Urban Heat Islands

Land Surface TemperatureThe image above shows ‘skin surface temperatures’ derived from satellite observations around the City of Boston. The Charles River and Boston Harbor are blue (cool) while roads and other impervious surfaces appear red (warm). Satellites can give us an estimate of the ‘skin temperature’ of surfaces (e.g. the temperature of pavement or the tops of trees), but these skin temperatures can be very different from the air temperatures that we experience on the ground.  More research is needed to understand how different patterns of land cover influence air temperatures, and in turn, how these different air temperatures influence plants, soils, and people.

What are Urban Heat Islands?

Built-up areas are often hotter than surrounding vegetated or undeveloped areas.  The term “urban heat island” arises from this observation.  The annual mean air temperature within a large city can be 2-6 °F higher than in nearby rural areas.  In the evening and at night on any particular day, this temperature difference can be much larger (as much as 22 °F).  Higher summer temperatures around urban areas causes increased energy demand in homes and businesses, leading to higher energy costs and greater pollution from power plants.  For residents of cities and suburbs, increasing daytime high temperatures during the summer months are a serious concern due to heat-related impacts on human health.

Conversely, the increase in nighttime low temperatures associated with urban heat islands may have the greatest impact on plants and ecosystems.  The reason nighttime low temperatures, particularly in the spring and fall, are so important is that they determine the length of the growing season.  Even a small increase in nighttime temperatures can have a very large impact in the length of the growing season.  For instance, a 2-4 °F increase in nighttime temperatures may lead to a 2-4 week longer growing season near a large city compared to more rural areas beyond the city.  This is because warmer nighttime temperatures mean fewer and less severe frost events.  The survival and spread of invasive trees, plants, and insects from warmer areas may be enhanced in urban areas due to milder winter temperatures.

Recent research suggests that the amount of built-up area alone is not sufficient to predict the magnitude and spatial variability of urban heat islands.  Instead, it turns out that there are complex temperature patterns in urban, suburban, and rural areas that are controlled by a range of landscape patterns.


What is the purpose of this study?

The purpose of this study is to understand how different patterns of land cover influence air temperatures, and in turn, how those air temperatures influence plants, soils, and people.  Increased daytime temperatures in cities and suburbs lead to increased peak energy demand, greater smog and ozone formation, higher power plant emissions, and can negatively impact human health during the summer months.  Higher nighttime temperatures in the spring and fall can significantly influence plants, soils, and ecosystems by moderating frost events and altering the length of the growing season.  Our research seeks to understand the temperature patterns around built-up areas and how these patterns influence people and ecosystems.



1) An air temperature sensor (left photo) and the solar radiation shield (center photo) that surrounds it are shown below.  The compete setup (right photo) is approximately 6 inches tall and 8  inches wide and is typically attached to a tree trunk using a fabric strap that can be loosened as the tree grows.  This instrument measures air temperature and relative humidity.

Sensor - Temperature Solar Radiation Shield - 2 Temperature Sensor on Tree

2) A soil moisture sensor is shown below.  The small metal “fork” portion of the sensor is installed in the soil, while the data logger (white) remains above.

Soil moisture sensor

3) There is a soil flux ring in each of the lawns pictured below.  A soil flux ring is merely a plastic ring that allows us to measure gas exchange between the soil and the air over a known ground area.  These rings can be installed flush with the soil surface so regular mowing and other activities can proceed normally.

soil gas ring - difficult to see    Soil Flux Ring


Frequently asked questions:

Question:  How can I help with this study?

Answer:  For those who have been contacted by our research group, allowing us to install a temperature sensor on your property would be enormously helpful for improving our understanding of how different patterns of land cover influence temperature patterns in cities – and in turn how those differences alter vegetation and soils.  We would also like to install soil moisture sensors and soil flux rings at some sites.  Please let us know if you would be willing to host these additional instruments on your property.


Question: What do the sensors and other instruments look like?  Can you put them in a discrete (fairly hidden) location on my property?

Answer:  A small temperature sensor will be housed inside a white casing (6 x 8 inches) and installed on a tree trunk or a post.  The white casing, which is more formally known as a solar radiation shield, prevents direct sunlight from heating up the sensor and encourages ventilation to ensure an accurate air temperature reading.  We would endeavor to place the temperature sensor in an unobtrusive location on your property.  Please refer to the photographs on this webpage.

If you are interested in participating, we may ask if you would also allow us to install a soil moisture sensor (for measuring soil moisture) and a soil flux ring (for measuring exchange of water vapor and gases from the soil) on your property.  Both soil moisture and gas exchange play a role in influencing nearby air temperatures.  For an idea of what these look like, please refer to the photographs on this webpage.


Question:  Do I have to be home when you visit my property? Can you tell us exactly when you will be visiting?

Answer:  You are welcome, but not required, to be home when we visit your property. We will be visiting many sites, and while we can tell you what day we will try to visit your property, it will be hard for us to estimate an exact time.  Sometimes we are unable to make it to all of the properties on our list for that day and have to return another day.  If it is important to know exactly when we will visit your property, or if your presence is required for us to visit, please let us know and we will do our best to accommodate you.


Question:  How long will it take to install the sensor (or sensors)?  How often would you revisit?

Answer:  Our initial visit should only take a few minutes.  At that time we would look for an appropriate location to install the temperature sensor (usually a tree trunk), and with your permission, install it.  In the future, we would return a few times per year (e.g. 3-4 times) for just a few minutes to download data and replace the batteries in the sensors.  With your permission, we would like to collect data for at least one year (preferably longer).


Question:  What steps will you take to ensure the privacy of property and property owner information?

Answer:  We will never disclose the exact location where the sensor data were collected or any information about the property or property owner.  The location of each site will remain obscured in any references to the data for this study.  Some of the findings of this study may be published in the form of scientific articles.  No individual location or property information will be identifiable in the published data.  Only aggregate patterns and results from the entire study will be released.  The privacy of all individual property owners and managers will be strictly protected.



If you would like more information about this project or would like to assist with this study, please contact us using the link below.