What is Pollutant Drift and Inversion Layers?

The movement of industrial airborne particles such as smoke, dust or odour, is often much more complex than wind directions and speeds indicate.  In most cases it is desirable for the pollutant to be highly dispersed, thereby staying under allowable limits, and avoiding issues with your neighbours.  The factors that are most likely to effect the dispersion of a polutant are net wind speed and direction, the variability of wind speed and direction and inversion layers.

Having a weather station on site at your industry is the best way to detect if an inversion layer is present, as well as record the wind speed and direction to determine the characteristics of dispersion.

Inversion layers

The air temperature is normally warmest at the earth’s surface and steadily reduces as you go up into the atmosphere.

Hot air is less dense (lighter) than cold air.  Therefore, normally, the warm air at the surface slowly rises taking any air pollutants with it.  This helps disperse the pollutants so that they are undetectable to humans and not a nuissance to immediate neighbours.

However, if a still or stable layer of colder air sits above warmer air, it forms a blocking layer or inversion layer that prevents the rise and dispersion of the warm air and the pollutants.

This is only likely to occur in low or no wind conditions.

Weather Stations and Inversion layers

Ground weather stations can measure the temperature at 2 heights (typically 1.5 metres and 10 metres) and this can be used to indicate the presence of an inversion layer.

Normally, the uppermost sensor will be at the same or a slightly lower temperature.  But in an inversion condition, the 10 metre high temperature sensor will indicate a HIGHER temperature as this sensor is in the warm air BELOW the layer of cold air (the inversion layer).

In the atmosphere, the typical rate of cooling of ‘dry’ air (the ‘Adiabatic lapse rate’) is one degree per 100 metres.  This difference would amount to about 0.1 degrees difference over the 10 metre height difference of the two temperature sensors.  However, at the earth’s surface, rates are typically much higher, which means the difference between the weather station temperature sensors at 1.5 and 10 metres is much more pronounced.

Other effects also occur near the surface, such as ‘cool air drainage’, where pockets of cold air tend to ‘flow’ downhill.  These are called ‘katabatic’ winds, and are typically in the evening or early morning.

Sigma Theta

As still air is also a necessary condition for an inversion layer to occur, the variability of the wind direction can be used as an indicator of the amount of turbulence and therfore mixing of air.  The term used is ‘sigma theta’, where sigma is a standard measure of variability and theta is a Greek letter commonly used to represent an angle in mathematics.

Sigma Theta can be calculated from Environdata weather station data as it is a built in function in the EasiAccess program, and uses either the ‘sum of sine and cosine’ of the wind direction or the ratio of the vector (directional) wind speed to the scalar (non-directional) wind speed from the weather station itself.

Typical values for stable air are 0 – 3 degrees and for unstable air 20 – 30 degrees.