The atmosphere is a complicated physical system, with air streaming this way and that, moisture condensing and falling as various kinds of precipitation, and the spinning of the earth affecting large-scale motions of air masses.
But some approximations and observations can be made that simplify our understanding of the weather.
The Mathematics of the Atmosphere
An analytical solution to the seven equations in seven variables that govern the atmosphere is well beyond the techniques of current — and probably future — mathematics. Mother Nature has found the solution She prefers, but we will have to be content with approximations and simplifying assumptions in order to understand it.
In addition, we can describe the atmosphere according to what we measure.
The Atmospheric System We Observe
Any rudimentary observation of the atmosphere in three dimensions reveals a remarkable thing: A powerful stream of air surrounds the globe in the mid-latitudes near an elevation of 18,000 feet, racing at up to 200 miles per hour in the direction of the earth’s rotation. We call it the jet stream. Waves, both long and short, called Rossby waves, propagate within the jet stream.
Atmospheric Potential Energy
The question ‘Why is there a jet stream?‘ has a two-word answer: energy transfer.
There are various ways that potential energy can be transformed into other forms like heat and motion. A water vapor molecule condensing is an example of the former; a rock rolling downhill demonstrates the latter.
The warm equator and the cold pole represent a state of high potential energy — warm on top of cold has lower potential energy. The atmosphere has chosen a rather complicated method of releasing the tension of high potential energy, and the jet stream is an important part of it.
The total energy balance of the atmosphere can be described as follows: Storms in mid-latitudes convert potential energy to kinetic energy of wind; this energy is dissipated as heat into the ground at more northern latitudes. The jet stream is the intermediary of this process.
A careful analysis of the jet stream shows that the average orientation of a u-shaped dip (trough) is slanted from lower left to upper right. That means that air molecules moving northward (in the northern hemisphere) have higher east to west velocity than molecules moving south. This asymmetry in the trough and ridge pattern keeps the jet stream flowing.
Long and Short Waves in the Jet Stream
Waves of different lengths propagate along the jet stream. The shorter waves move from west to east and cause most of the short-term weather such as local wind, rain, and temperature variation. Longer waves move very slowly, sometimes ‘retrograding,’ — moving westward. The longer waves tend to be stabilized by mountain ranges, particularly those in the United States and Eurasia.
The Current Long Wave Weather Pattern
The severe winter in the United States is a result of a pronounced long wave trough, pressed southward by the polar vortex, that has persisted over the center of the country, occasionally meandering east or west. The cold air beneath the jet stream has pushed far south and has met the warm, humid air from the Gulf of Mexico. This is a lethal mixture; when a short wave trough comes along providing spin and energy, snow, freezing rain, and wind are the result.
Winter and Summer Patterns
The solar radiation difference from equator to pole, which causes the jet stream, diminishes in the summer. As a result, we can expect the long wave to ease its grip on the continental United States as spring approaches. None too soon, many will say.