Boundary Layer Height HPBL:
The boundary layer is that part of the atmosphere near the Earth's surface. Its height (HPBL) ranges from several meters to several kilometers. This layer is characterized by Turbulence due to the distribution of heat on the surface of the Earth by heating from the sun and ground radiation or as a result of upward movement of air. All this play an important role in the distribution of pollutants. When this layer is high this mean that weather conditions help to disperse pollutants. And when this layer is low this mean that weather conditions help to concentrate the pollutants to rise above the acceptable or allowable limit.
It is said that the weather is stable when the distributions of temperature with height resist any upward movement of air. In this case there is no ascending or descending air currents. When the air is humid, the stability helps to fog formation at the surface of the earth, but if the air is dry, the stability help on concentration of dust, sand, and smoke near the Earth's surface layer. Then it is clear that stable weather works on decreasing the horizontal visibility near the earth's surface.
It is said that the weather is unstable when the temperature distributions with height help the upward movement of air. In this case there is ascending or descending air currents. When the air is humid, the instability helps to form low and medium clouds and thunderstorms, but if the air is dry, the air currents caused occurrence of air bumps and they help to raise sand and dust, according to the nature of the ground.
Period of increasing concentrations (critical period):
It is the period when weather conditions help to concentrate the pollutants to rise above the acceptable or allowable limit.
Thermal inversion (Temperature inversion layers):
They are areas where the normal decrease in air temperature with increasing altitude is reversed and air above the ground is warmer than the air below it. Inversion layers can occur anywhere from close to ground level up to thousands of feet into the atmosphere.
Inversion layers are significant to meteorology because they block atmospheric flow which causes the air over an area experiencing an inversion to become stable. This can then result in various types of weather patterns. More importantly though, areas with heavy pollution are prone to unhealthy air and an increase in smog when an inversion is present because they trap pollutants at ground level instead of circulating them away.
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