The Urban Heat Island Effect Around the World

The Urban Heat Island (UHI) Effect can be simply defined as an effect that takes place in urbanized areas in which the urbanized area experiences higher temperatures than surrounding rural areas. With this higher temperature, comes an increase in heat-related health risks. There are two main types of UHIs: when an urban area experiences higher temperature during the night as compared to surrounding areas, or when an urban area experiences higher surface temperatures as compared to surrounding areas (US EPA, 2021). As we analyze UHI across the globe, both kinds of UHI will be examined and discussed.

Seven major cities from around the world were chosen to be analyzed. The cities were chosen based on similar population sizes and have data pertaining to the fact that each city is a heat island. The cities chosen include New York City, New York; Paris, France; Lagos, Nigeria; Hong Kong, Hong Kong.

Due to Lagos' location in the tropics, it is important to understand Lagos's UHI Growth as even the smallest changes in temperature can increase heat-health issues.

There is a lack of data concerning long-term climate effects and changes in Lagos due to its location within a developing country. However, as it is now considered a growing megacity, there are recent studies that have analyzed UHI in Lagos.

One way that a UHI takes effect is through increased urbanization, and the increase of urbanization can be noted through the change in land use and land cover in a city over time.

(Bassett et al, 2020)

UHI in Lagos indicates a higher temperature at night, compared to the surrounding rural areas. This was determined through the calculation of Lagos' Normalized Difference Vegetation Index (NDVI) which indicates the presence of vegetation and green space through the reflection of near-infrared radiation. The inverse of NDVI was calculated to show the increase of non-green space areas and the increase of the absorption of near-infrared radiation in Lagos from 198 to 2016. It has also been found that surrounding rural areas are experiencing warming, as the warmth from Lagos exceeds the city limits. (Bassett et al, 2020).

Secondary effects of the UHI in Lagos also include higher wind speeds within the city and a height increase in the boundary layer of the troposphere. However, due to the recent nature of data collected concerning Lagos, there is not much literature concerning the solution to Lagos's UHI and UHI Intensity (UHII). Though it is clear that the effects of UHI are increasing in intensity, there is spatial expansion of the effects of UHI. (Bassett et al, 2020).

Due to Lagos' location as a city within a nation in the Global South, there is a lack of data that covers UHI and its effect on Lagos in regard to a large temporal scale. This points to UHI as being a multiscalar issue. UHI is a global phenomenon; however, there is variability in how it is studied on a local scale. As a mega-city undergoing fast-paced urbanization in a developing country, the socioeconomic status of Lagos is reflected in the data collected concerning UHI. The data collected, and the research performed is a recent development due to the fact that Lagos is only recently reaching high levels of urbanization.

There is also an indication that the land cover change occurring in the urban parts of Lagos is affecting how UHII is affecting surrounding non-urban areas in Lagos. UHII moves to affect the non-urban land surrounding Lagos (which includes forests, marshlands, and agricultural lands) (Braimoh, 2007) and this shows how land cover change works at a broader scale as it is not just the city center or urban areas that are facing a warming phenomenon.

Hong Kong is a densely populated subtropical city in China. The UHI effect is a concern here due to an increase in land surface temperatures and weather conditions. There is a specific correlation between land use and land surface temperatures.

Temperatures in Hong Kong range from 17.28 ^o C to 37.79 ^o C (63.10 ^o C to 100.02 ^o C).

The different land use/land cover types in Hong Kong are: water body, bare land, built-up land, forest, cropland and grassland, and semi-bare land.

Temperatures are higher in areas where the Normalized Difference Built-up Index (NDBI) and Normalized Difference Bareness Index (NDBaI) are high. Conversely, the temperatures are lower in areas where the Normalized Difference Vegetation Index (NDVI) and Normalized Difference Water Index (NDWI) are high.

Areas of high land surface temperatures are located in urban zones where the built-up index is high. Areas consisting of water bodies or forests, exhibit relatively lower temperatures.

Urbanized areas populated with buildings and people, and areas containing large zones of bare soil exhibit the highest temperatures in Hong Kong. Areas consisting of forests, plots of vegetation, and water bodies exhibit the lowest temperatures in Hong Kong.

Also, temperatures in Hong Kong are effected by wind speed and elevation. Higher wind speeds and elevation lead to lower land surface temperatures.

