Inside a Stevenson screen at a meteorological station, a dry-bulb thermometer reads 28^\ci… — JAMB UTME

Inside a Stevenson screen at a meteorological station, a dry-bulb thermometer reads $28^\circ\text{C}$ while a wet-bulb thermometer reads $22^\circ\text{C}$ , yielding a wet-bulb depression of $6^\circ\text{C}$ . Which atmospheric variable is determined using this temperature difference, and which weather instrument would be mistakenly selected if an observer attempted to measure this variable by recording atmospheric pressure instead?

Relative humidity is determined from the temperature difference; a barometer would be mistakenly selected because it measures atmospheric pressure.Answer
B
Atmospheric pressure is determined from the temperature difference; a hygrometer would be mistakenly selected because it measures humidity.
C
Wind speed is determined from the temperature difference; an anemometer would be mistakenly selected because it measures wind direction.
D
Solar radiation intensity is determined from the temperature difference; a Campbell-Stokes recorder would be mistakenly selected because it measures sunshine duration.

Answer

Relative humidity is derived from the depression of the wet-bulb thermometer (

28^\circ\text{C} - 22^\circ\text{C} = 6^\circ\text{C}

) using psychrometric tables. Mistakenly using a barometer targets atmospheric pressure instead of atmospheric moisture.

Relative humidity is the ratio of actual water vapour present in the air to the maximum amount the air can hold at that temperature. It is calculated using the depression of the wet bulb (the difference between dry-bulb and wet-bulb thermometer readings). A barometer is designed specifically to measure atmospheric pressure, so choosing a barometer to assess atmospheric humidity represents an instrument-parameter mismatch.

Step-by-Step Solution

Identify the weather element evaluated by a wet-and-dry bulb psychrometer.

The dry-bulb thermometer records ambient air temperature (

28^\circ\text{C}

), while evaporation from the moist muslin sheath lowers the wet-bulb temperature to

22^\circ\text{C}

The rate of evaporation and resulting cooling (wet-bulb depression of

6^\circ\text{C}

) depends directly on the relative humidity of the air.

Identify the mistaken instrument association.

A mercury or aneroid barometer measures atmospheric pressure (force exerted per unit area by the atmosphere).

Selecting a barometer when intending to measure relative humidity confuses pressure measurement with atmospheric moisture measurement.

Key Concept

Weather Elements and Instruments: Hygrometer/Psychrometer for Relative Humidity vs. Barometer for Atmospheric Pressure

Estimated Time:1m 30s

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