Competence for wind microclimate modelling

No licence exists for wind microclimate modelling. There is no competent person scheme, no protected title and no statutory requirement, so the question of who does wind microclimate modelling resolves into two practical checks: can they demonstrate that their method has been validated, and have they applied the right comfort and safety criteria to the right receptor locations? Everything else in the commissioning decision follows from those two.

That matters because the output of this work can change a building's massing. A tall building with a wind problem at its entrance does not get a mitigation condition. It gets a redesign.

What the assessment has to establish

Three findings, and the third is the one that stops schemes.

Pedestrian comfort across the site and its surroundings, assessed against a recognised set of criteria that relate wind speeds exceeded for a proportion of the time to the activity intended in each location. A bench needs calmer conditions than a pavement.

Pedestrian safety, assessed separately. Comfort criteria and distress or safety criteria are not the same test, and a location can be acceptable for walking while failing the safety threshold in strong conditions. Any exceedance of a safety criterion is a serious finding that has to be designed out rather than mitigated with a sign.

Seasonal and directional variation, because a terrace that is pleasant in July and unusable in February is a design failure that an annual average conceals.

Computational modelling and validation

Computational fluid dynamics is now the default for most schemes, and the quality varies enormously because the software is widely available and the judgement is not.

Ask for evidence on each of the following before appointing anyone.

• Which turbulence modelling approach was used and why, since steady state approaches handle separated flow around sharp-edged buildings less well than time-resolved ones
• Domain size and blockage ratio, and the roughness and inflow profile applied at the boundaries
• Mesh resolution near the pedestrian level and at building edges, with a grid sensitivity study showing the result is not an artefact of the mesh
• Convergence criteria actually achieved on the run, rather than the figures quoted in the method
• Validation against published benchmark cases or against wind tunnel data for a comparable geometry

That last point is the one to insist on. A modeller who cannot show validation evidence is asking you to take the result on trust, and a reviewing consultant acting for the authority will say so. Several authorities have published their own expectations for computational work on tall buildings, and they are written around exactly this list.

Wind tunnel testing and laboratory capability

Physical testing in a boundary layer wind tunnel remains the reference method, and for tall buildings in sensitive locations some authorities still expect it.

A scale model of the development and its surroundings is mounted on a turntable and tested across wind directions, with the approach flow conditioned to reproduce the atmospheric boundary layer for the site's terrain. Measurements at pedestrian level are taken with probes at a defined height, and the results are combined with long-term meteorological data to produce exceedance statistics.

What to check in a laboratory: that the tunnel is a boundary layer facility rather than an aeronautical one, the scale of the model and the extent of surrounding context included on the turntable, the number of wind directions tested, the probe height and locations, and the laboratory's own quality accreditation. Ask to see a redacted previous report.

Cost and programme differ substantially from computational work. Model making takes weeks, and changing the massing means a new model, so wind tunnel testing suits a design that is reasonably settled while computational modelling suits iteration.

Meteorological data and its analysis

The wind data is half the answer and gets a fraction of the scrutiny.

The assessment should use a long record from a station that is representative of the site's region, with the period of record stated, directional statistics presented as a wind rose, and corrections applied for the difference in terrain roughness between the station and the site. A short record, or a station in a sheltered inland location used for a coastal site, produces exceedance figures that are not wrong so much as meaningless.

Check that the analysis distinguishes seasons. Criteria are commonly applied on both an annual and a windiest season basis, and reporting only the annual case hides winter performance in amenity spaces.

Also check the exceedance convention. Criteria are expressed as a wind speed exceeded for a stated percentage of the time, and reports occasionally mix conventions between gust and mean speeds, which makes the comparison with the criteria invalid.

Context model extent and cumulative schemes

Surrounding buildings shelter and accelerate the flow, so the model has to include them out to a radius where they stop mattering, typically several hundred metres, with the extent stated and justified.

Four scenarios are usually needed, and a report missing the fourth is incomplete on a site with neighbours in planning.

Landscaping needs care in the modelled scenarios. Trees provide real shelter, but a deciduous tree offers far less in winter, and a newly planted one offers almost none for years. A mitigation case resting on planting should model the winter and establishment conditions, not the mature summer canopy.

Peer review and when authorities require it

Authorities with tall building policies frequently require wind assessment as part of the design evidence, and in London the tall buildings policy brings microclimate into the determination directly. Where a scheme is sensitive, expect the authority to commission its own reviewer.

That review will go to method before it goes to conclusions, which is why the validation evidence and the stated assumptions matter so much. Where the authority's reviewer and your consultant disagree, the argument is usually about mesh resolution, context extent or the criteria set rather than about the wind.

Report authorship should be explicit. The document needs a named author with stated experience, a date, and a clear statement of who carried out the modelling as distinct from who checked it. The reasoning behind a planning report carrying a named author and a date applies here more than almost anywhere, and where junior staff did the modelling the supervision chain behind work signed by a chartered author should be visible rather than implied.

Checking the criteria that were applied

Read the criteria section first when a draft arrives. Four questions.

Which criteria set was used, and is it the one the authority expects? Different published criteria produce different pass and fail boundaries for the same wind speeds, and switching between them mid-report is a red flag.

Were comfort and safety assessed separately, with the safety case reported for every receptor rather than only where comfort failed?

Do the receptor locations cover the places people will actually be? Building entrances, including secondary and service entrances, amenity terraces, play space, external seating, bus stops, cycle parking and the points where pedestrians round a corner at the base of the tower. A grid of points across an empty plaza is not the same as receptors at the doors.

Is the activity assumed at each receptor the activity the drawings show? A location assessed as suitable for business walking is not suitable for the outdoor seating the landscape drawing puts there.

A wind microclimate assessment drafted around those questions is useful to a design team. One that reports a grid of colours with no stated criteria, no validation and no receptor logic is not, whatever it cost. Where mitigation is proposed, make sure it is capable of being secured by condition and retained, because screens and canopies that disappear at value engineering stage take the permission's justification with them.

Daylight, sunlight and wind are usually assessed by different consultants on the same massing, and the mitigation for one can worsen another. Keep the wind work in step with the daylight and sunlight assessment rather than running them in sequence.

If you need a reviewer for a finished draft, the professional directory is the route to a review request and sign-off.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.