Atmospheric moisture concepts: Which statements regarding absolute humidity, relative humidity, and measurement methods are correct?

Introduction / Context:
Atmospheric moisture parameters are foundational in hydrology, irrigation scheduling, and weather analysis. Understanding absolute and relative humidity along with practical measurement approaches helps interpret evapotranspiration and rainfall processes.

Given Data / Assumptions:

• Standard thermodynamic definitions of humidity metrics.
• Air assumed to be a mixture of dry air and water vapour.
• Psychrometry used in field practice for humidity estimation.

Concept / Approach:
Absolute humidity is defined as mass of water vapour per unit volume of air (e.g., g/m^3). Relative humidity (RH) is the ratio of actual vapour pressure to saturation vapour pressure at the same temperature; equivalently, RH can be expressed as the ratio of actual to saturation mixing ratios or densities under the same conditions. Psychrometers (dry-bulb/wet-bulb thermometers) infer humidity from wet-bulb depression.

Step-by-Step Solution:
Recognize absolute humidity as a volumetric measure of vapour content.Define RH via vapour pressures: RH = e / e_s at the same T.Confirm measurement: sling or Assmann psychrometers provide RH through psychrometric charts or equations.

Verification / Alternative check:
Cross-check with dew point and temperature to compute RH; hygrometers (resistive/capacitive) are modern alternatives, but psychrometer remains a reference method.

Why Other Options Are Wrong:
Each individual statement (a–d) is correct; hence the combined correct choice is 'All the above.'

Common Pitfalls:
Confusing specific humidity, mixing ratio, and absolute humidity; using RH without specifying the temperature; instrument exposure errors affecting readings.

Final Answer:
All the above.