In applied climatology, the relative humidity of a region will be low under which of the following psychrometric conditions?

Introduction / Context:
Relative humidity is an important concept in weather and climate because it affects human comfort, evaporation, and cloud formation. A common instrument used to estimate relative humidity is the psychrometer, which has a dry bulb thermometer and a wet bulb thermometer. This question asks which condition corresponds to low relative humidity, using the readings of these two thermometers as a guide.

Given Data / Assumptions:

• A paired thermometer system is used: one dry bulb and one wet bulb.
• The readings of these two thermometers can be compared.
• We assume that the wet bulb is properly ventilated and supplied with water, as in a standard psychrometer.
• We must relate the size of the difference between readings to the level of relative humidity.

Concept / Approach:
When air is unsaturated and relatively dry, evaporation from the wet bulb is strong. Evaporation removes heat, lowering the wet bulb temperature compared to the dry bulb temperature. This creates a large difference between the two readings, indicating low relative humidity. When air is nearly saturated, evaporation is weak and the wet bulb temperature is close to the dry bulb temperature, so the difference is small or zero. Therefore, low relative humidity is associated with a large wet bulb depression, meaning a large difference between dry and wet bulb readings.

Step-by-Step Solution:
Step 1: Recall that the dry bulb thermometer measures the actual air temperature, while the wet bulb thermometer is cooled by evaporation. Step 2: Understand that stronger evaporation occurs when the air is dry and can absorb more moisture, which lowers the wet bulb temperature significantly. Step 3: This strong cooling causes the wet bulb reading to be much lower than the dry bulb reading, producing a large temperature difference. Step 4: When the air is nearly saturated, evaporation from the wet bulb is weak, so its temperature remains close to the dry bulb temperature, giving a small or zero difference. Step 5: Therefore, a large difference between wet bulb and dry bulb temperatures corresponds to low relative humidity. Step 6: On the other hand, equal readings on both thermometers would indicate 100 percent relative humidity, meaning saturated air.

Verification / Alternative check:
Psychrometric charts and tables are based on the difference between dry bulb and wet bulb temperatures, often called the wet bulb depression. The larger this depression, the lower the relative humidity for a given dry bulb temperature. Weather observers use these charts to convert the observed difference into a percentage humidity value. This standard practice confirms that a large difference in readings signals low relative humidity, while a zero difference signals saturation and high humidity.

Why Other Options Are Wrong:

• When the wet bulb and dry bulb thermometers show the same reading: This means no evaporation cooling and relative humidity at or near 100 percent, which is very high, not low.
• When the air temperatures are high regardless of moisture: High temperature alone does not guarantee low relative humidity; tropical regions can be both hot and humid.
• When the air temperatures are low regardless of moisture: Low temperature by itself does not define humidity; cold air can also be near saturation.

Common Pitfalls:
A frequent mistake is to associate low relative humidity with simply high temperature, ignoring the actual moisture content of the air. Another error is forgetting that the key indicator on a psychrometer is the difference between dry bulb and wet bulb readings, not the absolute values alone. To avoid confusion, remember that low relative humidity is signalled by strong evaporation, a much cooler wet bulb, and therefore a large wet bulb depression.

Final Answer:
Relative humidity is low when the difference between the readings of the wet and dry bulb thermometers is large.