Fog can form by which method?

• A. Moisture is added to the air
• B. Air is cooled to dew point
• C. Wind is rushed across terrain
• D. Temperature decreases with altitude

Answer: (A)

Fog forms when the air becomes saturated with water vapor, reaching 100% relative humidity. One direct way this happens is by adding moisture to the air—the introduction of water vapor can push the air past its capacity to hold moisture, causing the excess vapor to condense into tiny droplets and create fog. This mechanism is common in practical situations like steam or evaporation fog near bodies of water. While cooling the air to its dew point also leads to saturation and fog, the option that emphasizes moisture addition highlights a straightforward path to condensation. Temperature change with altitude or wind rushing across terrain doesn’t by itself produce fog.