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Indicator of our atmosphere's health

What is Haze?

The haze we see in our city skyline is caused by tiny particulates suspended in the atmosphere. At high concentrations, these particulates scatter and absorb sunlight resulting in diminished horizontal visibility thereby giving the atmosphere a characteristic opalescent appearance. Haze is not confined to urban environments; it may also be observed in rural areas.

Haze Particles - Their Origin

The particles that cause the haze phenomenon can originate from many sources, some of which are natural and some anthropogenic. Natural sources include the oceans, forests and ground surface. However the majority of the particulates are from human activities which include open burning, land clearing, vehicular use and combustion of fossil fuels in industrial boilers. Open burning Vehicle emissions

Atmospheric Conditions That Favour Haze Formation

In the tropics, wind speeds are generally low. During certain periods of the year, particularly during the Southwest Monsoon season, the atmosphere is very stable and horizontal as well as vertical air movements are reduced. The subsiding air mass does not favour active cloud development, thus the weather is dry. Several factors such as prolonged dry weather, a stable atmosphere, and an abundant supply of pollutants from urban or rural sources are the ideal ingredients for the formation of haze. Particulates emitted into the atmosphere are trapped within the stagnant air mass causing the particulate concentration to increase thus producing a hazy condition. In Peninsular Malaysia, haze is most likely to occur during the months from January to February and June to August.

On the other hand, during the Northeast Monsoon season, the moisture-rich northeasterly winds blowing from the South China Sea are generally stronger and therefore more effective in dispersing haze pollutants. The Northeast Monsoon is often associated with more frequent widespread rain, thus haze is seldom observed.

What Causes The Haze To Clear?

In our changing atmosphere, just as there are hazy days, there are also days which are clear and clean. What can bring about this change? Particulates in the atmosphere are removed through several processes. A proportion, particularly the heavier particles, settles to the ground or onto surfaces by gravitational sedimentation.

The finer particles are removed by a process called "rainout" involving condensation of water vapour onto minute particles to form water droplets eventually producing clouds. Particles are also removed below cloud by collision and adhesion with falling raindrops through another process called "washout".

Yet another removal mechanism involves the role of turbulence in transporting the haze particulates up to the higher levels of the atmosphere for more effective dispersion. In this case, widespread heavy rain/thunderstorms with strong updrafts and downdrafts are more effective, whereas light rain showers of short duration are usually quite inefficient in cleansing the atmosphere in a severe haze situation.

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