Introduction

Learning outcomes

• Understand the importance of identifying UHIs in a spatial planning context (what are the consequences if, in a spatial planning context, the UHIs are not identified?)
• Understand how EO surface temperature time series can support identification of UHIs
• Understand how to interpret EO surface temperature time series to identify UHIs
• Understand how to derive maps from EO surface temperature data, to be integrated with other relevant information related to UHI identification in a spatial planning context
• Understand how to analyse information derived from EO surface temperature data integrated with other relevant information related to UHI identification in a spatial planning context
• Understand the limitations of the EO surface temperature data due to the spatial resolution

What are Urban Heat Islands?

• a difference in temperature of urban and suburban areas compared to their outlying rural surroundings
  • E.g. the annual mean air temperature of a city with one million or more people can be 1 to 3°C warmer than its surroundings,
  • Usually the difference decreases as city size decreases.

Urban Heat Island: the phenomenon

Source: M. Magoni, C. Cortinovis, «Interventi di mitigazione delle ondate di calore in contesti urbani», 2014

History:

Urban heat islands have been recognized and described a long time ago:

“The generally higher temperatures inside towns have been noted for a long time, indeed Luke Roward (Roward, 1818), a pioneer of urban climatology though more famous for his work on cloud classification, first measured temperature differences in London as early as 1809. And yet, in spite of the numerous studies in urban temperatures, it still remains to quantify, even relatively, the heat exchange mechanisms leading to the excess heat of towns. There are obviously four contributory factors: changes in the thermal characteristics (albedo, heat conductivity and thermal capacity) of the surface following the substitution of buildings and roads for farms and fields; changes in the airflow patterns with a reduced diffusion of heat from streets and courtyards; changes in evaporation rates and heat losses and, the heat added by humans and human activities. These seem to be of differing importance in different cities so that the character of temporal variations also varies. In many cities of western Europe, heat islands are strongest in summer whilst in central and northern Europe there is, apparently, little difference between summer and winter intensities. In several Japanese cities, maximum heat island intensities occur in winter.”

(T. J. Chandler, Urban climatology - Inventory and prospect, Urban climates: proceedings of the WMO Symposium on Urban Climates and Building Climatology, WMO, 1970)

Why analyse urban heat islands?

(Heat Island Impacts, https://www.epa.gov/heat-islands/heat-island-impacts ; 'Heat island' effect could double climate change costs for world's cities, https://phys.org/news/2017-05-island-effect-climate-world-cities.html )

The consequences of Urban Heat

Source: M. Magoni, C. Cortinovis, «Interventi di mitigazione delle ondate di calore in contesti urbani», 2014

State of the art:

Recently, the dependency of UHI on a city’s layout could be shown:

“The arrangement of a city's streets and buildings plays a crucial role in the local urban heat island effect, which causes cities to be hotter than their surroundings, researchers have found. The new finding could provide city planners and officials with new ways to influence those effects.
Some cities, such as New York and Chicago, are laid out on a precise grid, like the atoms in a crystal, while others such as Boston or London are arranged more chaotically, like the disordered atoms in a liquid or glass. The researchers found that the "crystalline" cities had a far greater buildup of heat compared to their surroundings than did the "glass- like" ones.
The study […] found these differences in city patterns, which they call "texture," was the most important determinant of a city's heat island effect.”

(Urban heat island effects depend on a city's layout, https://phys.org/news/2018-02-urban-island-effects-city-layout.html

How to analyse urban heat islands?

To detect UHIs it is used the land surface temperature (LST). Be aware that this is not equal to the air temperature that people will experience in that place. However, it is a good and easy-to-access indicator.

It is possible to use data from Sentinel-3, which has thermal bands included and thus provides LST. Another alternative would be Landsat-8 which has a higher resolution (Sentinel-3: 500m, Landsat-8: 100m).

Remote sensing

• the acquisition of information about an object or phenomenon without making physical contact
• used in numerous fields, including geography, land surveying and most Earth science disciplines (for example, hydrology, ecology, meteorology, oceanography, glaciology, geology); it also has military, intelligence, commercial, economic, planning, and humanitarian applications.
• repeating satellite orbits enable continuous monitoring of the object, phenomenon or area in general, i.e. gathering EO time series

EO time series

• Efficiently provide information on
  • Land cover change
  • Vertical and horizontal land movements
  • Sea level change
  • Ice sheet balance change
  • Temperature change over the season or different time
  • Specific events such as fires, floods, landslides, earthquakes, etc.
  • Enable global and local analysis (such as detecting and monitoring Urban Heat Islands, UHIs)

EO time series for UHI monitoring

Satellite sensors can observe surface temperature using thermal sensors Sensors available at different satellites

Vegetation – UHI relationship

• More the vegetation less the UHI effect
• Methods on quantifying this
  • NDVI (Normalized Difference Vegetation Index)
  • Vegetation fraction
  • LAI (Leaf Area Index)
  • Other methods
    • Handheld temperature sensors
    • Aerial temperature measurements

Case: City of Stockholm, Sweden

Population ~1.0 million Area 188 sq. km. Summer temperatures normally 14-22 deg. celcius Heatwave in 2018

What are we going to do?

• Screening of the city for LHI
  • Using surface temperature maps
  • Using time series
  • Make statistic measures from time series
  • Max temp
  • Mean temp
  • Locate and delineate main LHI
  • Locate cool structures
  • Describe how this can be related to planning issues

Data

TIRS time-series Max temperature June Aug 2014-2020 Buildings, residential areas Green infrastructure Pre-schools

Description of the selected case

Residential areas and people living in or close to LHI

Surface heat temperature map 2014-2020, Time series.

Max temperatures showing warm and cool areas Landsat 8 - Band 10 Thermal Infrared.

Assessing the impact of LHI by the use of Surface temperature data and GIS-layers

Buildings within LHI
• No green or cool structures nearby (>100m)
• Green or cool structures nearby (<100m)

Buildings outside LHI
• No green or cool structures nearby (>100m)
• Cool and green structures nearby (<100m)
• Green structures nearby (<100m)
• Cool structures nearby (<100m)

Southeast of city

Planning takeaways: Caution - Residents in red areas can be affected by heat stress and little access to green or cool structures

Inner city south

Planning takeaways: Caution - Population in red areas can be affected by heat stress and little access to green or cool structures

Southwest of city

Planning takeaways: Mainly industrial areas / workplaces Caution - Population in red areas can be affected by heat stress and little access to green or cool structures

Situation of the preschools in Stockholm

Location of preschools in South Stockholm

Southeast of city

Planning takeaways: Caution - many Preschools within or close to LHI - lack of access to green structures

Southwest of city

Planning takeaways: Better situation with preschools mostly outside of LHI and better access to green structures

Conclusions / Recap

Conclusions / Recap

Conclusions / Recap

Conclusions / Recap

• Examples of planning takeaways
  • Avoid further soil sealing and densification in areas close to heat islands
  • Increase the share of green areas in certain “Hot spots” in the urban environment
  • Access to green and cool structures may be even more important for residents in areas exposed to urban heat
  • Special consideration should be given to sensitive groups such as elderly, preschool children, etc

Limitations of data

“One of seven priority goals in The City of Stockholm Environment programme 2020-2023, is a climate-adapted Stockholm. Working with climate adaption and urban heat issues, we need to integrate many different data sources to further increase our knowledge. Surface temperature measurements from satellite can together with data on e.g. census, land use and urban planning be an important tool for improving our basis of decision.”

Peter Wiborn, Senior Project Manager - Ecosystem Services, Environment and health department, City of Stockholm