Weather and air quality

Direct effects of weather on air quality

What the weather is doing can have direct effects on air quality at a given location. For example, sunshine, rain, air temperature and wind can affect the amount of air pollution present:

• Sunshine - makes some pollutants undergo chemical reactions, producing smog.
• Rain - washes out water-soluble pollutants and particulate matter.
• Higher air temperatures - speed up chemical reactions in the air.
• Wind speed, atmospheric turbulence/stability, and mixing depth - affect the dispersal and dilution of pollutants.

Different areas in the Waikato region are exposed to different weather patterns and different levels of air quality. For example, coastal areas are exposed to consistent winds, which help disperse any pollutants. In inland areas, air pollutants can build up when there is little or no wind.

Wind speed

Wind carries air contaminants away from their source, causing them to disperse. In general, the higher the wind speed, the more contaminants are dispersed and the lower their concentration. However, high wind can also generate dust – a problem in dry windy rural areas.

Atmospheric turbulence/stability

As the ground heats during the day the air becomes more turbulent, especially in the middle of the day. Air turbulence causes polluted air to disperse as it moves away from its source.

In contrast, stable conditions often occur at night when the air is cooler. Air contaminants released in urban areas at night, such as from home fires, are not easily dispersed causing localised air pollution.

Inversion layers

Air usually cools with increasing height in the atmosphere. However, sometimes an upper air layer is warmer than a lower one. This is called an inversion. In the Waikato, inversions often form on clear, calm nights when the ground cools rapidly.

The upper layer of warm air acts like a lid. It traps a layer of cold air beneath it, and air contaminants, such as smoke from home fires, get trapped in the bottom, colder layer.  These trapped contaminants can cause health and nuisance effects. Air contaminants build up when inversion layers form close to the ground (mixing depth). 

Inversion layers are usually dispersed by wind or by warm air rising as the ground heats up. But if the inversion layer stays in place for a long time pollutants can build up to high levels.

The diagram below shows an inversion layer trapping smoke from a home fire.

Some towns in the region, such as Tokoroa and Te Kuiti, are especially prone to inversion layers. 

Indirect effects of weather on air quality

The weather can also affect air pollution by affecting people’s activities.

For example, in cold weather we tend to:

• light more fires - for home heating
• travel to work in motorised forms of transport more often.

In warm weather we tend to:

• use barbecues more often
• go away for weekend trips or holidays - using motor vehicles.