water depends on air pressure.The lower the pressure, the lower the boiling point. At 3000 metres water boils at 90 degrees celsius -- which is 10 degrees less than that needed for a decent cup of tea! (Tea house in valley of Marsyangdi River) |
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| 023. In addition, it takes twice as long (7 minutes) to boil an egg at 5000 metres than it does at sea level. This explains the popularity today of pressure-cookers as wedding presents, since they reduce cooking times and save on fuel. |
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| 024. Differences in temperature from north to south are less significant than the contrasts in annual rainfall, but they have some bearing on plant growth. Conditions are equable in the south of Nepal and allow crops to be grown all year-round, because there is no real cold season. (Winter crops on the Terai) |
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| 025. However, the annual range in mean monthly temperature increases as you go further from the moderating effect of the oceans. In Jomson, for example, on the Kaligandaki, the difference between the averages for the hottest and coldest months is three times as great as the range at Chitwan on the plains below, though the difference in latitude is minimal. (Jomson in late summer) |
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