Climate & Cities
Shade is a public health measure with a maintenance bill
Getting out of the sun is the most effective thing a hot city can offer on a street. Providing it reliably is harder than it sounds.
What follows is an argument about urban shade provision, and about where the received version of it stops being true.
The argument in brief
- Shade reduces the heat load on a body more than air temperature does.
- Tree shade and built shade fail in different ways.
- Continuous shaded routes matter more than shaded destinations.
Radiant heat is what a body actually feels
A person standing in a street exchanges heat with the sun, the sky, the ground and every surface around them, not only with the air. Direct sun and radiation from hot paving contribute far more to heat stress than the air temperature reading suggests.
This is why a shaded street at the same air temperature as a sunlit one feels dramatically different and is measurably safer. Measures of thermal comfort that account for radiation describe the experience much better than a thermometer in a shaded box. It follows that shade is the highest-value intervention available on most hot streets, ahead of anything that adjusts air temperature.
Trees and structures fail differently
Tree canopy provides shade and evaporative cooling together, and it takes many years to establish and can be lost in a single storm. Built shade from canopies, awnings and shade sails works immediately, requires no water and provides no cooling from evaporation. Trees need soil volume, water and protection from compaction, and the commonest reason urban trees fail is that none of these were provided.
Structures need maintenance, fixings that survive wind loading and replacement of fabric that degrades under the sun it is blocking. Cities generally need both, using structures where soil conditions or services underground make trees impossible.
Continuity is the property that matters
A shaded bench is pleasant, and a shaded route is what allows somebody to walk to a shop, a clinic or a bus stop in heat. Gaps in shade force a person to make a series of decisions about exposure, and a long unshaded stretch can make a route unusable. Planning shade as a network, with priority on routes to transit stops, schools and health facilities, produces more benefit than scattered provision.
This mirrors how walking networks are assessed generally, where the weakest link determines whether the route functions. Mapping shade at the hottest hour, rather than at midday, identifies the gaps that actually matter.
Buildings cast the cheapest shade
A street lined with buildings of reasonable height is shaded for part of the day at no cost, and the shading depends on orientation. Narrow streets in hot climates were built that way for exactly this reason, long before anybody described it as climate adaptation. Wide streets and low buildings maximise sun exposure at ground level, which is comfortable in cold climates and punishing in hot ones.
Arcades and colonnades formalise this by extending the building over the footway, providing shade and rain shelter together.
Requiring arcades on specified streets is a planning instrument available to any city, and it changes the frontage design considerably.
Shade has costs and objections
Shade in summer is shade in winter too, which matters in climates with cold seasons and is why deciduous trees are preferred there. Retailers frequently object to canopies and trees obscuring shopfronts and signage, and that objection recurs in every city.
Shade structures collect litter, provide shelter for activities some residents object to, and require cleaning. Trees drop leaves, fruit and branches, lift paving and interact with underground services, all of which generate maintenance work. These are manageable and they are the reasons shade provision stalls, so addressing them directly is more useful than ignoring them.
Local statute governs most of this, and it varies street by street.
Assessing shade on a street
Walk the route in the afternoon rather than at midday, since the hottest part of the day is usually later than the highest sun. Note the longest unshaded stretch, because that is the section that determines whether the route is usable. Look at the surface material in sunlit sections, as dark paving stores and re-radiates heat long after the sun has moved.
Check whether shade falls on the footway or on the carriageway, since trees planted in the wrong position shade the parked cars. Look for seating in shade, because a rest point without shade is not a rest point in hot weather.
The takeaway
Map the longest sunlit gap on the route. That gap is the whole problem.
Cities are built by a thousand small permissions, not one big plan.
Questions readers ask
Is shade more important than lowering air temperature?
On a street, generally yes. Radiant heat from the sun and hot surfaces dominates what a body experiences, so blocking it produces a larger effect than small changes in air temperature.
Are shade structures a substitute for trees?
Partly. They shade immediately and need no water, and they provide no evaporative cooling or habitat. Most cities need both, using structures where trees cannot be planted.
