WX BRIEF

Winter weather refresher

Learning synoptic scale charting for storms can eliminate many frigid air headaches.

By Karsten Shein
Comm-Inst, Climate Scientist


Aircraft being deiced at DCA in Jan 2010. Most adverse winter weather we encounter is connected to the passage of large-scale winter storm systems that track beneath the jet stream.

For much of business and commercial aviation, the months from November to March are filled with weather delays, deicing fluids and shivering early morning preflights. They are also months in which a number of aviation accidents occur in adverse winter weather.

They are caused by a variety of reasons, and the weather is sometimes listed as a contributing factor—but, except for blizzard winds flipping a parked aircraft or heavy snow collapsing a hangar on the aircraft inside, NTSB and other safety bodies rarely attribute weather-related accidents directly to the weather.

Rather, the lion's share of winter weather accidents are blamed on factors such as failure to obtain a weather briefing, failure to deice, poor decisionmaking and other such considerations. In other words, at least according to the accident investigators, it's usually the pilot's fault.

Fortunately, most winter weather is relatively predictable. With the exception of some mountain blizzards, most of what we consider adverse winter conditions is the result of reasonably well understood larger weather systems known as midlatitude cyclones. These develop around centers of low pressure and move along beneath the polar jet stream which has descended into the lower middle latitudes during the winter. With an understanding of the formation and life cycle of these systems, pilots can gain an upper hand on most winter weather.

The setup

After the summer solstice in June, the North Pole again begins to tilt away from the Sun, and after the autumnal equinox in September, the noon Sun is centered over the Southern Hemisphere. What this means is that the pool of cold air that resides over the Arctic is able to expand as more heat energy is lost to space than is received from the Sun. Because cold air is dense, it pushes equatorward, driving the boundary between itself and the warmer air toward lower latitudes.

The boundary also intensifies as the temperature difference between the tropics and the poles increases. Throughout the year, temperatures in the tropics do not vary a great deal, since the Sun is more or less overhead year round.

In the summer, the high latitudes are also receiving a lot of sun, meaning the temperature difference is not quite as great, and most of the midlatitudes falls under the influence of the warmer subtropical air. But in winter, the arctic temperatures are far colder relative to the warmer air at lower latitudes. This enhanced difference is the setup for some significant large-scale storm systems.

For us this means 2 things. First, far above the surface boundary between the warm and cold air—known generally as the polar front—is the circumpolar vortex, more commonly called the jet stream. In a most basic sense, the jet stream is a core of rapidly moving air that exists because at the top of the troposphere, due to the temperature difference, there is a very strong change in pressure over a very small distance above the polar front.

Second, the force created by a change in pressure over a given distance in any direction is what results in air movement. The greater the change, the greater the force and the faster the wind. Since temperature and pressure are related to one another, pressure differences in the atmosphere are generally a result of differences in temperature caused by the uneven heating of the Earth by the Sun.

Because the temperature difference between warm and cold air is greatest in the winter, the resulting pressure difference that drives the jet stream is also strongest in the winter, meaning faster wind speeds.

Convergence

Winter storms generally begin beneath the downwind region of a trough in the jet stream. Here diverging air aloft creates a way for surface air to be evacuated to the upper troposphere, forming a surface low.

But simply having a strong jet stream overhead doesn't cause winter storms—one additional factor is necessary. The waves that move through the jet stream change the direction and speed of the wind. This results in the wind either converging or diverging. It is similar to driving along a road and coming upon a work zone.

As the cars ahead of you slow down, the distance between you and them decreases as you converge with the traffic. The convergence is worsened if there is an entrance ramp at the start of the work zone and more cars are trying to join the flow. Toward the end of the work zone, the cars exiting are accelerating, increasing the gap between them and the cars still in the work zone.

If there is an exit near the start of the work zone, something else happens. Some of the cars will get off the road to avoid the backup. Likewise, the increasing gaps between cars at the exit of the work zone permit new cars to enter the road easily at that point. The same thing is going on in the jet stream as a result of the crests and troughs of the waves that move through it.

As air flows toward a trough—the U-shaped low spot in the jet stream—with the base of the trough representing the "work zone," the air converges as it must change direction to flow through. As it exits the base of the trough, it is able once again to accelerate and diverge.

The increase in air mole­cules in the convergence zone means greater pressure over the surface, and the formation of a surface high. Conversely, the decrease in air molecules in the divergence zone means the establishment of lower pressure at the surface. In a nutshell, this is how the massive low pressure systems that paralyze our airspace in winter get their start.

The wave that creates these convergence zones does not have to be large. Most storm systems are due to relatively small disturbances moving along the larger jet. But the larger jet pattern is what guides the storm once it forms. A favorable setup for storm formation is a prominent jet stream trough through which smaller waves can flow. For example, a deep stationary trough over the eastern US can spawn a string of nor'easters as a series of short waves migrate through it—as we saw last winter.

The low forming at the surface creates a new pressure force that draws in surrounding air. As the air spirals in, the divergence aloft gives it a place to go. If there is more divergence aloft than air converging at the surface, the low will deepen, strengthening the developing system, but if the opposite is true—which is often the case when the surface low moves out from beneath the trough exit—the low will kill itself off.

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