Anna Answers: Why have we seen fog frequently this summer?

Weather Wisdom

Question: Why have we had fog so many nights this summer?

Answer: Here in the Twin Tiers we commonly see radiational fog. Since there are valleys here, a prevalent form of radiational fog we see here is valley fog.

Radiational fog forms when the air right at the surface cools as the sun sets and the dewpoint reached the temperature. As the air cools it also becomes more stable and the air cools and stabilizes, so it causes the air to become saturated. Once the air is saturated, the fog will form. This first starts at the ground surface level and can spreads as the air further above the ground cools and stabilizes. This frequently happens in valleys since it is sheltered from the wind. Winds considered less than about 10-15 MPH are considered light to prevent the dry air and the moist air from mixing and disrupting the fog.

Valley fog, which is a type of radiation fog, forms in valley areas where air along the ridges of hills cools after sunset. The air becomes too dense and sinks to the valley floor and cools. Valley fog forms as the sun sets-with the decrease of radiation from the sun, the temperature in the air cools. When the temperature is close to or identical to the dew point it causes the fog to form. If the air is cool and moist at the surface and there also are clear skies and light winds, it helps create the fog. The fog is sometimes so dense it can be seen on satellite! It also prevalently forms when the ground is saturated from rain. This summer we have had a lot of rain helping aid in the formation of this fog.

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