Two weeks ago, Hannah asked an excellent question: Will meteorology ever be an exact science? The quick and dirty answer is no, absolutely not. To illustrate why this is so, however, I am going to use a specific example usually taught in an introductory atmospheric science course. Before doing so, I am going to make a few definitions clear so that everyone is on the same page.
The atmosphere is a fairly complex system in its entirety, so we generally divide it into different layers, each of which has its own special attributes. The layer closest to the surface of the Earth is called the troposphere. In general, it extends ten to sixteen kilometers above the Earth, varying depending where you are at the surface. About eighty percent of the atmosphere’s mass is contained within the troposphere, making it (understandably) the layer of air most responsible for our weather.
The shallow layer of the troposphere closest to the Earth (the layer in which we reside) is often referred to as the atmospheric boundary layer. It typically extends no more than a kilometer above the surface of the Earth. If the troposphere is responsible for weather, then, by analogy, the boundary layer is responsible for mixing air in just the right proportions to make that weather possible. This makes studying the boundary layer extremely important to meteorology, more specifically forecasting.
Often times when atmospheric scientists study the boundary layer, they wish to see how thermal energy is being transported. Is heat traveling upwards in the boundary layer, making clouds and other phenomenon possible, or is heat traveling downwards, having the opposite effect? To quantify this transportation of thermal energy in the boundary layer, we calculate sensible heat fluxes. In a nutshell, sensible heat fluxes look at the amount of thermal energy passing through some area over a given time. If the sensible heat fluxes are positive, then we can deduce that thermal energy is moving upwards in the boundary layer. Conversely, if the sensible heat fluxes are negative, then thermal energy is moving downwards. Sensible enough, right?
Mathematically speaking, sensible heat flux QH is given by the following formula: QH = ρcp(θ’w’)avg. While there are other components in this equation, I want to focus our attention on the last quantity: (θ’w’)avg. This expression is what is known as a Reynolds average. In effect, it tries to take into account all of the slight up/down movements of thermal energy in the boundary layer to get a sense as to whether or not there is a significant net movement upwards or downwards of that thermal energy.
This, I hope you realize, is almost impossible to do. We would need a lot of data to notice all of the slight nuances of thermal energy movement in the boundary layer. Worse yet, if we did have enough data, these perturbations in thermal energy would change rapidly over time; the simplest actions…such as a butterfly flapping its wings…can disrupt energy balance in the boundary layer.
The inability to quantify or predict all variables influencing the boundary layer is a large outcome of what is known as The Butterfly Effect. Because we lack sufficient data and because the state of energy balance in the boundary layer is so chaotic, forecasting meteorologists find it impossible to predict weather phenomenon more than a few days out. While we may be able to add a few more days to the weekly planner in the future, there will come a time when we cannot forecast any further out, and it all comes down to that Reynolds average and its dependency on The Butterfly Effect.
As always, if you have more meteorology questions, feel free to post them in the comment section below.