How to Decipher Flight-Level Weather

Constant pressure charts not only help you with the quantitative aspects of your flight, such as finding the best altitude to minimize your headwind component, but they also help you with a few qualitative aspects, such as minimizing your exposure to uncomfortable clear air turbulence if you know what to look for. 

The primary training you received likely focused a fair amount of time on how to interpret surface analysis and surface prog charts for your preflight briefing. Sure, these do provide some important clues, such as the location of frontal boundaries, but they are essentially telling you about what will likely happen at the surface and give us with little or no guidance about what might be occurring aloft where you spend a majority of your flight time.

In other words, an area of low pressure or frontal system at the surface only tells a part of the weather story. Upper-air charts, also known as constant pressure charts, help to complete that story and provide those big picture weather details not available on other weather guidance you may use. 

First, a constant pressure chart depicts a surface that is represented by the msl height of a single pressure (e.g., 500 millibars [mb]). Meteorologists refer to this as geopotential height. Therefore, the lines on these charts are not called isobars since the entire chart describes a single pressure. Instead, they are lines of constant height, specifically, geopotential height. 

As you might imagine, the constant pressure surface is not an even surface. It varies with height for several reasons. First, the earth isn’t heated equally. The polar regions receive less insolation (solar heating) than the tropics. Second, the vertical and horizontal distribution of air density are created by differences in temperature, pressure, and moisture. Lastly, we live on a rotating body that has a profound effect on how these weather systems move throughout the hemisphere.  

Unless you are flying a spy plane, you are likely flying most of the time in the troposphere. Yes, airliners do fly in the lower part of the stratosphere. This part of the atmosphere is thin compared to the relative size of our planet. Think about holding an apple in your hand. If the apple represents Earth, then the skin of the apple represents the troposphere.

The point here is that these pressure surfaces vary with height, but not by all that much, maybe 600 meters from high to low on any given day. However, compared to the size of the aircraft you fly, the height differences are incredibly significant. If your cruise altitude is 18,000 feet or higher, you set your altimeter to 29.92 inches Mercury and you are essentially flying a constant pressure surface, thus, constantly changing your true altitude.  

 A good way to visualize a constant pressure surface is to picture a trampoline sitting on a gymnasium floor. The floor represents a msl height of zero. This is our base reference point since we can’t have “negative” heights. The trampoline mat represents the constant pressure surface. We can then measure the distance (height) between the mat and the floor anywhere under the mat. That becomes our vertical coordinate or geopotential height.

Sticking with the trampoline analogy, imagine if you and a few of your friends stand on the mat. This will cause the mat to sag at the places where you and your friends are standing such that the mat now has low points where people are standing and high points where they are not. Of course, this is all relative to the gymnasium floor (our reference point).

In the world of weather, the high and low points are called upper-level highs and upper-level lows, respectively. Here’s the typical confusion. These are not high and low pressures like you see on a surface analysis or surface prog chart. They are simply high and low “heights,” given that these pressure surfaces are constants. So, the height is changing on these charts, not the pressure.  

The height represented on these charts has a direct relationship to the density of air below the constant pressure level you are using (e.g., 500 mb). It is not affected, however, by what is occurring above this level, only below. In this context, the density of air is controlled by two primary factors, moisture and temperature, with temperature having the greatest contribution or impact. 

Cold air is denser than warm air. That means if you lower the temperature of the air below the pressure surface (a cold air mass moves in), the molecules will pack closer together (density increases) and the height of the pressure surface will decrease, and that will cause the trampoline mat to sag. The lowest point (lowest height), or the center of the upper-level low, will have the coldest column of air below it.

This does not mean that the coldest spot on the surface will be right under the center of the upper-level low. This is all based on the average temperature derived from the entire column of which can vary throughout the column. These upper-level features are called troughs, and they are characterized by cold, dense air.  

Conversely, if you bring in a warm air mass, this causes the air to expand upward, and the density of air decreases. These are the high spots on the trampoline mat that are called ridges and are shown as higher heights on the constant pressure chart. Ridges are characterized by warm, less dense air.    

One of my favorite charts that I often look at first before planning a flight is the 500 mb constant pressure chart. This pressure level is the waistline or belly button of the atmosphere. That is, half of the mass of the atmosphere is above 500 mb and the other half is below. That’s because at sea level the pressure is near 1,000 mb with the top of the atmosphere being 0 mb. So, 500 mb sits right in the middle. The average height of this pressure surface is around 18,000 feet msl.  

