The Urban Heat Island Effect

Urbanisation can modify the climate in cities resulting in the urban heat island (UHI) effect, where urban areas often experience significantly warmer temperatures than their rural surroundings. The UHI effect arises firstly, from the modification of land surfaces. Urban areas typically have high proportions of concrete and road surfaces which absorb more of the suns energy than surrounding landscapes. This absorbed heat is retained until night time when it starts to dissipate. Urban areas also tend to have less vegetation than surrounding landscapes so don’t experience the same cooling and shading effect that tree canopy cover provides. A secondary contributor to the UHI effect is the release of anthropogenic heat emissions. These emissions come from transportation like buses and cars, as well as from heating and cooling of buildings. Our bodies also release heat via our metabolism, which you can feel when in a crowd of people. Both increased absorption of the sun’s energy, and the release of anthropogenic heat emissions, results in urban areas experiencing much warmer temperatures than their surrounds. You can watch SWAQ Investigator Melissa Hart talking more about the UHI effect in the video below.

Why does this matter? 

The UHI effect is more pronounced at night time and can have significant health impacts for residents. Modified urban surfaces which retain heat during the day take much longer to cool down at night compared to vegetated areas, leading to higher overnight minimum temperatures in cities. Heat related illnesses and deaths have been shown to be more impacted by minimum temperatures than by maximum temperatures, because higher minimum temperatures mean the body no longer has the opportunity to cool down after a hot day. With more than half the world’s population now living in cities, the UHI effect is of considerable importance. Watch SWAQ Investigator Melissa Hart talk more about these impacts and mitigation options below.

A warmer city due to urban heating can result in an increase in energy use as we try to cool our buildings down, and this increased energy consumption can lead to a rise in greenhouse gas emissions. Consequently, cities contribute to the effects of both global changes (e.g., climate change) and regional changes, such as the UHI, and city residents are impacted by this double whammy of changes in climate. The SWAQ project can help us understand the UHI effect by collecting local weather data across urban Sydney, allowing us to monitor the effects of urbanisation on local weather and air quality while contributing to cutting-edge research into cleaner and healthier cities.

Extreme Weather Events

SWAQ instruments can also provide us local data for understanding extreme weather events. Extreme weather events are severe, unusual, unexpected or unseasonal weather such as flooding, storms, droughts, and heatwaves, that can have significant impacts.  A common extreme weather event experienced in Sydney, and indeed along the whole NSW coast, are east cost lows (ECLs). East coast lows are intense low pressure systems that form off the coast of Australia from south-east Queensland down to far eastern Victoria. They can produce damaging wing gusts and large seas leading to coastal erosion and sometimes inundation. The more intense east coast lows can create winds with strengths comparable to category 1 and 2 cyclones. They also move slowly causing prolonged periods of heavy rainfall which can lead to flash flooding. This video by the Bureau of Meteorology provides more information on east coast lows and how they form.

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