When are nocturnal jet maximum wind speeds typically observed?

When are nocturnal jet maximum wind speeds typically observed?

• A. 2 to 3 hours after midnight.
• B. At sunset.
• C. Mid-afternoon.
• D. Just before dawn.

Answer: (A)

The key idea here is the nocturnal low-level jet, which forms when the surface cools after sunset and a shallow, stable boundary layer develops. Friction near the ground slows the surface winds, while the layer slightly above remains more strongly influenced by winds aloft. This decoupling concentrates momentum in a narrow vertical region, creating a peak in wind speed a few hundred meters above the surface. That peak tends to reach its maximum in the pre-dawn hours. By around two to three hours after midnight, the inversion is well established and mixing is minimal, so the jet can intensify before sunrise. Once the sun rises and surface heating stirs the boundary layer, the jet is eroded and speeds decline. So the typical timing for the strongest nocturnal jet winds is about two to three hours after midnight.

The key idea here is the nocturnal low-level jet, which forms when the surface cools after sunset and a shallow, stable boundary layer develops. Friction near the ground slows the surface winds, while the layer slightly above remains more strongly influenced by winds aloft. This decoupling concentrates momentum in a narrow vertical region, creating a peak in wind speed a few hundred meters above the surface.

That peak tends to reach its maximum in the pre-dawn hours. By around two to three hours after midnight, the inversion is well established and mixing is minimal, so the jet can intensify before sunrise. Once the sun rises and surface heating stirs the boundary layer, the jet is eroded and speeds decline.

So the typical timing for the strongest nocturnal jet winds is about two to three hours after midnight.