对于气象学，我并无太深的了解，外文著述则研读更少。

　　《气象学》教科书有说，通常把不含水汽、液体和固体杂质的大气称为干洁大气，简称干空气。在大气热力学和化学中，指不含水汽的空气。

　　理想化的“干空气”可谓《气象学》的经典概念。

　　我想，大气中，除了水汽、液体和固体杂质外，还有各种污染气体，称为干空气犹可，称作“干洁”就十分勉强了。“洁”从何来？

　　用谷歌搜索，Dry air属于气象学名词，clear air已非气象学专门用语，主要地表示“清洁空气”了。

   传统的气象学与传统的水文学相似，主要研究大气或水体的物理变化，研究化学变化的大气化学与水环境学科都属于环境问题凸现后出现的新兴学科。

　　因此，《气象学》读物中的“通常把不含水汽、液体和固体杂质的大气称为干洁大气，简称干空气”似宜改为“通常把不含水汽、液体和固体杂质的大气称为干燥大气，简称干空气”或甩掉名不副实的“干洁大气”，干而脆之称为“干大气”或“干空气”。

附：http://usatoday30.usatoday.com/weather/wdensity.htm   05/17/2005 - Updated 09:46 AM ET

　　                                         Understanding air density and its effects

                                                    By Jack Williams, USATODAY.com

In simple terms, density is the mass of anything - including air - divided by the volume it occupies.

In the metric system, which scientists use, we usually measure density in terms of kilograms per cubic meter.

The air's density depends on its temperature, its pressure and how much water vapor is in the air. We'll talk about dry air first, which means we'll be concerned only with temperature and pressure.

In addition to a basic discussion of air density, we will also describe the effects of lower air density – such as caused by going to high altitudes – on humans, how humidity affects air density – you might be surprised – and the affects of air density of aircraft, baseballs, and even racing cars.

The molecules of nitrogen, oxygen and other gases that make up air are moving around at incredible speeds, colliding with each other and all other objects. The higher the temperature, the faster the molecules are moving. As the air is heated, the molecules speed up, which means they push harder against their surroundings.

If the air is in a balloon, heating it will expand the balloon, cooling it will cause the balloon to shrink as the molecules slow down. If the heated air is surrounded by nothing but air, it will push the surrounding air aside. As a result, the amount of air in a particular "box" decreases when the air is heated if the air is free to escape from the box. In the free atmosphere, the air's density decreases as the air is heated.

Pressure has the opposite effect on air density.

Increasing the pressure increases the density. Think of what happens when you press down the handle of a bicycle pump. The air is compressed. The density increases as pressure increases.

Altitude and weather systems can change the air's pressure. As you go higher, the air's pressure decreases from around 1,000 millibars at sea level to 500 millibars at around 18,000 feet. At 100,000 feet above sea level the air's pressure is only about 10 millibars. Weather systems that bring higher or lower air pressure also affect the air's density, but not nearly as much as altitude.

We see that the air's density is lowest at a high elevation on a hot day when the atmospheric pressure is low, say in Denver when a storm is moving in on a hot day. The air's density is highest at low elevations when the pressure is high and the temperature is low, such as on a sunny but extremely cold, winter's day in Alaska. (Related: Understanding air pressure)