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 How the weather affects
air pollution



The along with the number of souces and amount of smog produced, the weather plays a big part in the extent and effects of air pollution on any given day. On windy days, smog is blown away from the city but may effect country areas instead. On still days, pollutants are able to become more and more concentrated. Smog concentration is also increased by the following effects.  

 

Heat and Sunlight

Secondary pollutants are mostly the result of reactions involving heat and/or sunlight. As a result, photochemical smog reaches its highest concentrations on warm, sunny days when there is little wind. The photographs below demonstrate the difference such weather can make to visible smog (e.g. nitrogen dioxide) concentration.
 
A Winter day in Los Angeles
A Summer day in Los Angeles
 
 
 

Temperature Inversion
Sometimes pollution can be trapped close to the earths surface rather than rising up into the atmosphere as is usuallt the case with warm air. Temperature inversions can occur if the ground has cooled very rapidly (usually on a calm, clear night). A cold layer of air forms at ground level and is trapped by the warmer air above it. As peak hour arrives in the morning, pollution produced remains trapped in the lower, cooler layer of air, causing concentrations at ground level to become quite high.
 
 
 

 The Melbourne Eddy
 
Melbourne and some other cities have particular geographical features that make them more susceptible to smog than other cities. Melbourne is located on the shores of Port Philip Bay. This places the city in a sort of bowl shaped area made more effective by the presence of the Dandenong ranges on one side and other hills on the other. The result of this is a circulation of air as shown in the diagram on the left. This means that smog can stay in the area for up to 8 days. Luckily, this occurs only when there is an easterly wind which is not very often as the prevailing winds are from the west.
 
 
 

Summary - What to look out for on the weather forecast.
If you are one of those people most at risk from the effects of air pollution, you should beware of sunny, still days, especially in Summer. If you live in Melbourne, watch for days on which easterly winds are forecast. You can see this from the weather map - if there is a low pressure system north of a high pressure system and both are located to the east of the city, the wind will be coming from the east. 
 
 


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