Q.Why does the amount of water vapour decrease rapidly with altitude?

The amount of water vapour in the atmosphere decreases rapidly with altitude primarily because air temperature drops significantly with increasing height. As altitude increases, atmospheric pressure decreases, causing the air to expand and cool through a process called adiabatic cooling. Colder air has a much lower capacity to hold water vapour compared to warmer air. Consequently, as air rises and cools, any excess water vapour condenses into clouds and eventually falls as precipitation, effectively removing it from the atmosphere at higher levels. This leads to the concentrati …

Water vapour, an invisible gaseous form of water, is a crucial component of our atmosphere, playing a vital role in weather and climate. However, its distribution is far from uniform; it is most abundant near the Earth's surface and diminishes sharply as one ascends into higher altitudes. This rapid decrease can be understood by considering several interconnected atmospheric processes.

The fundamental reason for this distribution lies in the primary source of atmospheric water vapour: evaporation from the Earth's surface. Oceans, lakes, rivers, and moist land surfaces continuously release water vapour into the air directly above them. This means the greatest concentration of water vapour is naturally found closest to these sources, which are at ground level.

As with all atmospheric gases, water vapour is subject to Earth's gravitational pull. Gravity exerts a downward force, tending to keep the bulk of the atmosphere, including water vapour molecules, concentrated closer to the planet's surface. While lighter than other major atmospheric gases like nitrogen and oxygen, water vapour molecules are still heavy enough to be significantly affected by gravity, preventing them from easily escaping into the upper atmosphere in large quantities.

Perhaps the most significant factor contributing to the rapid decrease in water vapour with altitude is the change in temperature. In the troposphere, the lowest layer of the atmosphere where most weather occurs, temperature generally decreases with increasing altitude. This is known as the environmental lapse rate. Colder air has a significantly reduced capacity to hold water vapour compared to warmer air.

Consider these points:

• Cooling upon ascent: As air parcels rise, they expand due to lower atmospheric pressure and, in doing so, they cool. This cooling causes the water vapour within them to reach its saturation point more quickly.
• Condensation and Precipitation: Once the air cools sufficiently, the water vapour condenses into liquid water droplets or ice crystals, forming clouds. If these droplets or crystals grow large enough, they fall back to Earth as precipitation (rain, snow, hail). This process effectively removes water vapour from the atmosphere at higher altitudes, preventing it from accumulating there. …