At dew point. RH is

Options

A. `10%`
B. `20%`
C. `50%`
D. `100%`

Answer

To solve the question "At dew point, what is the relative humidity (RH)?", we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Dew Point**:
- The dew point is defined as the temperature at which air becomes saturated with water vapor. This means that the air can hold no more moisture at this temperature.

2. **Understanding Relative Humidity (RH)**:
- Relative Humidity is the ratio of the current amount of water vapor in the air to the maximum amount of water vapor the air can hold at a given temperature. It is expressed as a percentage.
- Mathematically, it can be expressed as:
\[ RH = \left( \frac{P_{actual}}{P_{saturation}} \right) \times 100 \]
where \( P_{actual} \) is the actual vapor pressure and \( P_{saturation} \) is the saturation vapor pressure.

3. **Condition at Dew Point**:
- At the dew point, the air is fully saturated with water vapor. This means that the actual vapor pressure equals the saturation vapor pressure.
- Therefore, at the dew point:
\[ P_{actual} = P_{saturation} \]

4. **Calculating Relative Humidity at Dew Point**:
- Substituting the condition of saturation into the relative humidity formula:
\[ RH = \left( \frac{P_{saturation}}{P_{saturation}} \right) \times 100 = 1 \times 100 = 100\% \]

5. **Conclusion**:
- Thus, at the dew point, the relative humidity (RH) is 100%.

### Final Answer:
At dew point, the relative humidity (RH) is **100%**.

---