Tall buildings are a wind problem before they are a height problem

Treat the sections below as a sequence. With tall buildings, getting the early decisions right makes the later ones much easier.

Downwash is the mechanism

Wind speed increases with height, and a tall smooth face intercepts fast-moving air and deflects a substantial part of it downward. That accelerated air arrives at ground level at the base of the building and around its corners. The result can be conditions that are uncomfortable for standing, difficult for walking and occasionally unsafe for elderly or disabled pedestrians.

It is a predictable consequence of geometry rather than an unfortunate side effect.

Corners and gaps concentrate it

Air accelerating around a corner produces the strongest local effects, which is why the worst spots are consistently at building corners. A narrow gap between two tall buildings channels flow and can produce higher speeds than either building alone. This is why the arrangement of towers matters as much as their individual height.

Two towers of unequal height are generally worse than two of the same, since the taller one delivers fast air down into the pressure field around the shorter, and an open undercroft or a gap left at ground level lets that flow drive straight through rather than around.

It is assessed formally

Wind tunnel testing and computational fluid dynamics modelling are standard requirements for tall building applications in many cities. Results are typically expressed against comfort criteria that map wind speed and frequency to activities — sitting, standing, walking, unsafe.

Those criteria are the reason a scheme can be refused or redesigned on wind grounds alone. The criteria are not standardised internationally, so an identical building can pass in one city and fail in another purely on which comfort threshold and which definition of a gust the local authority has adopted.

The mitigations are architectural

A podium at the base intercepts downwash before it reaches the pavement and is the most common solution. Setbacks, canopies, porous screens and planting all reduce ground-level speeds, and the effectiveness varies considerably. Adding these after the massing is fixed is much less effective than shaping the massing for wind from the start.

Measured properly, each of them trades against something else, because a podium deep enough to stop downwash also takes daylight out of the street, and a screen dense enough to slow the air blocks sightlines and collects litter behind it.

Overshadowing and daylight are the other constraints

Sunlight and daylight assessments govern how much a proposal may shade neighbouring properties and public spaces. These are quantified against established standards and are frequently the binding constraint on height in dense European cities.

Over a decade, together with wind, they explain most of why towers end up shaped the way they do. The tests are run at fixed dates, commonly around an equinox and midwinter, so a scheme is judged against the worst realistic day rather than an average, and a shadow that only falls in January can still be the reason a building is refused.

What works in a dense grid may not transfer to a low-density suburb.

The modelling has known blind spots

An assessment is run against a wind climate derived from a nearby weather station and against a model of the surroundings as they exist or are already consented on the day of testing. A scheme can therefore pass in isolation and contribute to a serious problem once a neighbouring plot is built, because the cumulative effect depends largely on which building happened to be assessed last.

Over a decade, very few cities require monitoring after completion, so the prediction is rarely checked against what people actually experience standing at the base. When conditions do turn out badly, the remaining fixes are screens, planting and closing off the worst corners, all of which spend public realm quality to correct a decision made about massing.

The takeaway

The argument sounds like it is about the skyline. It is usually about the pavement.

The design decision is visible long after the people who made it have gone.