什么是 Saturation; relative humidity？

Example 4.4 (A summer morning)

At $20{}^{\circ}\mathrm{C}$ the air contains $11.4\,\mathrm{g}/\mathrm{kg}$. The saturation content at $20{}^{\circ}\mathrm{C}$ is $14.7\,\mathrm{g}/\mathrm{kg}$, so the relative humidity is $11.4/14.7 \approx 78\,\%$. The saturation content is $11.4\,\mathrm{g}/\mathrm{kg}$ at $16{}^{\circ}\mathrm{C}$: that is the dew point. In the afternoon, the same air warmed to $28{}^{\circ}\mathrm{C}$ (saturation content $24.1\,\mathrm{g}/\mathrm{kg}$) has a relative humidity of only $47\,\%$ — with exactly the same water in it.

Example 4.9 (An afternoon cumulus)

In the afternoon of Example 4.4, the air at the ground is at $28{}^{\circ}\mathrm{C}$ with a dew point of $16{}^{\circ}\mathrm{C}$. Rising bubbles become saturated at $125 \times 12 = 1500\,\mathrm{m}$: the cumulus have their flat bases about $1.5\,\mathrm{km}$ up. In the morning, at $20{}^{\circ}\mathrm{C}$, the same air would have condensed at only $125 \times 4 = 500\,\mathrm{m}$: cloud bases rise through the day as the ground warms.

Example 4.13 (The warm dry wind of the lee side)

Air at sea level at $20{}^{\circ}\mathrm{C}$, with a dew point of $12{}^{\circ}\mathrm{C}$, is driven over a ridge $2500\,\mathrm{m}$ high. It becomes saturated at $125 \times 8 = 1000\,\mathrm{m}$, at about $10{}^{\circ}\mathrm{C}$. From there to the crest it cools at the saturated rate, to about $2{}^{\circ}\mathrm{C}$, and loses as rain about a third of its water (from $8.7$ to $6.0\,\mathrm{g}/\mathrm{kg}$). Coming down the other side it warms at the dry rate over $2.5\,\mathrm{km}$: it reaches the plain at about $27{}^{\circ}\mathrm{C}$, seven degrees warmer than it started, with a relative humidity of about $27\,\%$. Such warm, dry winds blow in the lee of many mountain ranges.