The air pollution concentration in the winter season of Katmandu valley is very high compared with summer season, because the air pollution concentration was strongly influenced by the inversion layer that is formed at Katmandu valley. This mechanism was simulated by a water tank experiment and by numerical calculation. Thermal stratification was made at the start of the experiment, and the temperature on the bottom of water tank was changed with the period of 12 minute. The updraft wind on the slope and Benard convection occurred while the bottom temperature was maintained in high temperature. The downdraft wind on the slope occurred and the inversion layer was formed while the bottom temperature was maintained in low temperature. The calculated pollution concentration was qualitatively agreed with the field observation in Katmandu valley.
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