As a basketball player in grade school, my coach noticed that my defense was really awful. He pulled me aside during practice and said, “Scott, you are looking at their hands, feet, and head, and they are faking you out.” OK, coach, what can I do? He said, “Look at their belly button…Where their belly button goes is where they are also going.” With that guidance, my defense improved tremendously.  

The atmosphere is remarkably similar to playing defense. What is occurring at 500 mb is one of the best ways to diagnose how the atmosphere is poised. Unlike the surface analysis chart, the 500 mb chart can tell you about the intensity and evolution of the weather system that you might be facing. In fact, I make the most use of this chart for the extended-range forecast two, three, or even as much as five days prior to my planned departure. It helps to set my expectations for the weather I might face for any particular day and time so I can stack the deck in my favor. 

These troughs and ridges are a reflection that Mother Nature abhors extremes. Anytime there’s an imbalance, she attempts to move cold air from the poles southward (troughs) and warm air from the tropics northward (ridges). Of course, moving a bunch of air masses around to bring the atmosphere to a point of equilibrium is a never-ending effort. The end result is this ribbon of fast-moving air that meteorologists call the jet stream that meanders north and south around the globe. 

Flying toward the trough usually means flying toward deteriorating weather. The opposite is true. Flying toward higher heights usually equates to flying toward fair weather. While ridges have their issues sometimes in the form of haze or even radiation fog, it is the upper-level trough that creates the adverse weather we try to avoid that includes turbulence, airframe icing, and convection.   

The standard levels for constant pressure charts include 850 mb (~5,000 feet), 700 mb (~10,000 feet), 500 mb (~18,000 feet), 300 mb (~30,000 feet), 250 mb (~34,000 feet), and 200 mb (~39,000 feet). The heights here are approximate since they vary on these constant pressure charts.  

You can find a trough on the 500 mb chart as a region of lower heights with a counter-clockwise circulation. It can be a closed or open-wave trough. 

Closed troughs typically have one or more closed height lines and a more circular or oval shape. Open-wave troughs, on the other hand, have more of a “U” or “V” shape to them.

From a severe turbulence perspective, there are two signatures that you should look for—the split-flow pattern and the sharp V-shaped meridional flow pattern. These patterns are less common during the warm season and tend to be seen more often during late fall, winter, and early spring.  

The split-flow pattern is a fairly easy one to recognize on most any upper-level chart. It contains a northern and southern branch to the jet stream with a closed, counterclockwise circulation in between. Upwind of the closed circulation, you’ll see strong winds rapidly decreasing in velocity as they enter the region just north of the closed circulation. Winds will then increase in velocity as they exit this area. This is referred to as speed shear. 

But the real clue is the directional shear that occurs just north of the closed circulation. This is where the wind varies significantly with direction over a relatively small geographic region. This rapid change in wind velocity and direction can create a classic setup for moderate or greater clear air turbulence. 

In addition to the winds changing speed (speed shear), their direction is also very chaotic. In this rather small region from northwestern Colorado to western Missouri, the wind direction is all over the place. This resulted in numerous reports of severe turbulence and prompted the Aviation Weather Center to issue a SIGMET for severe turbulence. [Courtesy image: UCAR]  

Typical west-to-east, upper-level flow is fairly benign and doesn’t usually produce any significant clear air turbulence. This is what meteorologists call a zonal flow pattern. However, the other signature to look for on a constant pressure chart is one that has a more north-and-south component that meteorologists call a meridional flow pattern. When this flow has a very sharp, nearly V-shape pattern, it will often produce moderate or greater turbulence when flying through the trough itself. 

Typically, with this kind of pattern the lines of constant height are packed close together. This gradient creates very strong wind speeds. This is similar to the strong surface winds you may encounter when you see tightly packed isobars on the surface analysis or prog chart. In this case, the wind must take a very sharp turn through the trough axis, which also creates a significant directional shear on each side of this trough. The axis is the “low spot” within the trough itself where the southern winds turn to the north.   

It’s not likely you’ll find these constant pressure charts within any of the heavyweight aviation apps, such as Garmin Pilot. However, these can be found on the National Center for Environmental Prediction (NCEP) at mag.ncep.noaa.gov. Choose the “Model Guidance” option and then select the GFS model within the CONUS region. From there, find the “500_vort_ht link” under “Upper Air.” This will take you to a page that allows you to loop the forecasts over a 16-day period.   

Lastly, just to connect the final dots, that surface analysis or surface prog chart you look at all of the time, where the pressure varies throughout the chart, is actually a constant height chart. It has a msl height of zero. Write that down.

This feature first appeared in the September Issue 950 of the FLYING print edition.