A small difference between the readings of the wet and dry bulb thermometers is an indication of

Analyze the question: The wet and dry bulb thermometers together make up a hygrometer (psychrometer). The question asks what a small difference between their readings tells us.

Explain the concept:

• The dry bulb measures the actual air temperature.
• The wet bulb has a wet muslin wick around it. Water evaporates from the wick and cools the bulb, so it reads lower than the dry bulb.
• How much evaporation happens depends on how dry the air is: • Dry air takes up lots of moisture, so evaporation is fast. The wet bulb cools a lot, giving a large difference and low relative humidity. • Moist air is already near its moisture limit, so evaporation is slow. The wet bulb cools only a little, giving a small difference and high relative humidity. • Saturated air (100% RH) allows no net evaporation, so the difference is zero.

Solution:

• A small difference means little evaporation, so the air is already holding a lot of water vapour. That means high relative humidity.
• Option A (low RH) is wrong because that would give a large difference.
• Option B (condensed air) is not a proper meteorological term.
• Option C (saturated air) would need the difference to be zero, not just small.
• Option D (high relative humidity) fits a small difference.

Answer: D. high relative humidity

Tip: Remember "Small gap = Sticky air". A small gap between the thermometers means humid, sticky air. A big gap means dry air. If the gap is zero, the air is saturated.