如何证明空气中存在水蒸气？
水蒸气是地球大气中最有效的温室气体。空气中可容纳的水蒸气的最大容量取决于空气的温度。暖空气可以包含更多的水蒸气。
水在大气中不断循环。它从地球表面蒸发。在这个过程中，它会在温暖的上升气流中上升到大气中。相反，它又凝结回地球表面形成云。然后它被风吹走，从而以雨或雪的形式到达地球表面。蒸发和冷凝水蒸气循环是将热量和能量从地球表面转移到大气的重要手段。
空气中存在水蒸气
大气中所含的温度高于 100 o C 的水蒸气称为水蒸气。水的液相通过两个主要过程进入气相：蒸发和沸腾。这两者都是物理变化。蒸发是水分子从容器表面进入大气的过程。沸腾是将热能传递给水分子的过程。这两个过程都涉及相变。
这种水蒸气变回液态的过程称为冷凝。在所有这些过程中，主要是蒸发、沸腾和冷凝，水的化学性质保持不变。
涉及水的物理状态变化的生物循环称为水循环，如下图所示：
影响空气中水蒸气的因素
- 水蒸气被改变，即由空气温度设定。
- 地表的温度会影响水从地表蒸发的速度。这导致低层大气中的水蒸气浓度增加，从而吸收和发射长波辐射。
绝对湿度与相对湿度：空气中所含的水蒸气称为绝对湿度。空气中的水蒸气与空气所能容纳的水蒸气相比，称为相对湿度。大气中含有水的空间因温度和压力而异。
Experiment to show the Presence of water vapour in the Air
Water vapour is contained in the atmosphere, which can be easily depicted using the following experiment :
Apparatus – A glass beaker and ice cubes
Procedure – The following procedure can be performed to show the presence of water vapour in air :
1. Take the glass beaker and dry it from outside.
2. Place the ice cubes in the beaker.
3. Leave the ice along with the beaker untouched for some time.
Observations –
Water droplets collect on the outside of the glass beaker. Take a dry blue cobalt chloride paper. In the presence of water, the paper turns pink in color.
示例问题
问题1：为什么最闷热的日子会发生在炎热的夏季？
回答：
Since the warmer air contains more water vapour in it. Therefore, the muggiest days happen at the height of summer heat. When the temperature decreases, the air can contain less quantity of water vapour. A part of it may turn into liquid water.
问题 2：在凉爽的夏日早晨，即使没有下雨，草地上也能看到水滴。为什么？
回答：
The water droplets are visible on the grass on a cool summer morning even in the absence of rain most likely came from the water vapours which eventually lead to the formation of water droplets when it cooled to the dew point. At the dew point, water begins to condense out of the air.
问题 3：解释水蒸气的产生过程。
回答：
The water vapour is produced as a result of the boiling liquid water. This is known as evaporation. It may also be produced from the sublimation of ice. It is removed from the atmosphere by the process of condensation under typical atmospheric conditions.
问题4：水蒸气如何影响地球变暖？
回答：
Water vapour is considered to be one of the Earth’s most vital greenhouse gas. It constitutes about 90% of the Earth’s natural greenhouse effect. This in turn enables to keep the Earth very warm to support life. Thus, water vapour is crucial to the weather and climate.
问题 5. 解释水蒸气进入大气的不同方式。
回答：
Following are the different ways by which water vapour is put into the atmosphere
- Evaporation of water contained in oceans, lakes and ponds due to atmospheric heat.
- Steam production due to factories and thermal power stations.
- Transpiration of water vapour by plants.
- Excretion of water vapour through animals.
问题 6. 说明水蒸气的任何特征。
回答：
Water vapour molecules are basically water droplets in gaseous form. These molecules are lighter than the molecules of nitrogen and oxygen which constitute to approximately 99% of the atmosphere.