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Why air density increases with increase in temperature , but this is opposite in case of liquid.

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    $\begingroup$ What makes you think it does? Have a look at Density of air. $\endgroup$ – Bianfable Nov 30 '19 at 17:47
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    $\begingroup$ Density decreases as temperature increases. "Density altitude" increases as temperature increases because the altitude where you find the currently less dense air is would normally be found at higher altitude. The density altitude is just a reference height in comparison to standard conditions $\endgroup$ – Jan Nov 30 '19 at 18:21
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    $\begingroup$ Down voted because the question makes a false assumption. $\endgroup$ – Michael Hall Nov 30 '19 at 18:26
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    $\begingroup$ Maybe the OP is confused with Boyle's law, a law stating that the pressure of a given mass of an ideal gas is inversely proportional to its volume at a constant temperature. In the case of aircraft flying in the Earth's atmosphere the volume is not constrained. $\endgroup$ – Transistor Nov 30 '19 at 18:36
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    $\begingroup$ @Transistor Yes you are right, I read this line in book on Avionics where they explained concept of duct propagation in radio waves using above mentioned air temp and density relation. $\endgroup$ – Vikas Singh Nov 30 '19 at 18:47
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As clarified in the comments, you are confused with Boyle's law, a law stating that the pressure of a given mass of an ideal gas is inversely proportional to its volume at a constant temperature.

In the case of aircraft flying in the Earth's atmosphere the volume is not constrained so pressure does not have to increase with temperature.

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Your premise is false. Air density decreases with increasing temperature.

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