Abstract: The density of nitrogen and oxygen in atmosphere changes because of the existing moisture.In this paper,the decreasing of various air density Will be expressed as the following equation:a_i=e/p;the pressure decreasing eff- ect caused can be expressed as △P=-ap;and the temperature in creasing effect can be expressed as △T=T(a/1-a);using the atmospheric pressure P observed in the station,and water vapour pressure e can calculate the chan- ges of oxygen density,the results can be indirectly used for getting oxygen content changes in water.The values got in this way could be used as an index for taking measures to prevent fish from floating its head caused by the deficiency of oxygen supply and suffocation.

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