Dr Peyman Zawar-Reza, Geography Department at the University of Canterbury, explains how temperature inversion occurs due to the heating of land during the day and cooling of air at night. He explains that, in Christchurch, temperature inversion is greater because the surrounding hills cause the cold air, which is denser, to drain down the slopes of the hills into Christchurch.
DR PEYMAN ZAWAR-REZA
The normal state of the atmosphere – warm close to the ground, much colder as you go up in altitude. Temperature inversion is just the reverse of that. That is why it’s called an ‘inversion’. It’s when it’s actually cold at the surface, and as you go up in height, it gets warmer or like an inverted atmosphere. The temperature of the air is controlled by two different sorts of wavelengths of light – one that we can see, which is the visible light, and one that we can't see, which is the infrared light. That other thing you need to know is that the Sun’s rays don't actually heat up the air directly, because the air is very transparent to our Sun’s rays. So almost everything that comes from the Sun just passes through the air – it doesn't really warm it up. The fact is that the Sun comes up and what it does is it heats the land – that's why the days are warmer. So that is the visible light heating the land. At night, the land air cools. It cools because of the type of radiation that we can't see, but it’s out there – it’s called infrared radiation. So when the Sun goes down, the only source of radiation that is left is the surface, and the surface just gives off all the heat that it gains during the day through infrared radiation, which we can see. When the land cools, it takes all the heat back from the air, cooling the air. So that is why the nights tend to be colder, because the ground is sucking out the heat from the air, and when it does that, it cools the air close to the ground forming the temperature inversion. You can get temperature inversions anywhere. What is special with the Christchurch region is we are situated close to sloping terrain. Cold air happens just to be dense, and what happens when you have a dense thing over a sloping terrain, it drains down to the lower elevation. So in the case of Christchurch, we have the Port Hills, which are sloping terrain, and we also have the Southern Alps, which is a sloping terrain. So this cold air comes and drains over Christchurch and it kind of enhances the effect of temperature inversion. It makes it a bit stronger than it might have been if these features weren't around.