ANS:B - wet bulb temperature

For water evaporating into unsaturated air under adiabatic conditions and at constant pressure, the wet bulb temperature remains constant throughout the period of vaporization. The wet bulb temperature represents the lowest temperature that can be reached by evaporative cooling in a given environment. During the process of water evaporating into unsaturated air, the wet bulb temperature remains constant because the process occurs adiabatically (no heat exchange with the surroundings) and at constant pressure. The wet bulb temperature is a key concept in meteorology and thermodynamics, representing one of the measures used to assess the humidity of the air. It refers to the lowest temperature that can be achieved by evaporative cooling of a water-saturated surface, such as the surface of a thermometer bulb covered with a wet cloth or wick. Here's how it works:

1. Setup: To measure the wet bulb temperature, a thermometer bulb is covered with a wet cloth or wick. The cloth or wick is saturated with water, allowing for evaporation to occur from its surface. The thermometer is then exposed to the air whose humidity is being measured.
2. Evaporative Cooling: As water evaporates from the wet cloth or wick, it absorbs latent heat from the surrounding air. This process of evaporation cools down the thermometer bulb and, consequently, the mercury or liquid inside the thermometer. The rate of evaporation and cooling depends on the humidity of the air: higher humidity slows down the rate of evaporation, while lower humidity accelerates it.
3. Equilibrium Temperature: Eventually, the rate of evaporation reaches an equilibrium with the rate of condensation back onto the wet cloth or wick. At this point, the temperature of the thermometer bulb stabilizes, reaching the wet bulb temperature. This temperature represents the lowest temperature that can be achieved through evaporative cooling under the given conditions of humidity and air movement.
4. Relation to Humidity: The wet bulb temperature is influenced by the moisture content of the air. Higher humidity levels result in slower evaporation and less cooling, leading to a higher wet bulb temperature. Conversely, lower humidity levels allow for faster evaporation and greater cooling, resulting in a lower wet bulb temperature.
5. Use in Meteorology: Meteorologists use the wet bulb temperature, along with the dry bulb temperature (the regular temperature reading), to calculate various humidity-related parameters such as dew point, relative humidity, and heat index. It provides valuable information about the moisture content and cooling potential of the air.