Question

The fundamental principle of evaporation from a free surface is a 'function of the difference in the vapour pressure of water and vapour pressure of air' was given by...................

• A. Dalton
• B. Thronthwaite
• C. Penman
• D. Jensen

Hint:

Remember ______Dalton's equation______ ($E \propto e_s - e_a$).
It proves that wind speed and humidity are the key atmospheric drivers of evaporation, as wind carries away humid air to maintain a high vapor pressure gradient.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Evaporation is the phase transfer of water from liquid to gas.
The physical driving force behind this process is the vapor pressure gradient between the evaporating surface and the atmosphere.
Key Formula or Approach:
The fundamental relationship is expressed by Dalton's Law of Evaporation:
\[ E = C \cdot (e_s - e_a) \]
where $E$ is the evaporation rate, $C$ is a constant incorporating wind speed, $e_s$ is the saturation vapor pressure at the water surface, and $e_a$ is the actual vapor pressure of the air.

Step 2: Detailed Explanation:
In 1802, John Dalton formulated the physical principle governing evaporation from a free water surface.
He stated that the rate of evaporation is directly proportional to the difference between the saturation vapor pressure at the temperature of the water surface ($e_s$) and the actual vapor pressure of the ambient air ($e_a$).
This difference ($e_s - e_a$) is known as the ______vapor pressure deficit (VPD)______.
If the air is saturated ($e_s = e_a$), no evaporation occurs because the gradient is zero.
This fundamental Daltonian equation forms the basis of all modern aerodynamic and combination methods used to calculate evapotranspiration (such as the Penman-Monteith equation).

Step 3: Final Answer:
This fundamental principle of evaporation was given by Dalton, which corresponds to option (A).