Wind microclimate: the Lawson comfort criteria explained

The lawson criteria wind microclimate method answers one question: is the wind at this spot suitable for what people will be doing there. It does that by comparing the wind speed exceeded for a small proportion of the time against a set of bands drawn from the Beaufort scale, then matching each band to an activity. Sitting needs calmer conditions than standing. Standing needs calmer conditions than walking through. A café terrace that tests as suitable only for business walking is not a café terrace.

What the criteria measure

Each band is defined by a mean wind speed range and an exceedance frequency. The assessment reports, for every location tested, the highest activity category the conditions will support.

The Beaufort ranges behind the bands are these.

The exceedance frequency is usually expressed as a small percentage of the time, and the precise percentages differ between published versions of the criteria and between authorities. Agree the version and the exceedance basis with the authority before the modelling starts. Two reports using different bases are not comparable, and a consultee who spots the difference will ask for the work again.

Matching a band to each space

This is the part that decides whether the assessment helps or hinders. Build a plan of the external spaces and label each one with its intended use before the results arrive.

Entrances, bus stops and seating areas need the calmer categories. Footways and crossings need walking categories. Service yards and roof plant areas need almost nothing. Amenity terraces, children's play space and any space where people will linger need sitting conditions during the season they will be used.

Seasonal use matters. A roof terrace assessed as suitable for standing in winter and sitting in summer is perfectly acceptable, provided the report says so and the landscape design reflects it.

An illustrative example. A podium garden intended for residents' seating tests as suitable for strolling in winter, two categories short of the intention. The workable answers are a screen along the exposed edge, a change in the seating position, or a frank statement that the space is a summer terrace. The unworkable answer is silence. The categories here are illustrative.

Where the tested condition is one band worse than the intended use, you have a design problem rather than a reporting problem. Changing the label on the plan to match the result is the oldest trick in this field and officers recognise it instantly.

The separate distress threshold

Comfort is one test. Safety is another, and it is not negotiable in the same way.

The distress or safety criterion looks at strong winds, at the upper end of the Beaufort scale, and asks how rarely they may occur. The exceedance allowed is very small, measured in hours per year rather than percentages of the time, and the tolerance is tighter again for locations used by cyclists, elderly people or children.

A location that fails the safety criterion cannot be mitigated by relabelling or by a management plan. It needs a change to the massing, a physical screen, or the removal of the use from that location. Authorities treat a safety exceedance as a refusal reason, and they are right to.

Corners of tall buildings, gaps between towers and the downwash zone at the base of a tall slab are the usual places it appears. So is the gap between a new tower and an existing one, where the accelerated flow can be worse than anything either building produces alone.

Weather data and modelling method

The inputs decide the credibility of the output. Expect the report to state the meteorological station used, the length of record, how the data was corrected for terrain and exposure, and the directional analysis by sector.

A long record from a representative station, analysed directionally and seasonally, is the standard. A short record, or one from a station in a very different exposure, produces results that a reviewer can dismiss without engaging with the modelling at all.

On method, both boundary layer wind tunnel testing and computational modelling are used. Wind tunnel testing on a scale model remains the reference method for tall buildings, and some authorities require it rather than accepting simulation alone for schemes above a stated height. Computational modelling is faster, useful for testing options during design, and increasingly accepted where it is validated and reported transparently. Ask the authority which it expects before you commission either.

Reading the results plan

A results plan is a set of coloured contours over the site plan, one per season, usually for three scenarios: existing, proposed, and proposed with mitigation. Cumulative scenarios including consented neighbouring schemes are requested in dense areas, for the same reasons they are requested for daylight.

Read them in this order.

1. Existing conditions, to understand the baseline. Some sites are already windy and your scheme may improve them.
2. Proposed without mitigation, to find where the scheme creates a problem.
3. Proposed with mitigation, to see what the measures actually achieve.
4. The table of tested locations, which is where the numbers live.

The contour plot is illustrative. The tabulated results at labelled locations are what the condition will be drafted around, so check that every important space has a labelled test point. A terrace with no test point has not been assessed, whatever colour it appears on the plan.

Mitigation and how it is secured

Massing is the first and most effective lever: podium levels that separate the tower from the street, setbacks, chamfered or rounded corners, and orientation that avoids channelling between buildings. All of it has to be settled early, which is why wind should be tested at concept stage rather than at the end of a planning application.

After massing come canopies at entrances, porous screens at exposed corners, and planting. Evergreen planting works in winter when deciduous does not, and mature trees take years to reach the size the model assumed, so state the assumed condition and check the landscape specification matches it.

Mitigation is then secured by condition, usually requiring the measures shown in the assessment to be installed before occupation and retained thereafter. Where planting does the work, the landscape and management condition carries it, and the maintenance obligation matters as much as the initial planting.

A Wind Microclimate Assessment is a report type councils expect a suitably qualified specialist to author, and professional review before submission is strongly advised. It sits alongside the Daylight and Sunlight Assessment and the Townscape and Visual Impact Assessment in the environmental package for a tall building, and all three respond to the same massing decisions. The related method for testing sun on the ground at the equinox is worth reading alongside, because a space can pass on sunlight and fail on wind.

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

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