Describe the behavior of water vapor in the atmosphere. What will be an ideal response?

---

Air is never completely dry; water vapor, the gaseous form of water, can occupy as
much as 4% of its volume. Sometimes liquid droplets of water are visible as clouds or
fog, but more often the water is simply there—invisible in vapor form, having entered
the atmosphere from the ground, plants, and sea surface. The residence time of water
vapor in the lower atmosphere is about 10 days. Water leaves the atmosphere by
condensing into dew, rain, or snow. The temperature and water content of air greatly
influence its density. Because the molecular movement associated with heat causes a
mass of warm air to occupy more space than an equal mass of cold air, warm air is
less dense than cold air. But contrary to what we might guess, humid air is less dense
than dry air at the same temperature—because molecules of water vapor have less
mass than the nitrogen and oxygen molecules that the water vapor displaces. Near
Earth’s surface, air is packed densely by its own weight. Air lifted from near sea level
to a higher altitude is subjected to less pressure and will expand. Air becomes cooler
when it expands. Air descending from high altitude warms as it is compressed by the
higher atmospheric pressure near Earth’s surface. Warm air can hold more water
vapor than cold air can. Water vapor in rising, expanding, and cooling air will often
condense into clouds (aggregates of tiny droplets) because the cooler air can no
longer hold as much water vapor. If rising and cooling continue, the droplets may
coalesce into raindrops or snowflakes. The atmosphere will then lose water as
precipitation, liquid or solid water that falls from the air to Earth’s surface. These
rising-expanding-cooling and falling-compressing-heating relationships are important
in understanding atmospheric circulation, weather, and climate