Evolution of the boundary layer - 6PM

6 PM:

  • surface heating diminishes

    • surface temperatures start to drop
    • thermal turbulence shuts off
    • hence, clouds begin to diminish
    • nocturnal boundary starts to develop

  • winds begin to weaken at the surface

  • mechanical turbulence is moderate - weakens with time

Summary: boundary layer thermal and mechanical turbulence intensity depends on:

QUESTION FOR THOUGHT:

A pilot enters the weather service office and wants to know what time of the day she can expect to encounter the least turbulent winds at 760 m above central Kansas. If you were the weather forecaster, what would you tell her?

QUESTION FOR THOUGHT:

Why is the difference in surface wind speed between morning and afternoon typically greater on a clear, sunny day than on a cloudy, overcast day?