Unless your flying is limited to local sightseeing in good weather, chances are you’ve used winds aloft charts at some point. For many commercial and military pilots, they’re a staple of the preflight weather briefing, and they’re easily found on sites like aviationweather.gov. These charts are constructed by weather centers at a series of designated heights, such as 12, 18, 24, and 30,000 feet. These charts obviously have utilitarian value in helping you plan fuel burn, visualize tailwinds and headwinds, and give you some idea of how you can adjust your route. However these charts also paint a detailed picture of what the weather is doing. As an aviation forecaster I’ll give you an idea of what mental picture we see when we look these charts over, and show you what kinds of hazards and trouble spots they might illustrate.
Understanding
To put the winds aloft in their proper context, it’s important to have an understanding of how the atmosphere works. We’ve covered some of the nuts and bolts already in prior articles such as “Weather Deconstructed” (June 2020), which explains at length how the global circulation develops in response to surface heating and how the upperlevel patterns organize. In this installment we’ll explain the weather systems themselves. The single most dominant feature we see on upper-level charts is the vast amount of high pressure in the tropics. Look at any winds aloft chart, like the sample below, and we can see that there is a strong tendency for the westerly upper level winds in the tropics to rotate clockwise. If you put a large propeller along the bottom margin of the chart and let the winds rotate it, it would be pushed in opposing directions in some areas, but the vast majority of the time this propeller spins clockwise. This paints out anticyclonic circulation. In the Northern Hemisphere, clockwise rotation always implies the existence of high pressure, whether at the surface, the troposphere, or the stratosphere. Likewise, cyclonic circulation has a counterclockwise spin in the Northern Hemisphere. The only exception to these rules occurs when rotation is unusually fast over small scales of space or time, making the Corolis contribution negligible. This would include circulations such as tornadoes (which get their spin from larger scale interactions), as well as mountain breezes, dust devils, and even the spin in a toilet bowl. But on large scale weather maps, the anticyclonic and cyclonic rules of thumb hold true. So the clockwise tendency in the tropics identifies areas of high pressure. Standard meteorological analysis charts, which evaluate geopotential height with respect to pressure, also reflect these areas as having “high heights,” equivalent to high pressure. This high pressure is caused by the great volume of the heated tropical air. Chances are you’ve opened a gasoline container on a hot day and noticed a release of pressure. This is due to the molecules of gasoline vapor becoming more active, building pressure in the head space and requiring more volume. It works exactly the same way in the atmosphere. The easiest escape valve for the tropics is expansion in an upward direction. Pressure levels (the level to which we would have to raise a barometer to find a specific value) rise, and we create high pressure aloft. This tendency for high pressure increases with respect to height. The opposite effect takes place in the polar regions. The relatively inactive air molecules settle down closer to the ground, taking up less volume, and leave a void aloft. So we find an increasing tendency for low pressure with respect to height. This void doesn’t just cover the North Pole; its effects extend well into the temperate latitudes. This gives us a polar vortex, which is technically not the killer storm that made headlines in winter 2014, but for meteorologists is the giant, permanent area of low pressure found aloft in the polar regions. It’s always there, even as you read this page, and it spans thousands of miles.
