Wind microclimate criteria for a new public space
Comfort criteria decide whether wind conditions suit the intended activity in each part of a public space. Safety criteria are absolute. Testing at massing stage is what lets the building form respond.
Wind microclimate assessment criteria come in two families, and a new public space has to satisfy both. Comfort criteria say whether the wind conditions suit the activity intended in each part of the space, from sitting outside a cafe to walking briskly to a station entrance. Safety criteria say whether strong gusts could knock someone over, and they are absolute: a space that fails a safety criterion anywhere has to be redesigned, whatever its comfort performance.
Designers who treat wind as a late-stage technical check end up with canopies and screens bolted onto a finished scheme. Testing at massing stage gives you the form instead.
Why wind is assessed as an amenity matter
Planning control reaches wind because the conditions in a new public space determine whether it gets used. A plaza that is unusable for six months of the year is a failed piece of public space, and where a tall building creates downdraughts onto a footway the effect lands on people who get no benefit from the development.
In London the London Plan expects wind microclimate to be considered for tall buildings and for the quality of new public space, with the assessment informing design rather than following it. Outside London the requirement comes from local plan policy on amenity and design, and from officers asking the question on schemes tall or exposed enough to raise it.
The output is not a pass or fail certificate. It is a map of conditions across the space, tied to intended uses, with mitigation where the two do not match.
The criteria used in practice
The Lawson comfort criteria are the framework most UK assessments use. They sort locations into bands by the wind speed exceeded for a given proportion of the time, and each band is associated with activities it suits.
| Band, in ascending wind speed | Activity it suits |
|---|---|
| Lowest | Sitting for long periods, outdoor dining, reading |
| Low to moderate | Sitting for short periods, waiting |
| Moderate | Standing, window shopping, brief waiting |
| Moderate to high | Strolling and general pedestrian movement |
| High | Business walking, purposeful movement between destinations |
| Highest | Unsuitable for pedestrian activity, requiring mitigation |
The numeric speeds and the exceedance frequencies attached to each band are set out in the criteria themselves, and different practitioners apply slightly different versions, so state the version used and the exceedance basis in the report. Agreeing that with the officer or their adviser at scoping stage prevents an argument about method later.
The safety criterion works differently. It tests whether a high gust speed is exceeded more often than a very small proportion of the year, and an exceedance indicates a risk of a person being blown over. Vulnerable users raise the bar, so a space beside a school, a care home or a step-free entrance needs the stricter treatment.
Then match bands to the design intent. A cafe terrace drawn on a plan in a band suited only to business walking is not a cafe terrace, and the honest answer is either mitigation or a different use for that corner. Our note on the Lawson comfort criteria sets out the bands and their origins in more detail.
Wind tunnel testing against computational modelling
Both methods are accepted, and the choice turns on the stage and the stakes.
Boundary layer wind tunnel testing uses a physical model, typically at a scale around one to three hundred, with the surrounding context reproduced for several hundred metres in every direction, mounted on a turntable so each wind direction can be tested. It is the method most authorities trust for tall buildings, it produces measurements rather than predictions, and it takes weeks including model construction.
Computational fluid dynamics builds the same geometry numerically. It is faster to iterate, which makes it the better tool during massing studies, and it can visualise flow in ways a tunnel cannot. Its weakness is that results depend on the modelling choices, so a CFD assessment needs its turbulence model, mesh, boundary conditions and validation stated, and an officer's adviser will look for exactly that.
The pragmatic sequence on a tall building scheme is CFD during design to find and fix the problems, then wind tunnel testing on the resolved scheme for the application. On a mid-rise scheme with a modest public space, CFD alone is usually accepted.
Either way, the meteorological input matters as much as the model. Long-run hourly data from a representative weather station, corrected for terrain and reference height, is what converts a flow field into exceedance statistics. State the station, the period of record and the corrections applied.
Seasonal and directional analysis
A single annual figure hides the problem. Wind conditions vary by direction and by season, and public space use varies the same way.
What a report should present:
- A directional wind rose for the site, showing the frequency and strength of each sector.
