The presence of water vapor in the air, even at temperatures below 100°C (the boiling point of water at sea level), is due to the concept of vapor pressure and the behavior of water molecules.
Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase at a given temperature. It is a characteristic property of a substance and represents the tendency of molecules to escape from the liquid or solid phase and enter the gas phase.
Even at temperatures below the boiling point of water, some water molecules have sufficient energy to overcome intermolecular forces and transition into the gas phase as water vapor. This process is known as evaporation. The rate of evaporation depends on factors such as temperature, surface area, and humidity.
As the temperature increases, the average kinetic energy of water molecules rises, leading to a higher rate of evaporation. However, it is important to note that at any given temperature, there is a distribution of molecular energies. Some water molecules possess energies higher than the average, allowing them to break free from the liquid phase and enter the gas phase as vapor.
Conversely, water vapor in the air can also condense back into a liquid when it encounters surfaces or air masses with lower temperatures. This process is known as condensation. When the rate of condensation exceeds the rate of evaporation, visible moisture, such as dew, fog, or clouds, can form.
In summary, even at temperatures below 100°C, there is water vapor in the air due to the continuous process of evaporation and condensation. The amount of water vapor present in the air at a specific temperature depends on factors like humidity, temperature, and the balance between evaporation and condensation processes.