New US Turbulence Map, Based on 1 Million Pilot Reports

Turbulence is hard to predict. It depends on many fast-changing weather factors. But some others are relatively stable: the thunderstorms in the equator, the jet streams at mid-latitudes, the mountains breaking down air flows, etc. As a result, some regions have more chances of developing turbulence than others, just because of their geographical location.

In this article we analyze the Pilot Reports (PIREPs) to present you the regions in the US where you have higher chances of encountering turbulence.

What are the PIREPs?

PIREPs are reports on the meteorological conditions encountered by pilots: turbulence, icing, storms, volcanic ash, etc.

Pilots are not obliged to report all the encounters of these conditions. But they are “urged to cooperate voluntarily” to help air traffic controllers and other pilots to understand the current conditions. In some cases pilots can be asked to report a PIREP if the area they are flying in is expected to contain hazards.

Once the pilot sends the report, organisms like the Federal Aviation Association add them to the weather reporting system. This data is used by air traffic controllers to propose better flight routes and by flight service stations for pilot briefings before departure. Extreme weather reports might trigger the issuance of global weather advisories.

Pilots know that sharing data will help them and their colleagues on the flight. Therefore, PIREPs have become a very common practice which helps the aviation industry build a network for communicating possible hazards.

How long are these reports?

Quite short. A standard PIREP might look as:

201907060501,F,A321,TUS UA /OV TUS045060/TM 0501/FL330/TP A321/TB MODERATE CHOP, 32.73477876456618, -110.21241123303858

which translated to human-readable text becomes:

On July 6th 2019 an Airbus 321 encountered moderate turbulence while flying 33,000 feet above Arizona.

As we can see the report is a single line of data with different pieces of information regarding the date, plane model, meteorological conditions and coordinates. The meteorological conditions are separated into different types using abbreviations such as /IC for icing, /TB for turbulence, etc.

The reports do not need to cover all the weather parameters. Some might only include data on ice, others on turbulence, etc. The description of each parameter also varies between pilots, which can refer to moderate turbulence as /TB MOD, /TB MODERATE, /TB MDR, etc. Although there are guidelines on how to prepare a PIREPs, there are no strict standards on how to phrase the weather conditions. The FAA actually encourages pilots to “not be overly concerned with strict format or phraseology”.

Getting all the PIREPs since 2015

In this article we used the PIREPs database from Iowa State University, which has been archiving them since January 2015. The database contains over 3 million PIREPs and about 1 million of them include information on turbulence at cruising altitudes (above 20,000 feet).

We can see that most of the pilot reports are inside US mainland. Routes towards Europe and Asia also have some reports. But their spatial resolution is too low to derive a turbulence map from them.

The patches without PIREPs are due to restricted airspaces. There are many areas in US with such restriction. They can be as large as Area 51 in Nevada or as little as Camp David and its surroundings. All the restricted regions are regularly updated by the Federal Aviation Administration in this map.

We noticed some star-shaped patterns of the reports around Texas. Also, northern regions such as Minnesota show concentric circles. We don't really have an explanation for these patterns. It's cannot be caused by planes flying in perfect concentric circles. Most probably they have to do with some adjustments of the coordinates done by the plane systems.

Converting PIREPs to turbulence levels

The turbulence reports were converted to numerical values in a range from 0 to 100, where 0 represents null turbulence and 100 extreme.

In some cases pilots include a range of turbulence levels. In this case the average between the two bounding levels was computed. Reports adding the word “CHOP” were added a 12.5 increment. Some examples of the conversion from reports to numbers are shown below:

•   /TB LIGHT = 25

•   /TB LGT-MOD = 37.5

•   /TB MODERATE CHOP = 62.5

The aircraft model can have an important effect on the turbulence felt inside the aircraft, but for this article we will not look into that.

Finally, to get the averaged turbulence within a region, the US map was split into squares of 0.75^o, and the PIREPs found at each region were averaged to obtain a representative value.

