The correct option is The temperature is higher than 0°C..
Explanation
Precipitation is the process where condensed water vapour falls to the Earth's surface. The state (liquid or solid) in which this water reaches the ground depends primarily on the temperature of the air layers through which it passes.
Analysis of Options:
- The temperature is lower than 0°C. is Incorrect: When the temperature is lower than 0°C (freezing point), precipitation typically occurs in solid forms such as snow, where water vapour condenses directly into ice crystals.
- The temperature is higher than 0°C. is Correct: Rainfall is defined as precipitation in the form of liquid water droplets. This occurs when the temperature is higher than 0°C, preventing the droplets from freezing into ice before reaching the ground.
- The air is unsaturated. is Incorrect: Precipitation occurs when the air becomes saturated (100% relative humidity) and can no longer hold moisture, leading to condensation. Unsaturated air promotes evaporation rather than precipitation.
- The dew point is below freezing. is Incorrect: If the dew point is below freezing, water vapour converts directly into ice (deposition), resulting in frost or snow rather than liquid rainfall.
Key Takeaway: Rainfall is the liquid form of precipitation, which necessitates an ambient temperature above the freezing point (0°C); temperatures below this threshold result in solid forms like snow or hail.