Terrestrial Stilling

Terrestrial stilling means a long-term decline in the average speed of winds near the ground over land. It describes a weakening of near-surface atmospheric flow, not the disappearance of wind or a necessary weakening of winds throughout the atmosphere.

What the term measures

Stilling concerns trends in surface wind speed, rather than a temporary calm spell or a change in wind direction.

• Surface wind observations are conventionally made at 10 metres above ground, with suitable exposure to minimise local obstructions.
• The trend is not uniform across all land regions, and its direction can change over time.
• Near-surface winds are affected by friction and obstacles, so they must be distinguished from winds higher in the atmosphere.

Why near-surface winds can weaken

Two important influences are changes in large-scale atmospheric circulation and changes in surface roughness.

• Changes in pressure gradients and circulation patterns can alter the strength of winds reaching a region.
• Buildings, trees and other obstacles increase drag and can reduce wind speeds close to the ground.
• The relative importance of these influences varies by location; terrestrial stilling should not automatically be attributed to a single cause.

Why the distinction matters

Weaker near-surface winds can reduce ventilation and pollutant dispersal. Holding other conditions constant, they can also reduce evaporative demand by slowing the removal of moisture from above a surface.

• A surface-wind trend alone does not establish the trend at wind-turbine height, where wind conditions may differ.