Lawson wind comfort criteria and how to read the results
Comfort categories match wind conditions to activities, while a separate safety threshold deals with rare high speeds. How to review a wind microclimate assessment.
The Lawson comfort criteria wind classification sorts pedestrian level wind conditions into categories tied to activities, from sitting through to walking, by asking what wind speed is exceeded for a given proportion of the year. A separate and much stricter criterion deals with safety. Both appear on the results plans in a wind microclimate assessment, and a reviewer's first job is to work out which version of the criteria the assessment used, because more than one formulation is in circulation and the thresholds differ between them.
Get that wrong and you will compare a result against the wrong benchmark.
What the criteria classify
The criteria are applied to a threshold wind speed at pedestrian level, usually taken at 1.5 metres above the ground, derived from a long-term meteorological record for the area and adjusted for the site.
The output is a category at each receptor location. The category depends on the speed exceeded for a stated proportion of the time, conventionally five per cent, and the categories map onto what a person could comfortably do there. Frequent sitting needs the calmest conditions. Occasional sitting needs a little less. Standing and entrance use needs less again. Strolling and then business walking tolerate progressively more.
The figures commonly applied in UK assessments run from around 2.5 metres per second for frequent sitting up to around 10 metres per second for business walking, with intermediate bands for occasional sitting, standing and strolling. Confirm the exact set used in the report you are reading, and confirm the exceedance probability, because a result quoted at a different exceedance is not comparable.
Comfort categories and the activity they are matched to
The practical question for a reviewer is not whether a location falls into a category. It is whether the category matches the use the drawings propose for that spot.
A podium terrace designed as outdoor seating for a cafe, with tables on the plan, needs a sitting category. If the assessment reports it as suitable for strolling, the two documents disagree and the landscape drawings will have to change. The same applies to a children's play space, a roof terrace shown with planters and benches, a bus stop, a cycle parking area where people stand while securing a bike, and the forecourt in front of a main entrance.
So read the landscape and ground floor drawings alongside the wind results, and check the receptor list against them. An assessment with twenty receptors on the pavements and none on the amenity terraces has measured the wrong places.
Safety and distress thresholds
The comfort categories say nothing about whether a location is dangerous. That is a separate test, applied at a much lower exceedance probability because it is concerned with rare events.
The distress or safety criterion asks whether a high threshold speed, commonly taken as around 15 metres per second, is exceeded for more than a very small fraction of the year, which is often expressed as a figure equivalent to a couple of hours annually. A less restrictive threshold is sometimes applied to locations used only by able-bodied adults rather than by the general public, including the frail and the very young.
Three points matter when reviewing this. The distress result should be reported separately from the comfort result and on its own plan, because a location can pass on comfort and fail on safety. An exceedance at an entrance, a crossing point or a stair is more serious than the same exceedance in the middle of a plaza. And a distress failure is not a matter for conditions. It has to be designed out, which means the massing or the podium has to change.
Wind tunnel and computational methods
Two methods are in use and they are not equivalent in the eyes of every authority.
Wind tunnel testing uses a physical scale model of the development with a proximity model of the surrounding context out to a defined radius, in a boundary layer tunnel that reproduces the vertical wind profile. It measures speeds at the receptor positions directly. For tall buildings in London it remains the method reviewers expect, and the London Plan expects wind microclimate to be addressed for tall building proposals.
Computational fluid dynamics builds the same geometry numerically and solves the flow. It is faster, it allows many design options to be tested, and its reliability depends entirely on the modelling choices. A reviewer should ask what turbulence modelling approach was used, how many wind directions were simulated, how the domain and mesh were set up, and whether the model was validated against measured data for a comparable case.
In practice the sensible sequence is computational work during design development to compare options, then wind tunnel testing on the preferred scheme for the application. An assessment that uses only computational work for a tall building should explain why.
Seasonal and annual reporting
Wind conditions vary through the year, and so does what people want to do outside. Assessments therefore report more than one period.
Expect annual results and separate summer and winter results, or annual plus the worst season. Summer matters because that is when amenity terraces and play spaces are used. Winter matters because that is when the distress threshold is most likely to be approached.
