Wind microclimate and pedestrian comfort at street level

Wind microclimate assessment answers one question: will the ground around this building be comfortable and safe for the activity intended there. It is judged against pedestrian comfort criteria that map wind speed and frequency onto activities such as sitting, standing, strolling and walking, with a separate safety threshold that applies whatever the activity. A terrace that fails the sitting criterion is not dangerous. It is just a terrace nobody will use.

Tall buildings are the usual trigger, because a tall flat facade intercepts faster wind at height and drives it down the elevation to street level, where it accelerates around corners and through gaps.

When an assessment is required

Most authorities ask for one where a building is tall relative to its surroundings, where a scheme creates new public space, or where an existing wind problem is known locally. In London the requirement sits in policy for tall buildings and for the quality of streets and spaces, and boroughs apply it routinely to anything that breaks the prevailing height.

Height alone is not the test. An exposed coastal or riverside site, a building on a ridge, a scheme creating a long north-south canyon, or a podium with a gap under it can all produce conditions worth assessing at modest heights. So can a building that sits upwind of an existing entrance.

Ask the question at concept stage. The cost of a concept-stage study is a fraction of the cost of redesigning a podium after a consultee response. An assessment commissioned after the massing is fixed can only recommend bolt-on mitigation, which works less well and looks worse.

Comfort and safety criteria explained

The criteria most commonly used in the United Kingdom sort conditions into categories by activity, each defined by a threshold wind speed that must not be exceeded for more than a stated proportion of the time.

Long-term sitting has the lowest threshold, then short-term sitting, standing, strolling and finally business walking. A location is assigned the category it meets. The assessment then compares that category with what the design intends: a cafe terrace needs a sitting category, a pavement needs walking, a bus stop needs standing because people wait there.

The safety criterion is separate and works differently. It is expressed as a gust threshold that should not be exceeded more than a small number of times a year, and it applies everywhere regardless of the intended use. A distress condition at an entrance is a refusal issue, not a comfort issue.

State both results for every receptor. A report that gives only comfort categories has left out the test that matters most.

Wind tunnel testing against computational modelling

Boundary layer wind tunnel testing uses a scale model of the proposal with the surrounding context, on a turntable, with the approach flow shaped to represent the terrain upwind. It measures at discrete points and it handles the complex separated flows around buildings well, which is precisely the regime that matters here.

Computational fluid dynamics gives results across the whole domain rather than at points, produces attractive contour plots, and can test options quickly. Its weakness is turbulence modelling in separated flow, where the choice of model and mesh changes the answer.

The practical position taken by most reviewers is that computational modelling is well suited to early option testing and to schemes of moderate sensitivity, while wind tunnel testing is expected for tall buildings and for schemes where the result is contested. Some authorities say so explicitly. Check before committing, because the cost and programme differ substantially.

Whichever is used, the meteorological data matters as much as the model. Long-term records from a representative station, corrected for terrain and for the height of the measurement, are what turn a wind speed into a frequency.

Seasons, directions and receptor points

Run the analysis for the full year and for summer separately. Amenity terraces, play space and outdoor seating are used in summer, and an annual result can hide a summer problem or overstate one.

Directional analysis is where design insight comes from. A result that says a corner fails is less useful than one that says it fails on south-westerlies, because the second tells you which part of the massing to change.

Choose receptor points deliberately and list them: every entrance, every amenity terrace, seating areas, the bus stop, cycle parking, the crossing, the through route, and the neighbouring pavement. Adding the points an objector would choose is better than leaving them out and being asked.

Mitigation through massing, canopies and planting

The mitigation that works is the mitigation designed into the form.

• Setbacks and podiums that break the downdraught before it reaches the pavement
• Chamfered or rounded corners, which reduce corner acceleration
• Avoiding through-building gaps at ground level, or closing them
• Deep canopies at entrances, sized to cover the approach rather than the door
• Lobbies and revolving doors, which also help the building's energy performance
• Porous screens and planting, which work when they are dense, tall and evergreen

Planting deserves a caution. A landscape drawing showing mature trees delivers nothing in year one and nothing in winter for deciduous species. If the assessment relies on planting, say what species, what height at planting, and what the condition is in winter.

Reporting results a committee can read

Wind reports are hard to read and easy to dismiss. The presentation problem is the same one we cover for daylight evidence in presenting daylight results a committee can read.

Give a plan showing the receptor points with the comfort category at each, in summer and annually, using a consistent colour scheme with a legend a non-specialist can follow. State the safety result separately and plainly. Then a short table: location, intended use, required category, predicted category, and whether mitigation is needed.

A Wind Microclimate Assessment is a report type where councils usually expect a suitably qualified author, and professional review before submission is strongly advised. It also needs to be consistent with the Daylight and Sunlight Assessment, because both models use the same massing and reviewers do compare them.

Conditions that secure the mitigation

Mitigation only counts if it is built. Where the acceptable result depends on a canopy, a screen or a planting scheme, that element should be named in a condition and tied to a drawing number.

Post-completion verification is worth accepting. It is usually a short monitoring exercise or a confirmatory assessment against the as-built arrangement, and it gives the authority confidence to grant permission on the strength of modelled results. Resisting it tends to produce a refusal rather than a cleaner permission.

Two further points on conditions. Name the drawing revision, because a later revision can quietly lose a setback. And where phasing means the assessed context will not exist for several years, say which phase the result assumes, since a scheme assessed with a neighbouring block in place behaves differently while that block is a car park.

Write the mitigation into the design and the condition becomes trivial. Leave it to a screen the contractor may value engineer out and the condition becomes the only thing holding the scheme together.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

Browse the PlanScope report types to see which assessments a tall building proposal is likely to need.

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