Angle of geostrophic wind with the coastline

The most intensive Coastal Convergence by friction is induced when the wind blows parallel or at small angle onshore, the lower pressure being to the left of the wind component parallel to the coast. In the left image, the low-level wind streamlines form a small angle with the Dutch coastline

Low-level convergence

Convergence in the lowest layers in the atmosphere along the coast effects the vertical velocity field. Convergence-divergence on the coast. Resulting vertical velocity at low levels.

Potential instability and instability of an airmass can be recognised with radiosonde data.

Radio soundings show the unstable stratification of the atmosphere.

The effect of Coastal Convergence on vertical velocity requires unstable conditions. Especially crucial is sufficient instability in the lowest layers of the atmosphere. In unstable conditions (e.g. during cold air outbreaks over relatively warm sea) vertical motion caused by frictional convergence is enhanced, whereas in stable conditions the vertical motion induced by Coastal Convergence is generally restricted to low levels.

Larger scale ascending and descending motions may disturb and even mask vertical motion caused by frictional convergence.