If you are a new pilot or one that has a few thousand hours in your logbook, eddy dissipation rate or EDR is not likely part of your vocabulary. However, if you use the new EZWxBrief progressive web app (visit ezwxbrief.com), you have stumbled across this relatively new "aviation" term. Let's take a look at defining EDR and how it should be used.
When you received your primary training you were likely taught how to deal with turbulence while in flight. That is, you were taught when you are experiencing moderate or greater turbulence, there is a strong need to reduce the velocity of the aircraft to below maneuvering speed or what is sometimes called turbulence penetration speed. Moreover, you were also taught that maneuvering speed is dependent on the weight of the aircraft. The higher the weight, the higher the maneuvering speed. The goal by slowing down is to reduce the forces (or load) on the aircraft parts that could fail as the aircraft accelerates and decelerates in turbulence. Keep in mind that this is less about the wings falling off and more about the engine mount failing.
What is EDR? EDR is an aircraft-independent meteorological field expressed in m²/s³. Simply put, an atmosphere that causes eddies to dissipate rapidly is one that is likely turbulent. Most importantly, EDR is NOT a measure of the likelihood of turbulence as some pilots will make you believe.
In fluid dynamics, an eddy is the "swirling" of a fluid. Given that air has similar properties as a fluid, you can have eddies in the atmosphere. Most pilots have seen pictures of wake vortices swirling off the wingtips of a large turbofan aircraft as it passes through clouds or smoke. Pilots are taught to avoid these wake vortices (which are usually invisible) since they can cause a smaller aircraft following the heavy jet to enter an uncommanded roll. Therefore, this is often referred to as wake turbulence.
In that light, pilots are taught to pay close attention to the wind with respect to wake turbulence. This is because the prevailing current of wind will cause the eddies to "drift" downwind. If the wind is calm or nearly so, these vortices can persist over the area of concern for some time. Stronger winds will cause the eddies to move farther away from the area and eventually mix out with time.
Most turbulence in the atmosphere doesn't manifest itself like these wake vortices. However, imagine such a vortex or eddy in the atmosphere. A turbulent atmosphere will cause the eddy to quickly dissipate whereas one with little or no turbulence will allow the eddy to persist. Most important, the eddies must be on the scale of the size of the aircraft for you to feel "bumps" while in the cockpit. Nobody is generating eddies in the atmosphere and watching what happens to them, but instead, they are determining what conditions in the atmosphere cause it to mix. An atmosphere that is mixing quite a bit is one that is likely turbulent. EDR is just a measure of how "mixy" the atmosphere is...or how quickly the atmosphere will mix out these eddies, hence the s³ (seconds cubed) in the unit's denominator.
EDR is an observed or forecast value that is between 0 and 1. It is not critical to understand how this is determined. The EDR value used throughout the EZWxBrief progressive web app is actually EDR x 100. Multiplying EDR by 100 provides a way to turn this into an integer value from 0 to 100 to make it easier to use. For example, an EDR value of 0.23 m²/s³ will be adjusted to 23.
EDR is not a "one size fits all" kind of weather parameter. That's the tricky part of EDR. As mentioned above, the turbulence penetration speed is directly related to the weight of the aircraft. In other words, a heavier aircraft can fly at a higher airspeed when encountering an area of moderate or greater turbulence. So this means that a pilot flying a Cessna 152 is going to experience turbulence different than a pilot flying a Gulfstream G700. And a Gulfstream G700 will experience turbulence different than an Airbus A320. And an Airbus A320 will experience turbulence different than a Boeing 777.
This means an EDR value of 18 may produce moderate turbulence in a Cirrus SR20, but will be light for a Boeing 737. There are three aircraft weight classes in the table below that is also documented in the EZWxBrief Pilots Guide. These categories include Light, Medium and Heavy based on the maximum takeoff weight shown below.
Light < 15,500 lbs maximum takeoff weight (e.g. Cirrus SR22, Piper Cub, LJ23)
Medium (or large) 15,500 - 300,000 lbs maximum takeoff weight (e.g. A320, B737, G5, MD80)
Heavy > 300,000 lbs maximum takeoff weight (e.g., A330, A380, B787, B777)
This table is based on the results of a study done by turbulence researchers at the National Center for Atmospheric Research (NCAR). The idea was to compare pilot weather reports (PIREPs) with in situ EDR data for the same report. But they only had medium weight class data (e.g., B737) for comparisons, and the spread was quite large. So they used the medians for comparison. Then the researchers used a theoretical argument about aircraft response based on weight to expand the mapping to light and heavy aircraft. It is important to understand that the EDR ranges for light and heavy aircraft weight classes have never been verified, and would be extremely difficult to do without a lot of data that simply doesn't exist.
In the end, the EZRoute Profile shown above evaluates the route for turbulence aloft and color codes the EDR values based on the aircraft weight class in the table above. That is, if you have the aircraft weight setting as a Light aircraft, green colors on the profile represent EDR values from 13 to 15, brown/tan are values from 16 to 35, red are values from 36 to 63 and dark red are values from 64 to 100. Seeing a forecast for extreme turbulence is very rare. Any point without a green, tan, red or dark red color is an EDR value of 12 or less indicating it should be a smooth ride.
Most pilots are weatherwise, but some are otherwise™
Dr. Scott Dennstaedt
Weather Systems Engineer
Founder, EZWxBrief™
CFI & former NWS meteorologist
