Lawson comfort criteria tables in a wind microclimate report
Lawson categories pair an activity with a threshold speed and a frequency of exceedance. Here is what each one means, why distress is absolute, and the six checks to run on the plots.
The Lawson comfort criteria turn wind speed into activities. Each category names something a person might reasonably do in a place, and pairs it with a threshold wind speed that should not be exceeded more than a set proportion of the time. Sitting for long periods needs the calmest conditions. Business walking tolerates the most. When a wind microclimate report says a location is suitable for strolling, it is making a statement about frequency of exceedance, not about how it felt on the day of the site visit.
Read the tables and the assumptions behind them. The colour plots are the part everyone looks at and the part that misleads most reliably.
What the criteria measure
The criteria work on a threshold speed and a frequency. A location satisfies a category if the relevant wind speed exceeds the category threshold for no more than a small percentage of the time, conventionally five per cent, across a representative year.
The speed used is a gust equivalent mean, which combines the mean speed and the gustiness into a single figure so that a turbulent, gusty location is not rated as comfortable simply because its average speed is low. That is the single most important thing to understand about the method, because a podium edge with high turbulence and a modest mean speed will perform far worse than its average suggests.
The wind data comes from a nearby meteorological station, corrected for terrain and for the height of the site. Check which station was used and over what record length. A short record, or one from a station in very different terrain, weakens everything downstream.
Comfort categories and the activities they describe
The categories run from the calmest to the windiest:
- Sitting for long periods, such as outdoor seating at a cafe or in a garden
- Sitting for short periods, or standing, such as waiting at a bus stop or an entrance
- Strolling, covering general pedestrian movement in public space and park paths
- Walking, covering pavements and thoroughfares where people move with purpose
- Business walking, the windiest acceptable category, appropriate only to places people pass through quickly
The test is whether the measured or modelled condition matches the intended use of that location, and that is a planning judgement rather than an engineering one. A plaza designed with fixed seating and a cafe terrace that tests as suitable for walking has failed, even though walking is a perfectly good category somewhere else.
So the first thing to check in a report is the schedule of intended uses. If the assessor was given the landscape drawing and tested seat positions, entrance thresholds, the children's play area and the cycle parking, the results mean something. If they tested a grid of points at two metres above ground with no reference to the proposed layout, they do not.
The distress threshold and why it is absolute
Comfort is about amenity. Distress is about safety, and it is assessed separately against a much lower frequency of exceedance.
The commonly applied safety criterion is a gust equivalent mean speed in the region of 15 metres per second, which should not be exceeded more often than roughly once a year in a representative year. Where a location is likely to be used by cyclists, older people, children or people with mobility impairments, a lower threshold is applied.
There is no trading here. A location that fails the safety criterion is unacceptable regardless of how attractive the scheme is, and mitigation has to bring it into compliance rather than compensate for it elsewhere. Officers and their advisers treat it that way, and so should the design team.
Where a report shows distress exceedances at a building corner or a through-route between blocks, expect the massing to be asked to change, not the landscape.
Wind tunnel testing against computational modelling
Both methods are used, and the choice is usually driven by the authority and the stage.
Boundary layer wind tunnel testing uses a scale model of the proposal and its surroundings on a turntable, with the approaching flow conditioned to match the site's terrain. It measures at discrete probe positions for a set of wind directions, and it handles complex separated flow well. It is slower and costlier, and it gives results only where probes were placed.
Computational modelling gives full-field results and is far more useful during design iteration, because a massing change can be tested in days. Its weakness is the treatment of turbulence, and results depend on mesh resolution, the turbulence model chosen and the domain size.
| Question | Wind tunnel | Computational modelling |
|---|---|---|
| Results coverage | Only at probe positions | Full field |
| Turnaround for design changes | Slow | Fast |
| Treatment of complex separated flow | Handled by physical measurement | Depends on turbulence model |
| Typical role | Validation for consent | Iteration and early screening |
For tall buildings, many authorities and the London Plan's expectations for tall building proposals point towards physical testing at application stage, with modelling used through design development. A report that relies on modelling alone for a tall building should say why, and should state the model settings rather than hiding them in an appendix nobody reads.
Seasonal and annual reporting
Annual results are the default, and they under-represent the summer problem. A terrace that satisfies the sitting criterion on an annual basis may still be unusable on summer afternoons if the prevailing summer wind direction aligns with a gap in the massing.
Ask for results reported for the summer season separately where outdoor amenity space, play space or food and drink use is proposed. Where a scheme includes an Open Space Assessment or roof terraces counted as amenity provision, seasonal results are the ones that matter.
Equally, check whether the assessment considered the existing condition as well as the proposed. A site that is already windy is a different planning proposition from one the scheme makes windy, and the report should present both.
Mitigation by canopy, screen and planting
Mitigation that works is built mitigation. Canopies over entrances, porous screens at podium level, recessed doorways, setbacks at the base of a tower, and massing changes that break up a long uninterrupted facade all produce measurable improvement.
Planting is the one to interrogate. Trees provide porous shelter, but they take years to reach the canopy modelled, they are seasonal where the species is deciduous, and they can be removed. Ask whether the modelled trees were at planted size or at maturity, and whether the winter condition was tested. A mitigation strategy resting on mature deciduous trees is a strategy that fails in January for the first fifteen years.
Secure the mitigation by condition, drawn and dimensioned, rather than described in a report. The same discipline applies to visual mitigation, as photomontage methodology in a landscape and visual assessment shows.
Reading the colour plots
Work through the plots in this order.
- Confirm the legend: which criterion is plotted, at what frequency of exceedance, and for which season.
- Confirm whether the plot is the proposed scheme alone or with mitigation included, since the second is often presented without a label.
- Check whether cumulative schemes, meaning consented but unbuilt neighbours, were modelled.
- Find every seat, entrance, crossing and play location on the landscape plan and check the category at each, rather than reading the plot as a whole.
- Look for the distress plot separately, because it is sometimes presented only as a table.
- Check the height of the assessment plane, which should be pedestrian head height.
A Wind Microclimate Assessment that answers all six without being asked is doing its job. One that leads with a green plot and buries the table is not.
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Frequently asked questions
What is a gust equivalent mean wind speed?
A single figure combining the mean wind speed and its gustiness, so that a turbulent location is not rated comfortable merely because its average speed is low. Lawson criteria are applied to this figure rather than to the raw mean, which is why podium edges and building corners perform worse than their average speeds suggest.
How often can a comfort threshold be exceeded?
Conventionally no more than five per cent of the time across a representative year for the category to be satisfied. The category is then matched against the intended use of that location, so a seating terrace has to meet the sitting criterion rather than the walking criterion, whatever the plot looks like overall.
Can distress exceedance be mitigated by improvements elsewhere?
No. The safety criterion is absolute for the location that fails it. A gust equivalent mean speed in the region of 15 metres per second exceeded more often than roughly once a year is unacceptable, with a lower threshold where cyclists, children or people with mobility impairments are expected. Mitigation has to remove the exceedance.
Is wind tunnel testing always required?
Not always, but for tall buildings many authorities expect physical testing at application stage, with computational modelling used through design development. Modelling gives full-field results and fast iteration; the wind tunnel gives measured results at probe positions. A report relying on modelling alone should state the model settings and justify the choice.
Should trees count as wind mitigation?
Only with care. Ask whether the modelled trees were at planted size or at maturity, whether deciduous species were tested in their winter condition, and how the planting is secured. A strategy resting on mature deciduous canopy provides little shelter for its first years and none in winter.
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.