Should a wind survey distinguish leaf-on and leaf-off tree conditions?

The short answer

A survey should distinguish leaf-on and leaf-off conditions when nearby vegetation can materially affect the proposed rotor's exposure. The objective is not to assume a universal foliage penalty. It is to avoid treating one seasonal photograph or short measurement period as an accurate representation of every season.

Record the relevant vegetation geometry

Map the tree lines or woodland, their approximate height, distance and direction from each candidate position. Note whether the trees are deciduous and whether the observations were made before or after leaf fall. Preserve photographs with dates and viewing directions.

Also distinguish current vegetation from expected future growth. The assessment should identify which condition its conclusions describe rather than presenting one map as permanently valid.

Compare like conditions in the wind evidence

Examine whether important wind sectors and useful demand coincide with leaf-on or leaf-off periods. A sheltered summer sector can matter greatly for a summer process even if annual wind is strongest in winter. Conversely, not every visible tree line has the same importance in every direction.

Avoid assigning a fixed loss percentage from appearance alone. Ask the assessor what measurements, flow interpretation or sensitivity cases support the vegetation treatment, particularly when the turbine would remain close to substantial obstacles.

Document an assessment update trigger

State when the siting conclusion needs review: substantial growth, planting, removal or a change in the selected location. Record who is responsible for noticing those changes during project development and operation.

If the business case depends on future trimming or removal, establish whether that action is actually feasible before counting the resulting exposure as the baseline. A resource estimate should describe the site the project can use, with realistic vegetation conditions, rather than an unobstructed landscape that exists only in the energy model.