What Affects my Air Quality?


High Pressure – Responsible for Pretty Days – but Sometimes Polluted Ones, Too

High pressure is the primary weather system responsible for conditions that lead to poor air quality. High pressure affects a number of weather conditions, which contributes to an increase in the concentration of air pollutants. Pollution concentrations tend to increase on the tail end of high pressure systems, when the center of the high has passed by. Temperatures and humidity increase, so the air tends to be more polluted.

  • Temperature: In general, higher temperatures promote chemical reactions. This applies to particle pollution as well as ozone. On hot humid summer days, poor visibility is the result of small particles in the atmosphere.
  • Wind speed: As pressure builds over an area, winds become lighter. Light winds- or absence of wind- allow pollutants that create ozone and particle pollution to build up, and provide a more favorable environment for the chemical reactions necessary to create ozone and particle pollution to take place.
  • Relative humidity: This term refers to the amount of moisture in the air. Moisture helps clouds form by causing air to rise and cool. When air is dry, it does not move as much, and pollutants build up. For example, on days when ozone is high, the relative humidity is often very low. Humidity adds water to the atmosphere, and this moisture is absorbed by particles, causing them to swell and impair visibility even more. Therefore, poor visibility on humid days is the result of particle pollution and moisture interactions.
  • Sinking air: Air sinks in a high pressure system, which prevents it from cooling and forming clouds. This sinking creates days with abundant sunshine -- a key ingredient needed to start the chemical reactions that form ozone.
  • Inversions: Sometimes a layer of cooler air is trapped near the ground by a layer of warmer air above. This is called an inversion, and can last all day, or even for several days. When the air cannot rise, pollution at the surface also is trapped and can accumulate, leading to higher concentrations of ozone and particle pollution. There are different conditions that cause inversions to form. The most common is a nighttime inversion, when clear skies allow air at the surface to cool faster than the air above.


Wind Direction – Responsible for Transport and Travel of Pollutants. Large weather systems dictate the predominant wind direction, which can have a considerable effect on the quality of air in a specific city or region.

  • Upwind Sources: Air quality can worsen in your community if the wind is blowing from a region that contains numerous sources of pollution. If the winds are coming from areas with little or no pollution, they can make your air quality better. Very light winds or no wind, such as those in a strong high pressure system, can be a problem for urban areas, because all the pollution that a city creates stays in one place.
  • Recirculation: The clockwise rotation of winds in a high pressure system can create a phenomenon called recirculation. When this occurs, a dirty parcel of air in one location may circle back around to where it started -- after accumulating even more pollution. Sea breezes circulations can also cause pollutants to recirculate over a particular area. In coastal areas, the wind often blows from land toward the sea during the nighttime hours, pushing pollutants offshore. In the daytime, the wind shifts and begins blowing pollutants back onshore. Recirculation often plays a role in the worst air quality episodes.