Media likes to exaggerate China’s pollution to an unbelievable level. Pollution in China is a fact. However when we encounter pictures of Chinese people choking in smog in cities like Beijing, the media fails to mention that a lot of that smog has to do with geography and atmospheric conditions that are particular to these cities.
Natural phenomenon called inversion
There is a natural phenomenon called inversion that can lead to the kind of smog we see in the news. Usually air temperature at sea level (surface temperature) is higher than air temperature at high altitudes. This is because sunlight warms the surface of the Earth which then warms the air near the surface. Warm air rises, cold air falls. This is known as convection. Warm air carries pollutants up into the atmosphere.
Inversion happens when certain atmospheric conditions come together. Surface air becomes cooler than air at higher altitudes. When that happens, pollutants hug the surface because they are carried down by cold air. Sometimes there is a steep change in temperature gradient so that there are almost two distinct temperature layers. When this happen, there is a ‘cap’ or ‘glass ceiling’ that pollutants cannot escape.
In this picture above of Almaty, Kazakhstan, an inversion traps smog in the city. The picture shows two distinct layers. This is because there is a steep change in the temperature gradient. Cold air sinks to the ground trapping pollutants while the layer on top has warm, clean air.
Geography is another factor
Some cities are more prone to smog than others. This is because of geography. Experts found that cities that are surrounded by high ground such as hills or mountain range are more prone to smog. Imagine these conditions coming together:
- City sits in the centre of a ‘bowl’.
- Surrounding high ground acting as a ‘wall’ preventing pollution from escaping.
- When an inversion happens, pollution that are usually carried out of the city by hot air is instead trapped and pressed into the ‘bowl’ by cold air.
This picture of smog over Nuwa Roda in Poland clearly shows the city located in a valley surrounded by higher elevation.