Heat Islands: Introduction

In this lab, you will learn about urban heat islands and how they can be calculated from satellite measurements of thermal radiation from the Earth’s surface.

Learning Outcomes

Introduction to Theory

Urbanization involves the replacement of natural landscapes with built-up structures such as buildings, roads, and parking lots. This land cover modification also changes the properties of the land surface. These changes can range from how much radiation is reflected and absorbed by the surface to how the heat is dissipated from the surface (for example, removing vegetation for urban development reduces evaporative cooling). The changes in surface properties can modify local weather and climate (Kalnay and Cai 2003). The most-studied local climate modification due to urbanization is the urban heat island (UHI) effect (Arnfield 2003; Qian et al. 2022). The UHI is when a city is warmer than its surroundings or an equivalent surface that is not urbanized. We have known about the UHI effect for almost 200 years (Howard 1833).

Traditionally, the UHI was defined as the difference in air temperature, measured by weather stations, between a city and some rural reference outside the city (Oke 1982). One issue with this method is that different parts of the city can have different air temperatures, making it challenging to capture the UHI for the entire city. Using satellite observations in the thermal bands allows us to get another measure of temperature: the radiometric skin temperature, often known as the land surface temperature (LST). We can use LST to calculate a surface UHI (SUHI) intensity, including how it varies within cities at the pixel scale (Ngie et al. 2014). It is important to stress that the UHI values observed by satellites and those calculated using air temperature measurements can differ significantly (Chakraborty et al. 2017, Hu et al. 2019, Venter et al. 2021).

Land surface temperature can be extracted from derived products, such as the MODIS Terra and Aqua satellite products (Wan 2006), or estimated directly from measurements in the thermal band.

References