Answer:
A temperature inversion is a meteorological phenomenon that occurs when a layer of warm air is trapped above a layer of colder air, causing the normal temperature gradient (decrease in temperature with altitude) to be reversed. This can result in the air near the ground being colder than the air above it.
Temperature inversions are common during the winter months, when the air near the ground is cooled by contact with the cold ground surface. They can also occur in large cities due to the heat produced by the urban environment, which can create a layer of warm air above the colder air at ground level.
In large cities like Vancouver, Toronto, Montreal, and Los Angeles, temperature inversions can be a concern because they can trap pollutants near the ground, leading to poor air quality. When the air is stagnant, pollutants such as car exhaust and industrial emissions can build up, leading to increased levels of smog and other forms of air pollution. This can have negative impacts on public health, particularly for individuals with respiratory conditions such as asthma.