What is commonly referred to today as the GreenHouse Effect (GHE) is actually quite the opposite, it is an Anti-GHE. This may go against everything that most of us have learned in school, but we’ve all have heard that the GHE is not actually a greenhouse, so what is it?
A greenhouse raises the temperature by preventing Convection. What is Convection? Convection is the process of gravity dragging cooler more dense air beneath a less dense warmer air mass. Hot air does not levitate so much as cool air sinks ‘pushing’ the hot air upwards.
So here on Earth, the surface heats the air just above it through conduction causing that air mass to warm and expand. As it expands it becomes less dense than the air mass next to it and gravity pulls more strongly on the cooler air mass dragging under the warmer air mass. This causes the warmer air to rise. Since the Earth’s surface is highly variable with regards to how much a particular region heats up and cools, this causes convective currents, which can be thought of as rivers of air. Thermals that birds and gliders use are a great example, they are very powerful currents.
So a greenhouse prevents the warmer air from expanding and prevents cooler air to be dragged beneath it, thus having a GreenHouse Effect.
So what is this so-called Radiative GHE? This is the process of certain gases absorbing and emitting Infrared radiation that the Earth emits as it cools via radiation. Since these molecules radiate in all directions, on average half will be emitted back towards the Earth thus preventing or better yet slowing the rate of Radiation. Sound like a Greenhouse? Is it preventing Convection? Nope, it is actually encouraging convection. Here’s how.
If a subset of the radiation that is leaving the Earth is redirected back to the surface, then in effect the ability for the surface to radiate has been decreased; its emissitivity has been decreased. This in a vacuum would cause the surface temperature to rise. However, we are not in a vacuum and Convection states that if the surface temperature rises, then the rate of Convection will also increase. The air mass just above the ground heats up and expands more quickly and the cooler air masses are dragged more quickly below it. An increase in heating causes an increase in cooling.
Now does it sound like ‘trapping’ radiation acts like a Greenhouse? Not at all, it in fact does the opposite, instead of preventing convection, it actually encourages it.
Since a greenhouse acts by preventing or reducing a convection rate, that would really be known as a Greenhouse Effect. Similarly a prevention or reduction in a radiation rate (emissitivity) would be known as an Emissitivity Effect. No matter how you spell it, it is an Anti-GreenHouse Effect (AGHE).
The End is FAR,
Steve A Morris