Weather conditions have a very important influence on air pollution. This is due to several factors:

1. How rapidly and effectively any emitted pollutant is diluted by cleaner air. Usually, emissions from smokestacks or vehicles become mixed with cleaner air and pollutant concentrations remain low. However, if winds are very light and there is little vertical air motion (stable conditions) then the air volume for dilution and the mixing rate are both reduced, and concentrations may rise steadily as long as such conditions persist. Vertical air movement is greatly restricted by temperature inversions, conditions in which temperature rises with height instead of falling. Inversions are common in the Saint John region due to cooling of near-surface air by the waters of the Bay of Fundy. Another situation which may lead to pollutant build-up is when emissions are caught up in a sea breeze circulation near the coastline, usually in summer. The polluted air may travel offshore and return inland, possibly more than once, leading to adverse air quality episodes.

2. The pollution concentration in air brought into the region from elsewhere. New Brunswick is particularly affected in this way, as winds travelling from the west or southwest, which prevail most of the time, have often travelled over densely populated regions of the Northeastern United States or southern central Canada, picking up pollution en route. This long-range transport effect is significant for ozone, acid rain and fine particulate pollution, which also affects visibility.

3. Conditions of temperature and sunlight. Particularly for ozone-containing smog, the temperature and intensity of sunlight have a strong influence. There may also be some effect on particulate pollution. High temperatures increase the evaporation rate of VOCs and strong sunshine speeds up the chemical processes which generate ozone. This is why ozone smog almost always occurs in summer. The lower atmosphere is often stable during hot sunny weather, which tends to favour ozone formation as well, by restricting the dilution of NOx and VOCs.