The correct option is Decrease in air temperature.
Explanation
Condensation is the phase change process where water vapour in the atmosphere transforms into liquid water. This occurs when the air becomes saturated, meaning its relative humidity reaches 100%. The state of saturation is achieved either by adding moisture to the air or by reducing the air's capacity to hold moisture.
Detailed Analysis:
- Decrease in air temperature (Correct): The moisture-holding capacity of air is directly related to its temperature; warmer air can hold more water vapour than colder air. When air temperature decreases, its capacity to hold moisture drops. Once the temperature falls to the dew point, the air becomes saturated, and excess water vapour condenses into liquid form. This cooling mechanism is the most common cause of condensation in the atmosphere (leading to clouds, dew, or fog).
- Increase in air volume keeping temperature constant (Incorrect): Increasing the volume of air while keeping the temperature constant decreases pressure but does not reduce the moisture-holding capacity. Consequently, it does not force the air toward saturation.
- Decrease in pressure with constant temperature (Incorrect): A decrease in pressure alone, without an associated drop in temperature (adiabatic cooling), does not promote condensation. In fact, lower pressure allows water molecules to remain in a vapour state more easily if the temperature remains unchanged.
- Removal of moisture from the air (Incorrect): Removing moisture decreases the specific humidity and relative humidity of the air. This moves the air state further away from saturation, making condensation impossible.
Key Takeaway:
The most favourable condition for condensation is a decrease in air temperature, which lowers the air's moisture-holding capacity until it reaches the dew point.