Heat Islands

Understanding and Mitigating Heat in Urban Areas

Lisa Mummery Gartland

• Publisher: Routledge
• Year: 2012
• eBook ISBN: 9781136564208
• Topic: Architecture
• Subtopic: Aménagement urbain et paysager

Heat islands are urban and suburban areas that are significantly warmer than their surroundings. Traditional, highly absorptive construction materials and a lack of effective landscaping are their main causes. Heat island problems, in terms of increased energy consumption, reduced air quality and effects on human health and mortality, are becoming more pressing as cities continue to grow and sprawl.

This comprehensive book brings together the latest information about heat islands and their mitigation. The book describes how heat islands are formed, what problems they cause, which technologies mitigate heat island effects and what policies and actions can be taken to cool communities.

Internationally renowned expert Lisa Gartland offers a comprehensive source of information for turning heat islands into cool communities. The author includes sections on cool roofing and cool paving, explains their benefits in detail and provides practical guidelines for their selection and installation. The book also reviews how and why to incorporate trees and vegetation around buildings, in parking lots and on green roofs.

1

What is a Heat Island?

Heat island definition

Urban and suburban areas have long been observed to have heat islands, a ‘reverse oasis’ where air and surface temperatures are hotter than in their rural surroundings.The heat island phenomenon has been found in cities throughout the world.

The first documentation of urban heat occurs in 1818 when Luke Howard’s groundbreaking study of London’s climate (see Figure 1.1) found ‘an artificial excess of heat’ in the city compared with the country (Howard, 1833). Emilien Renou made similar discoveries about Paris during the second half of the 19th century (Renou, 1855, 1862, 1868), and Wilhelm Schmidt found these conditions in Vienna early in the 20th century (Schmidt, 1917, 1929). Study of heat islands in the US began in the first half of the 20th century (Mitchell, 1953, 1961).

Source: www.cloudman.com/luke_howard.htm.

Figure 1.1 Luke Howard (1772–1864) of London, an amateur meteorologist, was the first serious practitioner of urban climatology

Heat islands form in urban and suburban areas because many common construction materials absorb and retain more of the sun’s heat than natural materials in less-developed rural areas.There are two main reasons for this heating. First, most urban building materials are impermeable and watertight, so moisture is not readily available to dissipate the sun’s heat. Second, dark materials in concert with canyonlike configurations of buildings and pavement¹ collect and trap more of the sun’s energy. Temperatures of dark, dry surfaces in direct sun can reach up to 88°C (190°F) during the day, while vegetated surfaces with moist soil under the same conditions might reach only 18°C (70°F). Anthropogenic heat, or humanproduced heat, slower wind speeds and air pollution in urban areas also contribute to heat island formation.

In colder cities at higher latitudes and elevations, the winter warming effects of the heat island are seen as beneficial. In some urban areas during the summer, shade around buildings can even create cooler areas for parts of the day. But in most cities throughout the world, the effects of the summer heat island are seen as a problem. Heat islands contribute to human discomfort, health problems, higher energy bills and increased pollution. On top of the effects of global warming, heat islands are further reducing the habitability of urban and suburban areas. Considering that more than 75 per cent of the world’s population lives in these areas (United Nations, 2002), heat island impacts are extremely consequential.

This book focuses on the negative effects of heat islands and gives strategies for reducing their impacts. In this first chapter, heat island impacts are briefly reviewed, and examples from cities around the world are used to demonstrate heat island characteristics. Subsequent chapters examine the causes of heat islands; how to measure heat islands; current land use characteristics and construction practices; and the three mitigation strategies, cool roofing, cool paving and trees and vegetation, that can cool communities. The final two chapters cover the community-wide benefits that heat island mitigation can bring, and put forward an action plan communities can follow to reduce their heat island impacts.

Impacts of heat islands

Why should we care about heat islands? Because their negative impacts affect so many people in so many ways. Heat islands do not just cause a bit of additional, minor discomfort. Their higher temperatures, lack of shade and role in increasing air pollution have serious effects on human mortality and disease. They waste money by increasing the need for energy use, for building and infrastructure maintenance, for the management of stormwater run-off and for the disposal of waste. In addition, the barren construction techniques that foster heat islands tend to be unattractive, unappealing and unhealthy for urban flora and fauna.

As shown throughout this book, the benefits of mitigating heat islands are very large. The implementation of cool roofing, cool paving and trees and vegetation bring many direct impacts to the owners and occupants of the spaces where they are implemented.These direct benefits are described for each measure in Chapters 5,6 and 7. When implemented on a wider scale, these measures can affect entire communities, and these community benefits are presented in Chapter 8.

