Airports at higher altitudes require a longer airstrip due to lower air density.

在高海拔地区建机场，由于空气稀薄的原因，需要一个较长的跑道。

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That engine would carry the hypersonic craft to an altitude where air density and resistance are less.

引擎将高超音速飞机带向空气密度、阻力比较稀少的高度。

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Even when the pressure stays the same, great changes in air density are caused by changes in temperature.

即使压力不变，气温的变化也能引起空气密度的巨大变化。

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So, instead of having the periodic variation of water, or the periodic variation of air density here we're talking about an electric field.

不像水或者空气密度的,周期变化，我们这里,讨论的是电场。

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Since He is much less dense that air (He has only 14% of the density of air) the drag and flutter of the platters in the drives is reduced.

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We have low population density, clean air, fresh water, fertile land, an ocean on either side and a grand total of two nations on our borders. (One of which is a also a huge, stable and peaceful nation with clean air, fresh water, and fertile land and even lower population density).

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For example, if you eat when you are stressed, try to choose foods that are very low-calorie density like air popped popcorn or vegetables and hummus to keep calories under control.

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