Wind microclimate assessment: who runs the model
Wind assessments for tall buildings are written by specialist wind engineers. Here is how tunnel testing compares with computational modelling, and what a reviewing officer checks first.
A wind microclimate assessment is written by a specialist wind engineering consultancy, not by the project architect or the transport consultant, and on a tall building scheme the choice of who runs the model matters as much as the result. The wind microclimate assessment author needs access either to a boundary layer wind tunnel or to computational fluid dynamics modelling set up for pedestrian level wind, plus long-term meteorological data properly mapped onto the site's terrain. Councils in London and in other high-rise centres routinely ask for the modeller's credentials and for the method statement before they accept the conclusions.
Get the appointment wrong and you buy a report that the council's own reviewer will discount.
When a council asks for wind assessment
The trigger is usually height relative to surroundings rather than an absolute number of storeys. A building that rises well above its neighbours accelerates flow down its windward face and around its corners, and the effects land on pavements, entrances, terraces and any public space at the base.
In London, Policy D9 of the London Plan requires the environmental effects of tall buildings to be assessed, and wind microclimate sits inside that. Outside London, local plan tall buildings policies and design codes carry similar wording, and the City of London's published wind guidelines are widely borrowed by other authorities even where they have no formal status.
Expect the request whenever a scheme includes a building substantially taller than its context, a new public square or roof terrace, a podium with an exposed corner, or a cycle parking entrance on a windward elevation. A Wind Microclimate Assessment is also commonly asked for where an earlier consent is being amended and the massing has changed.
Wind tunnel testing against computational modelling
Both methods are legitimate. They answer slightly different questions and they cost very different amounts.
A boundary layer wind tunnel test uses a physical model, typically at a scale around 1:300, of the proposal and the surrounding context within a radius of several hundred metres, mounted on a turntable and tested across the full compass in increments of 10 or 15 degrees. Probes at pedestrian height record wind speeds, which are then combined with long-term wind statistics from a representative meteorological station.
Computational fluid dynamics solves the flow numerically instead. Steady-state Reynolds-averaged models run fast at low cost and tend to under-predict the peaks around corners and in passages. Large eddy simulation handles those transient effects better and costs considerably more time to run.
| Question | Wind tunnel | Computational modelling |
|---|---|---|
| Accepted for a tall building at application stage | Usually, and often expected | Sometimes, with justification of the setup |
| Early design iteration | Slow and expensive per variant | Fast, useful for comparing options |
| Transient gusts and corner peaks | Measured directly | Depends heavily on the turbulence model |
| Auditability | Physical model, recorded runs | Mesh, solver settings and convergence data |
The practical pattern on a large scheme is computational modelling during concept design to compare massing options, then a wind tunnel test of the preferred scheme for the application. Where a council has published guidance preferring the tunnel for buildings above a stated height, follow it rather than arguing.
Comfort and safety criteria in common use
The Lawson comfort criteria are the standard in the United Kingdom. They classify locations by the activity they are intended to support, from sitting through standing and strolling to business walking, and each category has a threshold wind speed that must not be exceeded more than a set percentage of the time.
Two things follow from that structure. First, the assessment is only as good as the activity map, so the design team has to tell the modeller what each area is for before the runs happen. A space labelled as a thoroughfare when it is actually the entrance to a cafe terrace will pass on paper and fail in use.
Second, comfort and safety are separate tests. A location can meet the comfort criterion for walking and still fail the distress or safety threshold, which looks at strong infrequent gusts rather than typical conditions. A safety exceedance in a public area is the one result that will stop a scheme, because no amount of soft landscaping fixes it.
Seasonal reporting matters too. Summer and winter results differ, and a terrace that works in July and is unusable in January should be described that way rather than averaged into an annual figure.
Inputs, wind roses and surrounding context
Three inputs decide whether a result is credible.
The meteorological record comes from a long-term station chosen for its representativeness, and is corrected for terrain roughness between the station and the site. State which station, which period and what corrections were applied. A model driven by a short record or by an airport station with no terrain correction is not defensible.