Check the meteorological baseline too. The report should name the station the long-term record came from, the period covered, and the corrections applied for terrain and for the distance between station and site. A record from a coastal airport applied without correction to an inland urban site will overstate speeds.
Cumulative conditions are the other thing to look for. The assessment should test the proposed development with consented but unbuilt neighbouring schemes in place, because a scheme that performs well in isolation can fail once the plot opposite is built.
Mitigation that works, and mitigation that does not
Not all mitigation is equal, and reviewers should be sceptical of some of it.
| Measure | How reliable it is |
|---|---|
| Changing massing, setbacks and podium form | Most effective, because it alters the flow rather than screening it |
| Canopies and colonnades at entrances | Effective and durable, and they are built as part of the building |
| Porous screens and baffles | Effective where specified by porosity and height, and they need maintaining |
| Tree planting | Variable, with reduced effect in winter for deciduous species and no effect until established |
| Balcony glazing and screening | Useful for individual balconies, with little effect at ground level |
The weak point in many assessments is planting. A result that only passes with mature trees in leaf is a result that fails in February and for the first decade after completion. Where mitigation planting is relied on, ask for the results with and without it, and for the species, size at planting and establishment period.
Where the assessment is a reserved matter or a condition, think about whether the design can honestly be fixed later. The reasoning in our note on reserved matters and what cannot be reserved applies, because wind performance is driven by massing and massing is rarely a detail.
Reading a results plan
Work through the plans in a fixed order and the problems surface quickly.
- Check which criteria version and which exceedance probability the legend states.
- Check the receptor positions against the landscape and ground floor plans.
- Compare the existing, proposed and cumulative plans for the same season.
- Read the distress plan separately from the comfort plans.
- Establish whether the proposed plan includes mitigation, and ask for the unmitigated version if it does.
- Check the stated intended use of each failing location against what the drawings show.
That last step is where most of the useful findings come from. A category that would be acceptable on a pavement is not acceptable on a terrace with seating, and the mismatch between the wind report and the landscape drawings is the most common unresolved issue in a Wind Microclimate Assessment submitted with a tall building application.
The same discipline applies to the other environmental assessments that accompany tall buildings, and a Daylight and Sunlight Assessment should be read against the same drawings so the three documents describe one scheme.
PlanScope can draft the planning-stage narrative around specialist assessments like these, structured to UK planning policy, with flags where the specialist results belong. Councils expect a suitably qualified author for wind microclimate work, and professional review before submission is strongly advised. Browse the full set of report types to see what a tall building application usually needs. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
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 do the Lawson comfort categories measure?
They classify pedestrian level wind conditions by the threshold speed exceeded for a stated proportion of the year, conventionally five per cent, and map each band onto an activity. Frequent sitting needs the calmest conditions, then occasional sitting, standing, strolling and business walking. Confirm which version of the criteria and which exceedance probability the report uses before comparing results.
How is the distress or safety criterion different?
It applies a much higher threshold speed at a far lower exceedance probability, because it concerns rare events rather than everyday comfort. A location can pass on comfort and fail on safety, so the distress result should appear on its own plan. A distress failure has to be designed out through massing rather than addressed by condition.
Is computational modelling acceptable instead of wind tunnel testing?
It depends on the authority and the scale. Wind tunnel testing with a physical proximity model remains the method reviewers expect for tall buildings, particularly in London. Computational work suits option testing during design development. Where only computational work is submitted, ask about turbulence modelling, the number of wind directions, mesh set-up and validation.
Why should results be reported by season?
Because wind conditions and outdoor activity both vary through the year. Summer results matter for amenity terraces and play space, and winter results matter because that is when high threshold speeds are most likely. Check the meteorological station, the record period and the corrections applied for terrain and distance from the site.
How much weight should tree planting carry as mitigation?
Less than assessments often give it. Deciduous species provide reduced shelter in winter and none until established, so a result that passes only with mature trees in leaf fails in February and for the first decade. Ask for results with and without planting, plus the species, size at planting and establishment period.
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.