Why does the amount of water vapour decreases rapidly with altitude?
The amount of water vapour decreases rapidly with altitude due to two main reasons.
First, the ability of air to hold water vapour depends entirely on its temperature. As temperature decreases with increasing altitude, the capacity of air to absorb and retain moisture also decreases significantly.
Second, the moisture in the atmosphere is derived from water bodies through evaporation and from plants through transpiration. These primary sources of water vapour are located at or near the Earth's surface. Therefore, water vapour is most abundant near the surface and diminishes as one moves away from these sources towards higher altitudes.
Explanation
The textbook context clearly establishes two key facts: (1) 'The ability of the air to hold water vapour depends entirely on its temperature' and 'Increase in temperature increases water absorption and retention capacity of the given parcel of air' - this explains why cooler upper altitudes hold less moisture; (2) 'The moisture in the atmosphere is derived from water bodies through evaporation and from plants through transpiration' - these sources are surface-based, so water vapour concentration naturally decreases with distance from the source.