Wind microclimate assessment and the Lawson criteria

A wind microclimate assessment answers a narrow question: at each place where people will stand, sit or walk around your building, how often will the wind be strong enough to spoil that activity or knock somebody over. The Lawson criteria are the framework almost every English assessment uses to answer it, mapping pedestrian activities onto threshold wind speeds and reporting how often each threshold is exceeded.

Councils rarely have a policy setting numerical wind standards. They rely instead on the assessment itself, which puts a lot of weight on how honestly the receptor locations were chosen.

When a wind assessment is requested

Three triggers in practice.

Height relative to surroundings. A building noticeably taller than its neighbours accelerates flow down its windward face and around its corners, and tall building policies in local plans and in the London Plan expect a wind assessment as part of the design evidence.

New outdoor space. A scheme creating a square, a through route, a roof terrace or an outdoor seating area invites the question, because the scheme is proposing the activity as well as the building.

Sensitive receptors nearby. A bus stop, a school entrance, a level crossing of a cycle route or an existing park edge can put a wind assessment on the validation list at heights below the usual tall building thresholds.

Ask at pre-application. Being told at week six of determination that a wind tunnel study is needed costs three months, because model construction alone takes weeks.

The comfort and safety criteria and how they are read

The Lawson framework, in the form most commonly used in England, classifies locations by the activity intended there. Each category carries a threshold wind speed, rising in roughly 2 m/s steps from long duration sitting, through short duration sitting and standing, to strolling and finally business walking. A location passes for a given activity if its threshold is exceeded no more than about 5 per cent of the time.

Safety is assessed separately, using a criterion based on strong gusts, expressed as an equivalent mean speed that should not be exceeded more than a stated number of occasions per year. Failing the safety criterion anywhere accessible to the public is a refusal-grade finding, and no amount of comfort category argument fixes it.

Read the results the right way round.

That last row is where assessments get criticised. A result table showing every location as suitable, with no comparison to the existing condition, invites the question of what was left out.

Wind tunnel testing against computational modelling

Boundary layer wind tunnel testing uses a physical scale model of the proposal and its surroundings, rotated through wind directions, with sensors at the receptor points. Computational fluid dynamics solves the flow numerically.

Both are accepted. The choice usually turns on three things: whether the local authority has a stated preference, the complexity of the geometry, and programme.

Wind tunnel work remains the default for tall and complex schemes in London and several other cities, partly because the method is long established and partly because reviewers can inspect the model. It handles separated flow, vortex shedding and corner acceleration without the modelling assumptions that computational work depends on.

Computational modelling is faster to iterate, which makes it useful during design development, and it produces the coloured plan drawings that communicate results to a committee far better than a table of probe readings. Its weakness is turbulence modelling: a steady-state solution can under-predict gustiness, and the safety criterion depends precisely on gusts.

A sensible programme uses computational work to shape the massing and a wind tunnel study to verify it. Where only one is affordable, ask the council which it will accept before commissioning.

Receptor locations that matter

The receptor schedule is the part of the Wind Microclimate Assessment that determines whether the conclusions are worth anything.

Include every entrance, including service and refuse doors that staff use daily. Include balconies and roof terraces, which are frequently the worst locations in a tall scheme and the ones most often omitted. Include the bus stop, the taxi drop-off, the crossing point, the cycle stands and the seating shown on the landscape drawings. Include the neighbouring pavement, since the effect does not stop at the red line.

Then add the places the design does not intend people to go but they will anyway: the sheltered corner people smoke in, the short cut across the podium.

Where the Landscape Strategy shows outdoor eating, planting or play, the wind results and the landscape drawings must describe the same places. Reviewers cross-check them, and a terrace shown with café tables and reported as suitable for business walking only is an obvious inconsistency.

Mitigation through massing and landscape

Mitigation ranks in a clear order of effectiveness.

1. Massing changes: setbacks at podium level, breaking a continuous facade, reducing the height of the windward corner, chamfering or rounding corners.
2. Permanent built elements: canopies over entrances, colonnades, solid screens and baffles at the worst corners.
3. Porous screens and pergolas, which reduce speeds without creating a new accelerating edge.
4. Planting: evergreen where year-round performance is needed, with enough soil volume to reach the modelled size.

Trees are the mitigation most often proposed and the least reliable. A deciduous canopy delivers nothing in January, when wind speeds are highest, and a specimen tree modelled at mature size may take fifteen years to get there. If planting is doing real work in the assessment, model it at planting size as well as at maturity, and say what happens in the interim.

Downstands, screens and canopies are also design elements that a heritage or design officer may resist. Resolve those two conversations together.

Presenting results to a planning committee

Members do not read probe tables. What lands is a plan of the ground floor and each terrace, coloured by suitable activity, shown before and after mitigation, with the failing points named in plain words: the corner by the bus stop, the north entrance.

Say what changes for the existing pavement outside the site, because that is what neighbours ask about. And answer the safety question in one sentence rather than leaving it inside an appendix.

Where daylight and wind evidence both come to committee, keep the receptor language consistent between the Daylight and Sunlight Assessment and the wind study. Two documents describing the same terrace by different names is a small thing that makes both look careless.

Conditions requiring post-completion checks

Councils sometimes condition a post-completion verification, requiring measurement once the building is occupied and remedial measures if results differ from prediction. These conditions are difficult to write well. Wind measurement over a period long enough to be statistically meaningful takes a year or more, and the remedy for a failure is a physical alteration that may need permission of its own.

A more workable condition secures the mitigation itself: the canopy as drawn, the screen specification, the planting schedule with a replacement clause, and a restriction on removing any of it without approval.

For specialist technical reports, qualified professional input may be required. Wind microclimate work is normally carried out by a specialist consultancy, and councils usually expect that.

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