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During periods of cloudless weather over land in the northern hemisphere the…

  • A
    surface wind speed tends to be highest at night
  • B
    surface wind speed tends to be highest during the mid-afternoon
  • C
    angle between isobars and surface wind direction tends to be greatest in the mid-afternoon
  • D
    wind tends to back from early morning until early afternoon

THERMAL TURBULENCE/CONVECTION

Insolation gives rise to convection currents – which will cause mixing of the slower surface winds with the faster winds moving freely above the friction layer, giving an overall increase in the surface winds.

  • By day the presence of thermal currents will tend to reduce low level stability and hence increase the depth of the friction layer.
  • At night there is only mechanical turbulence so the stability will tend to increase because of the surface cooling and the depth of the friction layer will reduce.
So flight within the friction layer on a sunny day will be affected by variable speed vertical currents and hence wind shear giving turbulence. Thermal turbulence (and winds) is greatest around 1500 hrs on clear sunny days on anticyclonic conditions.

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  • Norway
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