Urban Food Garden

Urban Climate

Urban environments have a big impact on local climate patterns as they generate and store huge amounts of heat.  Understanding this impact will help you manage your garden’s microclimate.

The urban heat island effect

Cities of a million or more people can be 1–3°C warmer on average and as much as 12°C  warmer in the evening than the surrounding countryside*.  This is called the Urban Heat Island Effect.  While it is more pronounced in large cities even small regional towns will experience some form of this heat island effect.

* Source: U.S. Environmental Protection Agency.

Graph showing how the heat island effect impacts on an urban environment.

thermal mass and heat generation

There are two key ways air temperature is raised in an urban environment, by thermal mass and heat generation.

thermal mass

‘Thermal mass’ describes a material’s capacity to absorb, store and release heat.  Examples of materials that have a high thermal mass are buildings, concrete paths and bitumen roads.  These materials absorb the heat of the sun’s rays during the day and slowly give them off at night, thereby raising the air temperature in the immediate area.

Heat generation

This is heat that is generated actively by power sources such as motor vehicle engines, wood or gas fires, electric heaters and reverse cycle air conditioners.

Illustration showing examples of thermal mass and heat generation sources.

The impact of urban climate on your garden

If your garden is in an urban setting then the urban heat island effect may mean that you can plant earlier in the season and grow warmer climate plants than those with gardens in the nearby countryside.  But it could also mean that your garden will be hotter in summer.  To counter these hotter conditions see Altering A Garden’s Microclimate.

Local climate variations

There can also be factors influencing the urban climate at a local level.  For instance a leafy treelined street will be cooler in summer than a street without trees, which will in turn have an impact on the houses and gardens in that street.  Being near water (such as a lake or river) can also have a cooling effect as air passing over water will be cooled slightly.

A leafy treelined street will be cooler in summer than a street without trees.   (Image courtesy of the internet)