Characteristics of heat islands

Heat islands exhibit five common characteristics:

1 When compared to undeveloped, rural areas, a heat island is warmer in general, with distinct daily patterns of behaviour. Heat islands are often warmest, relative to rural surroundings, after the sun goes down, and coolest after the sun rises. Urban air in the ‘canopy layer’, below the tops of trees and buildings, can be as much as 6°C (10°F) warmer than the air in rural areas.

2 Air temperatures are driven by the heating of urban surfaces, since many man-made surfaces absorb more of the sun’s heat than natural vegetation does.

3 These differences in air and surface temperatures are enhanced when the weather is calm and clear.

4 Areas with the least vegetation and greatest development tend to be hottest, and heat islands tend to become more intense as cities grow larger.

5 Heat islands also display warmer air in the ‘boundary layer’, a layer of air up to 2000 metres (6500 feet) high. Heat islands often create large plumes of warmer air over cities, and temperature inversions (warmer air over cooler air) caused by heat islands are not uncommon.

These characteristics are described in detail in the rest of this chapter.

Hotter air temperatures

Heat islands have air temperatures that are warmer than temperatures in surrounding rural areas.The difference between urban and rural air temperatures, also called the heat island strength or intensity, is often used to measure the heat island effect.This intensity varies throughout the day and night. In the morning, the urban–rural temperature difference is generally at its smallest. This difference grows throughout the day as urban surfaces heat up and subsequently warm the urban air.The heat island intensity is usually largest at night, since urban surfaces continue to give off heat and slow the rate of night-time cooling.

Note: Temperature conversion: (°C × 1.8) + 32 =°F.

Source: Morris and Simmonds, 2000.

Figure 1.2 Summer and winter air temperatures in the central business district (urban) and airport (rural) of Melbourne, Australia

Figures 1.2 and 1.3 show air temperatures and heat island intensity for typical summer and winter days in a heat island. Figure 1.3 plots daily variations in air temperature in the central business district and at the airport of Melbourne, Australia (Morris and Simmonds, 2000).These daily profiles are averaged from hourly data for December 1997 and for January and February 1998 (summer) and for June, July and August 1998 (winter).This plot shows that temperatures are always warmer in the central business district than they are at the airport. From Figure 1.2, which plots the difference between the urban and rural air temperatures, it is seen that the heat island is strongest at night [2.4°C (4.3°F) differential at 8:00pm in winter, 2.2°C (4.0°F) at midnight in summer] and weakest during the day [1.0°C (1.8°F) at 11.00am in winter, 0.4°C (0.7°F) at 3.00pm in summer.

The daily pattern of the Melbourne heat island – with its intensity peaking overnight and gradually diminishing during the day – is characteristic of heat island behaviour in most cities of moderate climate and latitude. But the heat island intensity varies in its magnitude and the timing of its peak from city to city. Peak heat island magnitudes as large as 7°C (12°F) have been recorded (Moll and Berish, 1996). These peaks usually occur three to five hours after sunset (Oke, 1987), but are sometimes delayed until after sunrise.The timing of the peak depends on the properties of urban materials. Cities built of materials that release heat more quickly (such as dry soil and wood) reach peak heat island intensity sooner after sunset, while those made of materials that release heat more slowly (such as concrete and stone) may not reach their peak until sunrise.

In cold northern climates and some desert climates, the urban–rural air temperature difference during the day can actually be less than zero, creating a daytime ‘cool island’. For example, in Reykjavik, Iceland, the heat island magnitude on summer days can be as low as negative 4°C (7°F) (Steinecke, 1999), so the city is actually cooler than its rural surroundings. This occurs mainly because the relatively low summer sun casts long building shadows in northern cities. In the desert city of Phoenix, Arizona, a similar cooling phenomenon, called the oasis effect, has been noted. More landscaping and irrigation in developed areas keep peak daytime temperatures 1–2°C (2–4°F) cooler than in the surrounding rural desert (Brazel et al, 2000). However, the heat island is still a factor in Phoenix, since urban night-time air temperatures run 3–8°C (5–15°F) hotter than rural temperatures, and have been showing a steady increase of about 0.5°C (0.9°F) per decade over the past half-century (Brazel et al, 2000).

Source: Morris and Simmonds, 2000.

Figure 1.3 Summer and winter differenc...

Table of contents

• Cover Page
• Half Title page
• Title Page
• Copyright Page
• Contents
• List of Figures and Tables
• List of Acronyms and Abbreviations
• 1 What is a Heat Island?
• 2 Causes of the Heat Island
• 3 Measuring and Modelling Heat Islands
• 4 From Heat Islands to Cool Communities
• 5 All About Cool Roofing
• 6 All About Cool Paving
• 7 Cooling with Trees and Vegetation
• 8 Community Benefits from Heat Island Mitigation
• 9 Cool Community Action Plan
• Index