Understanding the UHI effect and it's relationship to land use/land cover patters is vital to being able to lessen and/or solve the issues it causes across all scales. The UHI effect can have adverse effects on human comfort, health, and overall the livability of the area being affected. These concerns are likely to grow with the upwards trend of urbanization and global climate change, and without proper use of the information on UHI's to implicate well-rounded solutions.

New York City is the largest city in the United States by population. Therefore, understanding the main causes and impacts of the UHI is vital to construct explanations and design solutions on local and global scales.

Studying the impact of spatial structures on urban heat patterns is extremely important for sustainable urban planning, growth, and human health.

Cities experiencing UHI continue to pose challenges for humanity's increasingly urban population.

Within NYC, research has shown that densely urbanized areas correspond to higher urban temperatures, while parts of the city/surrounding areas with vegetated landcover correspond to lower urban temperatures.

There are greater differences in urban temperatures compared to surrounding areas both during the day and night. However, In NYC, the difference of temperatures between areas is greater during the night than during the day, which is the typical behavior of the UHI.

Some studies explain this by using the canyon effect, where the heat accumulated during the day by the urban building materials isn’t easily dissipated during nighttime. Areas outside of the city or with vegetation typically cool faster at night.

UHI can depend on other factors like wind-speed and elevation. Land-sea breezes cool NYC on warm spring and summer days and warm NYC on cold nights in fall and winter. Elevation ridges block cooler land-sea breezes when moving from the coast inland, resulting in warmer surface temperatures.

Results show that surface temperature increases with increasing impervious cover (both impervious-medium and dark surfaces).

More impervious surfaces have a heating effect even in the presence of highly reflective bright surfaces.

Dark urban surfaces, mainly low-rise multi-family walk-up buildings, produce fewer shadows and contribute to the urban heat.

Trees and shadows cast by high-rise buildings have cooling effects. Although the building density is highest in Manhattan, the tall buildings with variable heights have shown cooling effects.

Buildings with lower heights (fewer floors) and less height variability are associated with higher surface temperatures.

Staten Island has the lowest mean surface temperature amongst all the boroughs, where the number of trees is more. The Bronx has the highest mean surface temperature and make up moderate building density, height, and height variability.

A conclusion was made that UHI in NYC is what you would typically expect. The research and many studies show correspondence to higher urban temperatures and densely urbanized areas with low shade, impervious surfaces, low tree canopy, and many other urban components. Areas outside of the city or areas with a larger urban tree canopy, vegetated landcover, permeable surfaces, and high shade correspond to lower urban temperatures.

Paris is one of the largest and most densely populated cities in Europe, with almost 21,000 people per square kilometer. This dense population, in addition to hard surfaces like concrete and asphalt, has caused Paris to have one of the most impactful urban heat island effects of all European cities, with temperatures inside the city sometimes being 4 - 5 degrees fahrenheit higher than the surrounding rural areas.

This 4 - 5 degree difference inside the densely-populated urban center of Paris is exacerbated during a heat wave, sometimes resulting in temperatures 10 degrees fahrenheit hotter in the city, such as in the heat wave of 2003 which killed 15,000 people in France alone. One of the most important findings of studies done on the heat island effect in Paris is how the heat island causes a much more significant increase in temperature at night than during the day.

The danger heatwaves pose to high-density urban centers such as Paris are only going to increase if steps are not taken to mitigate the impact of the urban heat island effect. High populations in dense areas with little green space or vegetation cause even deadlier heatwaves in the future.

Looking at the effects of UHI in cities around the globe, it can be seen that urbanization and UHI have a positive relationship with one another. Without efforts to improve urban planning to mitigate UHI, increasing urbanization can lead to negative effects that go hand in hand with UHI. The most prominent being heat-related health issues. Many cities that face UHI also face an increase in heat-related health issues (Heavenside, 2017).

Possible solutions proposed by many studies concerning UHI fall into two main categories: Increased urban greening, or improvement in initial urban planning.

City examples that aid in reducing heat-related illness/deaths:

• Prioritize most impacted neighborhoods for heat mitigation and structural interventions, such as home cooling assistance, tree planting and greening, green infrastructure, and electric grid resilience to decrease risk. Couple these measures with housing and energy policies that help longtime neighborhood residents benefit from improvements.
• Support the work of community-based organizations working to reduce the health impacts of heat and climate change in their neighborhoods and include them in decision-making in their communities.
• Continue to strengthen emergency response measures during periods of extreme heat, such as opening cooling centers and issuing heat-health warnings, prioritizing people and communities with the greatest need for these interventions.