Since pilots report turbulence mainly when it represents a hazard, the obtained averages might over-estimate the actual turbulence. Light turbulence is also reported, but mainly as additional data for other hazards. Therefore, we should look more at the differences between regions than at the actual absolute values in the turbulence map.

The US turbulence map

Putting together all the data gives us the figure below. We will go region by region and try to explain why they look this way.

The Rocky Mountains: The most turbulent area is the one of the Rockies. But why is turbulence triggered near mountains? The main reason is that the air is forced up and down as it passes through them, and this generates a wide range of turbulent structures that can shake the plane. This turbulence can propagate far from the mountains, sometimes in the form of rolling trains of vortices known as mountain waves. At cruising altitudes the wind in the US is from west to east. Therefore most of the turbulence should be located at the eastern side of the mountains. For example the turbulent patch near Mt Elbert.

The Great Basin: Surrounded by mountains and with many other tall mountains in it, the Basin is another hotspot for turbulence. East and west from Salt Lake City (SLC) seems to be the toughest area, probably due to the high density of mountains in this region. The area of Mt Whitney in Sierra Nevada has surprisingly low levels of turbulence. In theory, a range almost perpendicular to the winds from the Pacific should be a good turbulence generator. It could be that there is indeed turbulence, but that this one is transported towards the Great Basin and felt more there. Mt Rainer in the Cascade Range has a very clear turbulent patch at it's eastern side.

The Appalachian Mountains: This range of mountains is not as wide and tall as the Rockies. Therefore its overall turbulence levels are lower. A patch with high levels was found north from the Mt Mitchell. This patch does not lay on the mountain range itself, but on the plains on its western side. It could be that air masses pushed towards the Appalachians are also generating important levels of turbulence. The turbulence seen above Pennsylvania is most probably also induced by the jet stream, which begins to pick up speed at this region.

The Great Plains: This area presents the lowest levels of turbulence in the US. The flat terrain is an important factor. But also, the plains are sheltered from the Pacific and Atlantic winds by the Rockies and Appalachians. These two factors bring a peaceful patch of air with very low turbulence levels.

Florida: One would expect that such narrow peninsula exposed to the Atlantic winds would be a major turbulent hotpost. But it turns out that of that most reports in this region are only on low levels of turbulence. This could be due to being out of reach from the northern jet stream and to the absence of mountain ranges in this region.

The Mississippi and Texas: The turbulence levels in these areas are also large. For Texas, it seems that the turbulence from the Sierra Madre is propagating eastwards. This is a long stretch. But we have also seen that in the Rockies the turbulence from Mt Elbert also propagates quite a lot eastwards. For the Mississippi, we have the opposite behavior as for Florida. A flat river valley is not expected to be an important generator of turbulence. But somehow pilots report larger levels here, and we don't have a sound explanation for it.

Chose your routes

If you are up for a bumpy ride, your best chances are to take a flight from Denver to anywhere on the West Coast. Salt Lake City could be an equally good choice. Tip: buckle your seatbelt during the entire flight, even if the seatbelt sign is off. Turbulence can cause important injuries, such as the ones reported in this flight from Denver to Billings.

If you prefer a relaxing flight, take a trip from Minneapolis to Dallas. This will "hopefully" give you a quiet time. This said, remember that turbulence can always appear unexpectedly, even if the latest reports indicate light levels. Tip: buckle your seatbelt during the entire flight, even if the seatbelt sign is off.

Bumpy takeoffs and landings: another story

Bumpy takeoffs and landings can occur in the absence of turbulence. Here the important factor is the crosswind component (the one perpendicular to the plane direction).

As a general rule, landings or takeoffs are not allowed with crosswinds larger than about 45 km/h. This limit is only an approximate value, since the actual one depends on the aircraft type. Large airports usually have 2 perpendicular runways, so when the crosswind is large in one of them they simply switch the traffic to the other.