The context model has to include consented but unbuilt schemes as well as what exists today. A neighbouring tower with permission will change your results, and the council will ask about it. Run the assessment for both the existing and the future context where a cumulative case exists.
The proposal itself must be modelled with the landscaping and screening that are actually being applied for. Trees deserve care. A modeller who assumes mature canopies gives you a result you cannot deliver for 15 years, and deciduous planting does little in the winter months when the wind is strongest.
Mitigation that changes the architecture
Wind mitigation applied late is expensive and usually ugly. Applied early it is invisible.
Form-based measures come first: chamfered or rounded corners, setbacks at the podium, breaking the windward face rather than presenting a flat slab, and orienting the main entrance away from the prevailing direction. Then come built elements such as canopies over entrances, colonnades, porous screens and solid balustrades to terraces. Planting comes last, because it is seasonal, slow and dependent on maintenance.
Bring the wind consultant into the design team before the massing is fixed. Reviewing results after planning submission means either accepting conditions you cannot discharge or redesigning under time pressure. The same logic applies to the Design and Access Statement, which should explain the wind-driven design moves rather than presenting them as arbitrary sculptural decisions.
Competence, review and what the council checks
There is no statutory licence for wind modelling and no protected title, so competence is demonstrated by track record and by method rather than by a certificate. What a reviewing officer or their consultant looks for is a named author, a described method, the meteorological source, the context radius, the criteria applied, the activity map, and a clear statement of any location that fails.
Ask for a sample report before appointing. Check that it reports failures plainly rather than burying them, that it distinguishes comfort from safety, and that the mitigation it recommends is drawn rather than described. Where the council appoints its own reviewer at the applicant's cost, that reviewer will read the assumptions first and the conclusions second.
If you are commissioning several specialists at once, the professional review route is worth understanding early, because a draft that a competent reviewer can sign is worth more than a draft nobody will put their name to.
Conditions securing post-completion checks
Councils increasingly attach conditions requiring wind mitigation to be installed before occupation and retained, and sometimes requiring a post-completion verification exercise on site.
Read those conditions before permission is issued. A condition requiring measured verification against the modelled results is reasonable on a large public space scheme and unworkable on a small one, and the time to say so is during determination. Where mitigation depends on planting, expect a landscape management condition running for several years, with replacement of failed stock.
But the best protection is still the early appointment. A wind consultant engaged at concept stage changes three or four decisions that would otherwise cost a redesign.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.
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Frequently asked questions
Who is qualified to write a wind microclimate assessment?
There is no statutory licence or protected title. In practice the work goes to specialist wind engineering consultancies with either boundary layer wind tunnel facilities or established computational fluid dynamics capability for pedestrian level wind. Competence is shown by method, by the meteorological treatment and by track record on comparable schemes, so ask for a sample report before appointing.
Is computational modelling acceptable instead of a wind tunnel?
Often yes for early design comparison, and sometimes for application stage where the setup is justified. Steady-state models tend to under-predict peaks at corners and in passages, so large eddy simulation is preferred where transient effects matter. Where a council's published guidance prefers tunnel testing above a stated building height, follow that guidance.
What are the Lawson comfort criteria?
A classification that matches wind conditions to intended activity, running from sitting through standing and strolling to business walking, with a threshold wind speed for each that must not be exceeded more than a set proportion of the time. A separate distress or safety criterion deals with strong infrequent gusts. Comfort and safety are assessed and reported separately.
Do consented but unbuilt neighbouring buildings need modelling?
Yes, where they exist. A council will ask how the scheme performs in the future context as well as the existing one, because a neighbouring consented tower can change flow at your entrances. Run both cases and report them separately. Leaving out a known consent is one of the quickest ways to have an assessment sent back.
Can planting be relied on as wind mitigation?
Only with care. Trees take years to reach the modelled canopy size, deciduous species do little in winter when wind speeds are highest, and the benefit depends on maintenance. Form-based measures and built elements such as canopies, colonnades and porous screens are more reliable. Planting works best as a supplement rather than the principal measure.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.