AINsight: Limitations of Pilot Weather Apps

 - December 23, 2020, 10:00 AM

Pilots are increasingly relying on apps for preflight weather briefings. Built for convenience, these apps provide a host of information, including the latest weather conditions, forecast, radar, and charts.

As an example of their popularity, one radar app has sold more than 30 million copies. The problem is these tools might not provide the pilot with a complete weather picture in very dynamic scenarios such as rapidly-growing thunderstorms, and there are some known data latency issues with radar returns. 

The simplicity of these apps should not alleviate the in-depth knowledge of weather required to safely operate an aircraft. In general, a weather app is only as good as the source or model used to create that information. Likewise, these tools only provide a snapshot of the weather at a given time, and once the pilot straps into the aircraft this information becomes outdated.

One of the first and often overlooked steps in building a solid picture of convective weather activity is to look at the Convective Outlook (AC) note on the NOAA Storm Prediction Center (SPC) website. This forecast is produced by a team of experienced NOAA/NWS SPC meteorologists in Norman, Oklahoma.

The AC note graphically depicts areas of convective activity around the U.S—green areas depict air-mass thunderstorms, yellow areas point to a moderately unstable atmosphere, and red and magenta areas indicate a highly unstable atmosphere that can produce extreme and fast-growing thunderstorms.

One of the greatest features of the AC note is the “summary discussion” that is written in plain English and signed by the lead forecaster.

Armed with this knowledge, pilots can better plan their flight, warn passengers in advance of turbulence, and be on the lookout for extreme or supercell storms that can produce damaging winds, hail, or even tornados. These most severe thunderstorms often develop rapidly and can grow at 3,000 fpm. In an extreme case where a cell popped up in front of a Boeing 757 (injuring 22 people), the NTSB reported that the storm grew at 5,000 to 7,000 fpm.

Turbulence-related injuries are common and avoidable. As these storms grow, a turbulent “bubble” of air is formed 2,000 to 4,000 feet above the advancing cloud tops. Attempting to outclimb or fly over the top of a fast-growing cell is a mistake.

Understanding and interpreting the data presented on a weather app or airborne weather radar also requires more advanced training. The late Archie Trammell—a master of all things related to radars and thunderstorms—believed that “before any pilot can be considered a professional, he or she must be adept at making short range, minute to minute convective storm forecasts.” After many consultations with scientists, researchers, and meteorologists, Trammell created the Objective Storm Hazard Index Test (OSHIT—a great acronym) to identify the potential for a supercell. A supercell may have these characteristics:

Evidence of atmospheric instability—Check the AC note as the fastest-growing thunderstorms are most likely to occur in the red or magenta areas. En route, as you transit these areas, use caution with any echo displayed on your radar. Extreme turbulence and/or large hail may be encountered in any area of radar return (including the green areas) up to 10 to 20 nm outside of the echo.

Dew point of 10 degrees C or greater—Dew point is an indicator of the amount of moisture in the atmosphere. Water vapor is the fuel for a thunderstorm. The higher the dew point, the greater the chance of an atmospheric explosion.

Temperature/dew point spread greater than 17 degrees C—A high temperature/dew point spread indicates a dry atmosphere. In an area of atmospheric instability, this is a set-up for a massive microburst.

In this scenario, the water from a shower aloft falls into warm dry air below, evaporating and becoming water vapor. This sets up a vicious cycle of warm water vapor rising (leaving a vacuum below), condensing in the form of water droplets and falling back to the surface, evaporating, rising again and again until it explodes into a huge mass of water boiling up to above 40,000 feet and then falling back to earth as a giant microburst.

The southernmost cell in a line of thunderstorms (North America)—When evaluating a line of thunderstorms, the southernmost cell will have the highest water content. Remember, water vapor is the fuel for a thunderstorm; in the U.S., the Gulf of Mexico is a great source of water vapor. In other parts of the world, look for the large bodies of warm water as the source of water vapor and study the prevailing winds.

The size and shape of a line of thunderstorms can also tell a story. A bow-shaped line of thunderstorms—where a section of the storm is pushed out further east—is a characteristic of a potentially hazardous cell. A solid line covering a large area is hazardous and might be difficult to circumnavigate. Likewise, cells located to the north of a break in a line are also dangerous, since they contain a lot of energy.

Odd-shaped echoes—Storms with a lot of energy that suck in large amounts of air and water vapor will develop a notch on their southeast side. This is referred to as a bounded-weak echo region and can be extremely dangerous. Likewise, a storm with tops that are impacted by the jetstream will move faster aloft and slower over the ground resulting in a pendant-shaped echo. These are easily displayed on airborne radars. Always pass on the upwind side of these cells—20 to 30 nm should be sufficient.

Multiple echoes in a cluster—Three or more echoes in proximity are indicative of a concentration of moisture and instability. If located within 10 to 20 nm from an airport, these strong outflows and inflows cause sudden gusts and turbulence and can be hazardous during takeoff and landing.

An ATC warning of “extreme” echoes—Most ATC facilities have weather radar capabilities and will report the intensity of echoes as either light, moderate, heavy, or extreme. Extreme echoes should be avoided due to the possibility of hail and/or turbulence.

While the list goes on, these are some of the most common elements related to a supercell or severe thunderstorm and highlight the need to know more than the output of a popular weather app. Not mentioned is the need for more advanced training on airborne weather radar. Proper tilt management and understanding the operation and limitations of your radar are paramount to operating safely in areas of convective activity.

Picking up where Trammell left off, Erik Eliel of Radar Training International conducts seminars on airborne weather radar. Likewise, Trammell’s wife Mary continues to sell the original course material from her husband’s courses.

While the utility and popularity of weather apps are impressive, they are no substitute for knowledge of weather required to operate an aircraft safely. Bottom line: never confuse a weather app with a degree in meteorology or good airmanship.

Pilot, safety expert, consultant, and aviation journalist Stuart “Kipp” Lau writes about flight safety and airmanship for AIN. He can be reached at stuart.lau3@gmail.com.