How does the dew point spread influence fog formation and icing risk?

How does the dew point spread influence fog formation and icing risk?

• A. Spread has no effect on fog.
• B. Large dew point spread indicates high fog risk.
• C. Small dew point spread indicates high humidity and potential fog or frost.
• D. Any spread guarantees icing.

Answer: (C)

Dew point spread is the difference between the air temperature and the dew point. When this spread is small, the air is very close to saturation, so relative humidity is high. In that situation, a little cooling or added moisture can push the air to 100% relative humidity, which leads to condensation and the formation of fog near the surface. If the air temperature is at or below freezing, that same high moisture can frost surfaces instead of forming fog, or produce frost as a coating on wings and sensors. Fog itself implies a lot of water vapor present in the air, which also raises the potential for icing if you fly through it—especially when the temperature is near freezing and droplets are supercooled. Not every fog will icing, and icing depends on factors like droplet size and exact temperature profile, but a small dew point spread signals high humidity and the conditions under which fog or frost (and the associated icing risk) are most likely. In contrast, a large dew point spread means the air isn’t near saturation, so fog formation is unlikely, and the icing risk from fog is correspondingly lower.

Dew point spread is the difference between the air temperature and the dew point. When this spread is small, the air is very close to saturation, so relative humidity is high. In that situation, a little cooling or added moisture can push the air to 100% relative humidity, which leads to condensation and the formation of fog near the surface. If the air temperature is at or below freezing, that same high moisture can frost surfaces instead of forming fog, or produce frost as a coating on wings and sensors.

Fog itself implies a lot of water vapor present in the air, which also raises the potential for icing if you fly through it—especially when the temperature is near freezing and droplets are supercooled. Not every fog will icing, and icing depends on factors like droplet size and exact temperature profile, but a small dew point spread signals high humidity and the conditions under which fog or frost (and the associated icing risk) are most likely.

In contrast, a large dew point spread means the air isn’t near saturation, so fog formation is unlikely, and the icing risk from fog is correspondingly lower.