Geoscape Buildings and Urban Heat Islands

We’ve already seen that Geoscape Buildings can be used to estimate and aggregate the Solar Reflectance Index and assist in managing the impacts of roof selection in the Urban Heat Island.

This is a more technical deep dive into how we calculated the Solar Reflectance Index values for our previous article.

What is the Urban Heat Island Effect?

An urban heat island is an urban area or metropolitan area that is significantly warmer than its surrounding rural areas due to human activities. The temperature difference is usually larger at night than during the day, and is most apparent when winds are weak. UHI is most noticeable during the summer and winter. The effect increases energy costs (air conditioning), air pollution levels, and heat-related illness and mortality.

Buildings and their roof construction are significant contributing factor to the UHI effect. Geoscape Buildings includes attribution for roof_colour and primary_roof_material. These can be used to calculate the effect that buildings and whole suburbs have on the UHI.

How is it calculated?

One way to measure how ‘cool’ a roof can be is by calculating its Solar Reflectance Index (SRI)[1] which is the roof’s ability to reject solar heat[2]. SRI incorporates estimates for both:

• Solar Reflectance – the ratio between solar energy globally reflected by a surface and the total incident solar energy, and
• Thermal Emissivity – the property of a roof to radiate energy with respect to a black body at the same temperature, per unit area.[3]

SRI is defined such that a standard black (reflectance 0.05, emittance 0.90) has a value of 0 and a standard white (reflectance 0.80, emittance 0.90) has a value of 100. The higher the SRI value, the ‘cooler’ the roof is.

Solar Reflectance

Solar reflectance can be estimated using the roof colour supplied with the Geoscape Buildings 3.0 product. The value for roof colour is given in a CMYK hex value (ie #B5B7AB). This is a string representing the colour parts of Cyan 1%, Magenta 0%, Yellow 7%, Black 28%.

Using the roof colour we can calculate a representative Albedo value between 0 (black – full absorptance) and 1 (white, full reflectance) to use as a Solar Reflectance value.

Surface Emissivity

Estimating surface emissivity requires a little judgement, generalisation and estimation. A cool material’s high emissivity (close to 1.0) denotes surfaces that dissipate accumulated heat, without transferring it to the building. A high emissivity value is also important for reducing the UHI effect.

Emissivity values for some roof types can vary with temperature[4], roof angle, properties of the material such as thickness, density, surface finish, and the age of the surface so the values used are rough estimates based on industry averages.[5]

Geoscape Buildings 3.0 is produced with 4 attributes relating to primary_roof_material and we have used generic values estimating the Emissivity coefficient

• Metal 0.22-0.28
• Concrete 0.85-0.95
• Plastic/Fibreglass 0.90-0.97
• Tiles 0.97

Estimating Surface Temperature

Using SR and E it is possible to calculate an estimated surface temperature for each building’s roof using the formula:

Where:

• R = Surface Reflectance
• I = Insolation = 1000 Wm²
• E = Emissivity
• Sigma = Stefan-Boltzmann constant = 5.67e^-8 Wm² K^-4
• Tsurface = Our desired surface value to solve for
• Tsky = Sky temperature = 300K
• Hc = Convection coefficient (medium wind) = 12 Wm^-2 K^-1
• Tambient = Ambient air temp = 310K

Using a Python function we iteratively solved for T_surface between a black surface temp 355.6K and white surface temp 317.8K to obtain a 0 result for the equation within a practical tolerance.

Calculating SRI

Aggregating SRI

SRI can be aggregated based on building area (also provided with Geoscape Buildings) to gain an average SRI value per Locality. For localities in Sydney, we can see the effect of roof colour and material on UHI.

Camperdown has hotter rooves than the surrounding suburbs due mainly to the large buildings associated with the University of Sydney and the Royal Prince Alfred Hospital campuses.

By comparing the SRI calculation (Red to Blue) to the roof colour images below we can see that roof colour has a significant impact on the Surface Reflectance Index and consequently the Urban Heat Island effect.

There are a multitude of things we can do with the Buildings and other Geoscape datasets. What would you like to see us tackle next?

Author: Andrew Collins, Pre-Sales Engineer, Geoscape Australia