For unsaturated air (relative humidity less than 100%), how does the dew-point temperature compare with the wet-bulb temperature?

Introduction / Context:
Interpreting psychrometric measurements requires knowing the order of characteristic temperatures. In unsaturated air, these temperatures follow a consistent inequality that aids quick checks and diagnostics.

Given Data / Assumptions:

• Air–water vapour mixture at RH < 100%.
• Proper instrument use and steady conditions.

Concept / Approach:
For unsaturated air, the standard relationship is: dew-point temperature (DPT) < wet-bulb temperature (WBT) < dry-bulb temperature (DBT). Dew point is the temperature at which saturation just occurs for the existing moisture content; wet-bulb reflects adiabatic evaporation potential; dry-bulb is the actual air temperature.

Step-by-Step Solution:

Verification / Alternative check:
Plot a point inside the psychrometric chart’s field (not on the saturation curve): lines of constant DPT lie left of the state point, and constant WBT lines intersect with WBT greater than DPT but less than DBT.

Why Other Options Are Wrong:

Equality holds only at saturation (RH = 100%).“Greater than” contradicts established ordering in unsaturated air.“Unrelated” is incorrect; the values are physically linked.

Common Pitfalls:
Instrument lag or poor ventilation around the psychrometer wick can distort WBT readings; still, the theoretical order remains DPT < WBT < DBT when RH < 100%.

Final Answer:

Less than the wet-bulb temperature