Weather Influences Air Quality SWCAA News

Friday, August 01, 2014 - Weather influences our day-to-day air quality even though emission of pollutants is the source of air quality degradation, the weather largely determines the quality of the air we breathe day-to-day. As air pollution rises vertically and is transported horizontally, the less concentrated it becomes. Here are ways our weather influences our day-to-day air quality.

High Pressure

When a high pressure air mass moves into our region, it is accompanied by "fair" weather: light winds, lack of storms and precipitation with few clouds. High atmospheric pressure can be of particular concern for air quality because the weather conditions it brings usually inhibit motion in the atmosphere, both vertically and horizontally. These weather elements can allow air pollution to build to unhealthy levels.

Atmospheric Temperature

Vertical distribution of temperature in the atmosphere defines the atmospheric stability, which controls the depth of vertical mixing. Mixing depth is the vertical limit of the volume of air into which pollutants emitted near ground-level are dispersed. If there is a large decrease of temperature with height, that part of the atmosphere is considered unstable because cold dense air from above displaces the warmer and less dense air below as it seeks to settle into a more stable position. The best conditions for pollution dispersion occur with strong instability and deep mixing. The atmosphere is stable when there's an insufficient decrease in temperature with height to promote vertical mixing and good dispersion. Mixing heights are usually lowest late at night or in the early morning and highest during mid to late afternoon. This daily pattern often causes pollution to be concentrated in basins and valleys during the morning and dispersed aloft in the afternoon.

Temperature Inversion

When temperature increases with height, a condition known as a temperature inversion, the atmosphere is extremely stable. In a high pressure air mass, air slowly sinks, or subsides, causing it to compress and warm at levels relatively high in the atmosphere. The warmer temperatures create a temperature inversion aloft and also inhibit the formation of clouds. While the aloft temperature inversion inhibits air movement, the clear skies contribute to the formation of temperature inversions closer the ground. Conditions that favor strong inversions are calm winds, clear skies and long winter nights. Calm winds prevent warmer air from mixing down to the ground and clear skies increase the rate of cooling at the Earth's surface. Since the nights in the winter are much longer than in the summer, surface inversions are stronger and more common in the winter. During the day, surface inversion normally weaken and disappear with rising daytime temperatures as energy from the sun is absorbed by the Earth's surface and is released to the lower atmosphere. When temperature inversion persist, which can occur when high pressure remains over a region for an extended period lasting several days or even a couple of weeks, as we saw here in the Vancouver area in January, the low mixing heights can lead to air stagnation with very little vertical motion and a build-up of pollution trapped near the ground.

Wind

Horizontal transport of pollutants by the wind disperses pollutants, but is usually inhibited in a high pressure air mass. The stronger the wind, the more pollution is dispersed. Passage of a cold front typically improves dispersion with stronger winds and an unstable air mass that can scour away existing inversions. But, sometimes the strength of these disturbances is insufficient to effectively improve dispersion. Winds oriented along a valley can help scour stagnant air out of the valley. Under many surface-based inversions there is little transport wind. The winds that do exist are often terrain-driven and localized and serve only to move pollution around locally.

Using weather data to predict air quality

To prepare the daily air quality forecast, Southwest Clean Air Agency staff review the latest pollution levels and meteorological conditions, and may consult with the local weather service office. During the heating season, this data is also used to issue wood burning restrictions. To subscribe to our e-mail list use the Sign-Up button on our home page.