Friction, convergence & vertical motion

Why does surface wind cross the isobars?

Let friction change the speed, then reconsider the force balance.

Less speed means less Coriolis

Surface drag and turbulent momentum transfer reduce wind speed in the friction layer. The weaker wind has weaker Coriolis acceleration, so the pressure-gradient force is no longer balanced by Coriolis alone. Including drag produces a steady flow that crosses the isobars towards lower pressure. The surface wind is normally slower than the free-air wind and has an inward component towards a low in either hemisphere.

In the Northern Hemisphere, descending from geostrophic flow towards the surface normally brings a decrease in speed and a backing of wind direction, meaning the direction turns anticlockwise. Climbing through the layer normally brings increasing speed and veering, a clockwise change. In the Southern Hemisphere the directional changes reverse: surface wind veers relative to the free wind, and the wind generally backs with height while increasing in speed.

These are approximate tendencies for a friction-controlled layer. Thermal wind changes, fronts, terrain and low-level jets can alter the observed profile. Do not treat the rule as a guarantee that every measured wind must turn the same way with height. First identify whether the comparison is a friction-layer problem and whether the hemisphere has been specified.

What surface wind follows from a geostrophic wind?

Use the examination percentages and turn the direction towards the low.

State the prescribed approximation

For the EASA examination approximation over water, surface wind speed is about 70% of geostrophic speed and the wind crosses the isobars towards lower pressure at about 10°. Over land, use about 50% and 30°. The stated landscape changes both the speed and the crossing angle. These values are calculation conventions, not an observational forecast for every coastline or airport.

For Northern Hemisphere geostrophic wind 270°/40 kt, the land approximation gives 240°/20 kt: speed halves and the from-direction backs 30°. Over water it gives 260°/28 kt. In the Southern Hemisphere the same geostrophic from-direction gives a surface direction of 300° over land and 280° over water, because the turn towards the low is opposite. Keep wind from-directions throughout the calculation.

To reverse the problem, divide surface speed by the appropriate percentage, rather than adding the missing percentage to it. A 20 kt land wind corresponds to 20/0.5 = 40 kt geostrophic speed under the rule. A 28 kt water wind corresponds to 28/0.7 = 40 kt. Reverse the directional turn as well and wrap directions through 360° when necessary.

Why does the same airport have different winds by day and night?

Connect roughness and stability to the amount of turbulent mixing.

The layer is not a fixed thickness

Terrain roughness, wind speed and atmospheric stability are the main controls on the vertical extent of the friction layer. Rough ground and strong flow generate mechanical turbulence; daytime surface heating creates buoyant mixing. These processes transport the surface influence through a deeper layer. Smooth water, weak flow and stable stratification can limit vertical exchange. The depth may range from a shallow stable layer to a much deeper daytime mixed layer; a single memorised height is unreliable.

Over land on a fair-weather day, convective mixing commonly brings faster-moving air downwards from aloft, increasing surface wind and gustiness during the afternoon. At night, radiative cooling can create a stable layer that decouples the surface from stronger flow above. The near-surface wind then often weakens and the vertical shear increases. With a Northern Hemisphere friction-controlled profile, the daytime surface direction often veers towards the wind aloft and backs again as mixing weakens after sunset; the directional sense reverses in the south.

Do not confuse a weak surface wind with weak wind at circuit height. A nocturnal inversion may have calm air near the runway and a much stronger low-level flow above it. Strong mechanical forcing, cloud or advection can modify the usual daily cycle. Over the sea the surface temperature changes less through the day, so the thermal daily modulation is often smaller than over land.

How does horizontal convergence support ascent?

Use continuity through a layer rather than treating cloud as an automatic property of every low.

Horizontal inflow and outflow

Convergence is a net horizontal inflow of air into a region; divergence is net horizontal outflow. Streamlines can converge as a channel narrows, or flow can slow downstream so that more mass enters a region than leaves it horizontally. Conversely, spreading flow or downstream acceleration can contribute to divergence. Directional geometry and speed must both be considered; two arrows pointing almost parallel do not prove zero convergence.

Persistent low-level convergence needs a vertical outlet, so it commonly supports ascent above the boundary layer. Near-surface friction makes wind spiral into a low and out of a high, with ascent commonly associated with the low and subsidence with the high. If rising air is moist and cools to saturation, cloud and precipitation may develop; dry or strongly capped air can show a different result. Ascent is a mechanism, while saturation and stability determine the weather response.

Upper-level divergence can support rising air from below; upper-level convergence is commonly associated with descent. The pressure tendency depends on the net mass change of the whole column. An upper outflow exceeding the lower inflow removes mass and can deepen a surface low. If inflow and outflow balance, a circulation can persist without a pressure fall. It is too simple to say that any upper divergence must always reduce surface pressure by itself.

Can you apply the ideas?

Four original questions. Decide from the facts, then review the explanation.