- Results by season, at minimum summer and winter, because a terrace matters in summer and a station approach matters year round.
- The worst-case direction for each measurement location, identified rather than averaged away.
- Conditions with and without the surrounding consented schemes, since a neighbouring tower under construction changes the answer.
- The existing baseline, so the effect of the development can be separated from the conditions already there.
Point four catches schemes out on regeneration sites. Modelling the proposal in the current context and ignoring a consented tower next door produces a result that will not survive consultation, and re-testing later is expensive.
Mitigation, and where it changes the building
Mitigation runs from light touch to structural, and the light touch end has real limits.
- Porous screens and hedges, which reduce speed locally and work best at the edge of a seating area rather than across a route.
- Canopies and colonnades, which protect a defined edge and are the standard answer for an entrance.
- Tree planting, effective in leaf and much less so in winter, so evergreen or dense-branched species where winter performance matters.
- Balustrade and terrace detailing, where solid or partly solid balustrades change conditions on upper level amenity space.
- Entrance design, including lobbies and revolving doors, which addresses door operability rather than the space around it.
- Massing changes: podiums, setbacks at the base, chamfered or rounded corners, and breaking a long facade that accelerates flow along it.
The last row is the one that matters. Corner acceleration and downdraught from a tall flat facade are geometry problems, and screens at ground level rarely fix them either. Where testing shows a safety exceedance at a corner, the answer is usually a podium or a setback, which is why the assessment has to run early enough for the massing to move.
Trees deserve a caution. Counting immature planting as mitigation overstates the benefit for a decade, so model the planting at the size it will be at completion and note the improvement expected as it matures.
Reporting and securing the outcome
A Wind Microclimate Assessment that a case officer can use contains a plan of measurement locations, a table of results by location and season, the criteria version used, the mitigation assumed in the modelled case, and a comparison against the baseline and the future context case.
Be explicit about what is assumed. Where the results depend on a canopy, a screen or planting, those elements have to be secured, and the natural mechanism is a condition requiring them to be provided as modelled before the space is brought into use.
Two further practical points. Conditions sometimes require post-completion verification for tall buildings, so agree what that involves before accepting the wording. And where the space includes shared amenity areas for residents, the sunlight and daylight analysis should use the same layout as the wind work, because an Overshadowing Study showing a sunny courtyard that the wind assessment finds unusable is a contradiction officers notice. Our note on courtyard blocks and sunlight to shared amenity space covers that interaction.
PlanScope drafts wind and amenity documents structured to UK planning policy, flagging site-specific data that is missing rather than inventing it. Reports should be reviewed before submission, and we do not guarantee planning approval. Councils usually expect a suitably qualified author for wind assessment, and professional review is strongly advised.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.
Frequently asked questions
What is the difference between comfort and safety criteria?
Comfort criteria band locations by the wind speed exceeded for a given proportion of time and match each band to activities it suits, from long-term sitting to business walking. Safety criteria test whether a high gust speed is exceeded more often than a very small proportion of the year, and an exceedance requires redesign rather than mitigation alone.
Is wind tunnel testing always needed?
No. For tall buildings most authorities prefer boundary layer wind tunnel testing because it produces measurements rather than predictions. On mid-rise schemes with a modest public space, computational modelling is usually accepted provided the turbulence model, mesh, boundary conditions and validation are stated in the report.
Why does the surrounding context matter so much?
Because flow around a site is determined by neighbouring buildings as much as by the proposal. A model that ignores a consented tower next door will produce results that do not survive consultation. Report conditions in the existing context and in the future context including consented schemes, and identify the worst direction for each location.
Can planting be relied on as mitigation?
Only with care. Trees work in leaf and much less so in winter, so winter-critical locations need evergreen or dense-branched species. Model planting at the size it will be at completion rather than at maturity, and note the additional improvement expected later, otherwise the assessment overstates performance for a decade.
How is wind mitigation secured in a permission?
By condition requiring the canopies, screens or planting assumed in the modelled case to be provided before the space is brought into use. Tall building permissions sometimes also require post-completion verification, so agree what that involves and who pays for it before accepting the condition wording.
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.