Temperature inversion is an atmospheric phenomenon that often raises concerns about air pollution. In recent years, there have been many days when the air in metropolitan areas has been so polluted that it has led to the closure of schools and serious health warnings. On cold autumn and winter days, the blame is usually placed on the temperature. Here we explain what temperature inversion is and is pollution really inevitable? Inversion is an atmospheric phenomenon that can be clearly seen because it covers the city like a quilt. If you leave the polluted area of the city during the temperature reversal, you can see thick and gray layers of pollutants on the city from afar. The top border of this layer is usually a smooth line that simplifies its detection. Above this border, the air is very clean and clear.
A temperature inversion in Vancouver
There are two types of temperature: superficial inversion and permanent inversion. Before explaining the two, we must remember the physics of heat transfer. According to the phenomenon of convection, hot air rises and gives way to cold air. The air above a heat source, such as a heater, heats up continuously and moves upwards. In addition, the higher we go, the lower the temperature. This is why the top of a mountain is usually colder than the bottom. So it is natural to conclude that the airflow is always from the ground (warmer air) upwards (colder air). This vertical upward movement usually carries airborne pollutants into the atmosphere and away from the city. But when the temperature is reversed, the air in the sky becomes warmer than the air on the surface of the earth, disrupting the upward movement of pollutants. Therefore, pollutants get trapped near the surface and gradually increase in size.
Superficial inversion usually occurs on cold days, in autumn and winter. It is formed in the atmospheric layer of the Sepehr (troposphere), which is the layer of all meteorological phenomena. In cold seasons, long nights are an opportunity for the earth to lose its surface heat gained during the day. If the air is clear, cold, and dry (the prevailing air on winter nights), the heat radiation radiates much faster and the earth transfers its heat to the air. In addition, the sun shines in winter. So its radiation warms the atmosphere around the earth more than the surface of the earth. The sum of these reasons makes the air at higher altitudes warmer than the air close to the ground. The same thing disrupts the upward flow of air and the accumulation of pollutants in the city air. Permanent inversion occurs for the same reasons, but in the stratosphere, which is the upper layer of the sphere. Pollutant particles usually reach this layer by volcanic eruptions and remain suspended. The effect of permanent inversion is longer and wider and sometimes even global.
A temperature inversion in Salt Lake City
Air pollution is serious on days of temperature. Smoke, pollutants from the industrial exhaust, industrial exhaust fumes, and fire smoke are all trapped and accumulated near the ground. In addition, nitrogen oxides and volatile organic compounds in pollutants are converted to ozone gas when exposed to the sun, which is harmful to health by inhalation. On such days it should be remembered that inversion is not the cause of contamination. The best planning solution is to produce less smoke and particulate matter. Less car use, no lighting of fires except in emergencies, and less use of the smoke industry.
In order for inversion to occur, the following conditions are necessary:
Calm and windless air, because wind and turbulence in the air cause the air temperature to mix at different levels and prevent the formation of temperature. Clear and cloudless air in such weather will provide better conditions. Because the presence of cloud cover in the sky absorbs infrared radiation from the earth’s surface and reflects it back to the earth’s surface, causing the air to reheat from below. As a result, in cloudy weather, the probability of temperature is less. Long night. On long nights, the earth will have enough time to reflect the energy absorbed during the day and time will be provided to transfer heat energy to the higher layers, so the longer the night, the thicker the inversion layer will be. From the above conditions, it is clear that winter nights, especially when stable air masses are inhabited in an area, will create the conditions for radiative inversions more than ever. This type of inversion gradually disappears with the rising of the sun. The heavy cold air and the unwillingness of this air to climb and expand and combine it with the upper air on the surface of the earth condenses and causes an increase in the concentration of pollutants inside the human inhaled air and will lead to complications.
Turbulence means the creation of small turbulences in the air, which appear in both thermal and mechanical forms, which have two completely opposite effects on temperature. The first type occurs during the day with sunrise and results in the disappearance of temperature, and the second type occurs as a result of wind blowing on uneven surfaces such as bumps and high-rise urban buildings. This process causes rotations. Small inside the air in the labyrinth of buildings or between uneven shrinkage and as a result the lower part of the air is separated from its upper surfaces and this layer of air that does not lose its temperature overnight radiation is not mixed with the upper air and The face of a cold layer appears as an inverted layer below the warmer upper layers. This phenomenon is seen in large cities with tall buildings or natural surfaces or cities located on the edge of hills. Turbulence enhances radiation inversion.