3.6. Temperature Inversion

The temperature profile turns out if you go out now, is generally like this (as shown in the picture), approximately, to a certain height beyond that it will change. So, this height beyond that it will change is somewhere in the troposphere.

The temperature profile describes how temperature changes with height above the ground. Generally, as you go higher in the atmosphere, the temperature tends to decrease, at least up to a certain height known as the troposphere. This is crucial for understanding atmospheric conditions, as it influences how pollutants disperse.

Think of a warm soup in a tall pot. The soup near the bottom is hot because it's heated directly by the stove, while the air above the soup gets warmer as well but isn't as hot. If you spill some soup, the warm liquid (similar to a pollutant) will rise to the surface because that’s where the heat is concentrated.

During daytime the radiation heats up the soil or the land faster than it heats the air. So, the air above the surface heats up from the soil, it does not heat from the sun directly because the soil heats much more rapidly than the sun.

During the day, the sun warms the ground, which in turn heats the air directly above it. This creates a temperature gradient where the air near the ground is warmer than the air higher up. This can lead to a positive temperature gradient, promoting vertical movement of air known as convection.

Imagine standing on a beach. The sand gets very hot under the sun, while the air a few feet above it remains cooler. When you walk along the beach, you can feel that the air close to the ground is warmer than the cooler breeze at head height.

When there is no radiation, say at 7pm, the soil then starts cooling, it cools very rapidly. The air is still hot but the soil has started cooling.

As the sun sets, the ground begins to cool rapidly compared to the air above. This can create a situation where the air near the ground becomes cooler than the air above, leading to a phenomenon known as temperature inversion. In this scenario, temperature increases with height, which is the opposite of the typical lapse rate.

Think about a warm-up session in the morning at a gym versus the cool-down at the end. The gym cools down quickly once everyone leaves, while your body temperature stays elevated for a while longer. This difference in cooling rates can create pockets of warmer air above cooler air, resulting in a temperature inversion.

This region is called as the temperature inversion. The temperature inversion means generally in the daytime temperature is reducing as a function of height, but here temperature is increasing as height.

Temperature inversions can significantly affect how pollutants disperse in the atmosphere. When an inversion occurs, pollutants that are normally rising can become trapped under the warmer air above, leading to higher concentrations of pollution in a stagnant air mass.

Consider a lid on a simmering pot of soup. The lid traps steam, preventing it from escaping into the air. Likewise, a temperature inversion traps pollutants, preventing them from dispersing and leading to smoggy conditions.

If I release a parcel here, what happens is, if its temperature is higher, it wants to go up... If there is no exchange of energy, its volume also expands like this and it cools.

An air parcel that is warmer than its surroundings will rise due to buoyancy. As it rises, it expands and cools due to lower pressure at higher elevations, a process known as adiabatic cooling. This principle helps explain the vertical dispersal of pollutants.

It’s like releasing a helium balloon. The balloon rises because it is less dense than the surrounding air. As it climbs higher, the balloon expands due to lower air pressure, while also losing some heat, which may alter its properties as it ascends.

If you have an unstable environment, this height is very high, it has the opportunity to mix in a larger volume and therefore the concentration of pollutants is going to be smaller.

In unstable atmospheric conditions, air parcels can rise and mix effectively, leading to lower concentrations of pollutants in the atmosphere. Conversely, stable conditions can trap pollutants close to the ground, resulting in higher concentrations and poorer air quality.

Imagine a crowded room filled with smoke from a campfire. If the windows are open (unstable conditions), the smoke disperses easily. If the windows are closed (stable conditions), the smoke stays concentrated in one area, making the air difficult to breathe.

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