Examkin

Question

Difficulty: HardWeather Elements, Instruments, and Measurements

A weather observer at an agricultural station measures atmospheric moisture using a dry-and-wet bulb psychrometer. The dry-bulb thermometer reads 32∘C32^\circ\text{C} while the wet-bulb thermometer reads 24∘C24^\circ\text{C}. If the wet-bulb muslin sleeve dries out completely and is not re-moistened, what will happen to the wet-bulb temperature reading, and what parameter does the psychrometer assess when operating properly?

  1. The wet-bulb temperature will rise to 32∘C32^\circ\text{C}, matching the dry-bulb reading; the psychrometer assesses relative humidity via evaporative cooling.Answer
  2. B
    The wet-bulb temperature will drop to 0∘C0^\circ\text{C}, indicating minimum freezing threshold; the psychrometer assesses atmospheric pressure.
  3. C
    The wet-bulb temperature will remain static at 24∘C24^\circ\text{C} due to thermal inertia; the psychrometer assesses solar radiation intensity.
  4. D
    The wet-bulb temperature will fluctuate with ambient wind velocity; the psychrometer assesses wind speed and direction.

Answer

The wet-bulb temperature will rise to 32∘C32^\circ\text{C} to equal the dry-bulb temperature because evaporation ceases; the psychrometer measures relative humidity.
The lower temperature on a wet-bulb thermometer is caused by evaporative cooling taking heat away from the bulb. When the muslin wick dries completely, evaporation stops and the thermometer bulb absorbs heat from the surrounding air until it reaches thermal equilibrium with the dry-bulb thermometer (32∘C32^\circ\text{C}). A wet-and-dry bulb psychrometer is designed specifically to measure relative humidity by using the temperature difference (depression of the wet bulb).

Step-by-Step Solution

1
Analyze the mechanism of a wet-bulb thermometer
The lower reading (24∘C24^\circ\text{C}) on the wet bulb is maintained by sensible heat loss due to water evaporating from the wet muslin sleeve into surrounding air.
Evaporation requires latent heat, which is extracted from the thermometer bulb, lowering its temperature relative to the dry bulb.
2
Determine the effect of removing moisture from the muslin sleeve
When the muslin dries completely, evaporation stops entirely. With no heat loss occurring, sensible heat transfers from the air to the bulb until equilibrium is reached at 32∘C32^\circ\text{C}.
Both dry-bulb and dry wet-bulb thermometers exposed to the same ambient air will indicate the actual air temperature.
3
Identify the primary atmospheric parameter measured by a psychrometer
A psychrometer (hygrometer) measures relative humidity and dew point based on the wet-bulb depression (32∘C−24∘C=8∘C32^\circ\text{C} - 24^\circ\text{C} = 8^\circ\text{C}).
The rate of evaporation and resulting depression depend directly on the moisture saturation of the atmosphere.

Key Concept

Psychrometer Mechanics and Relative Humidity Measurement
Rate this question

Topics

Weather Elements, Instruments, and Measurements

More questions from this topic