How to cool a hot city

International research on urban heat islands provides solutions for the UAE that can drop city temperatures by several degrees

Buildings, roads, parking lots and pavements are the foundations of every city.

They are also responsible for raising summer city air temperatures, sometimes by as much as 4 degrees Celsius (between 5 and 7 degrees Fahrenheit) more than surrounding areas, creating an urban heat island.

This phenomenon affects a city, its government and its residents in a number of ways, including public health.

Poor air quality is an important side effect of urban heat islands, as hotter city air boosts the creation of smog, leaving more pollutants in the air.

Urban heat islands also prolong and intensify heat waves in cities, making residents and workers uncomfortable and putting them at increased risk for heat exhaustion and heat stroke.

Hotter cities are also more expensive, with both government and residents paying the energy costs of additional air conditioning.

The increased electricity use also takes its toll on the planet, by producing carbon dioxide and contributing to global warming.

Scientific institutes around the world have long documented urban heat islands and researched ways to mitigate their effects.

The Lawrence Berkeley National Laboratory (LBNL) in California, the oldest of the US Department of Energy's national laboratories and a leader in science and engineering research, is pre-eminent among them.

Dr Hashem Akbari, leader of the heat island group at LBNL, was in Dubai earlier this month to address the first International Conference on Green Buildings in the UAE.

He spoke to Gulf News about the Heat Island Group's findings, and outlined some of the measures cities in this region can take to cool down.

"Measures to cool heat islands are simple and have been known to human beings for a long time," said Dr Akbari.

"We just need to remind ourselves, and to put them back in practice."

Passive solar orientation

In hot climates, one of the most basic and effective ways to build a green city, Dr Akbari explained, is to orient it in the right direction, towards prevailing winds and away from solar heat gain.

For this region, buildings would be ideally positioned in a north-south orientation, with the majority of windows and other openings located in these directions.

The more limited the opportunity for direct sunlight on the east and west surfaces of a building, the better.

The greatest amount of heat gain is produced on the west side.

"Most streets in the Middle East were typically designed in a north-south orientation, with the side streets at an angle," said Dr Akbari.

"The understanding behind traditional building techniques and design still holds true today, but is often ignored. Some of the ideas like narrower streets may not be viable today due to population pressures, but the basic idea of minimising direct heat is still valid."

Taking advantage of wind patterns means not putting any obstructions in the way to ensure natural cooling goes as far as it can, he added.

Buildings that incorporate plazas and other features that allow wind to pass through take this concept one step further.

Obstructions like buildings can come in handy, however, to block hot winds coming in from the desert.

Cool roofs

One of the major components of the Heat Island Group's research involves cool roofs and cool pavements.

The underlying concept is a simple one: dark materials absorb more heat from the sun than lighter, reflective materials.

If those dark surfaces are roofs, some of that heat is transferred indoors.

In the sun, black surfaces can become up to 40 degrees Celsius (70 degrees Fahrenheit) hotter than the most reflective white surfaces.

Staying comfortable under a dark roof requires more air conditioning, meaning higher electricity bills.

Dark roofs also heat the air around them, contributing to the heat island effect.

Cool, reflective roofs can reduce this effect, save energy and make interior spaces cleaner and healthier.

Research in California and Florida showed that buildings with lightly coloured, more reflective roofs used up to 40 per cent less energy for cooling than buildings with darker roofs.

Cool roofs, therefore, are both financially and environmentally beneficial.

And while the finer details of the solar reflectivity of roofing materials are best left to architects and contractors, it is important to know that cool roofs are a function of both the materials used and their colour.

Research into cool roofing materials is ongoing at LBNL, with scientists trying to find the ideal roofing materials for different types of buildings in different climate zones and seasons, with different roof insulation levels, angles and orientations.

Cool pavements

Around the world, roads and parking lots are generally paved with black asphalt concrete and other dark materials that absorb most of the sunlight that falls on them.

Pavements get hot and heat the air around them, again adding to the heat island effect.

Researchers at the Heat Island Group have tried to determine exactly how black pavements are, how hot they get, the effect this heat gain has on city air and how to reverse these actions.

Their studies show that by a change to the asphalt coating can cause pavements and roads to reflect more heat, last longer and reduce city temperatures.

In one important experiment conducted last September in Berkeley, California, researchers measured the reflectivity of three types of asphalt - one newly laid, one aged and one with a prototype coating developed with the road construction industry - in the same area and at the same time.

The fresh asphalt recorded a temperature of 123 degrees Fahrenheit (50 degrees Celsius) with an albedo (reflectivity) level of 0.05; the aged asphalt 115 degrees Fahrenheit (46 degrees C) with an albedo of 0.15; and the coated prototype a temperature of 88 degrees Fahrenheit (31 degrees C) with an albedo level of 0.51.

For every minor increase in reflectivity, pavement temperature dropped several degrees.

Citywide simulations using cool pavement technologies showed an average decrease of about 3 degrees C (5.4 degrees F), and an estimated cost savings of $15 million (Dh55 million) annually.

A thin resurfacing with the prototype coating is all that is needed. Cities, say Group researchers, would have cooler summers at no extra cost if these materials were used to pave new roads and resurface old ones.

Leafy air conditioners

Vegetation is crucial in cooling cities, first by providing shade for buildings, cars and pedestrians, and secondly by reducing urban temperatures through a process known as evapotranspiration.

Trees placed strategically next to offices, homes, parking places and even paved areas, for example, minimise heat buildup in those areas and make air conditioning more effective by blocking direct sunlight and heat gain.

Researchers at LBNL estimate that properly planted trees can reduce cooling costs by an average of 10 to 20 per cent.

Over their lives, it is estimated that trees can be much less expensive than air conditioners and the energy needed to run them.

Plants also sweat or transpire water through pores in their leaves.

This water draws heat as it evaporates, cooling the air and lowering ambient temp