A Fresh Forecast
Aviation Weather Under the Microscope
By Nicole Hartman and Rebekah Waters, FAA Safety Briefing Magazine
Every summer for over thirty years, Paul Hamilton, a seasoned pilot with 38 years of flying experience, flies his family from the Washington, D.C., area to Sandy Island, N.H. This journey requires accurate timing because there is a scheduled boat to the island. Many parts of this trip are unchanging. Every year the departure airport, Potomac Airfield (VKX), and the preferred destination, Moultonborough Airport (4MB), are the same. Every year the van meets them at the airport to take them to the dock to meet the boat that will take them to the island. However, one thing that might change is the weather. Hamilton relies on weather data and forecasting tools before and during these flights to make sure he and his family arrive safely and on time. We will discuss these tools and check in with Hamilton again to see how his trip went last August.
The Field of Forecasting
Since the beginning of civilization, humans have used recurring meteorological and astronomical events to anticipate weather patterns and plan for seasonal changes. Originally based on mostly inaccurate observations of the sky, wind, and temperature, these forecasts have evolved to be more advanced and reliable. More recently, new technology has revolutionized the field of forecasting.
Today, almost everyone relies on weather forecasting to anticipate weather and schedule their day accordingly — particularly when it comes to flight planning. Weather conditions significantly impact aircraft flight time, performance, and safety. The weather we experience on the ground and in the air affects every decision made in aviation.
Meteorological Minutiae
Pilots cannot make good decisions based on incomplete or missing information. Fortunately, there are numerous weather and flight planning products available to aid in safe decision-making. These products assist with flight planning and highlight potentially hazardous weather.
Meteorological Aerodrome Report (METAR)
The METAR is the international standard code format for hourly surface weather observations. These reports are generated by an airport’s weather observation system and are specific for that aerodrome (aka, airport), depicting the weather conditions within a 5-mile radius of the center of the field. METAR reports are issued frequently (every hour at a minimum), and since they deal with current weather conditions, they are observations, not forecasts.
When the weather changes rapidly, more frequent updates to METARs are reported and referred to as special reports or SPECIs. When a METAR is labeled as a SPECI, pilots should take note that the winds, visibility, or precipitation levels have changed appreciably within a shorter timeframe than an hour.
🌦️ Coded Conditions ⚡
Here is an example of a METAR:
METAR KRDU 010150Z 10009KT 10SM -SHRA OVC050 23/15 A2982 RMK RAB40 FQT LTG DSNT SW SLP094
Translation:
Aviation routine weather report for Raleigh-Durham Airport, observation the first day of the month at 01:50 Zulu time, wind from 100 degrees true at nine knots; visibility 10 statute miles; light rain showers; ceiling 5,000 feet overcast, temperature 23 C; dewpoint 15 C; altimeter 29.82 inches. Remarks: Rain began at 40 minutes past the hour; frequent lightning to the distant southwest; sea level pressure 1009.4 Hectopascals/millibarH.
Terminal Aerodrome Forecasts (TAF)
TAFs are issued for specific airports and are valid for a 5-statute-mile radius from the center of the runway complex. They contain information on the expected surface winds, visibility, weather, obstructions to vision, and cloud coverage and heights. TAFs are issued four times a day and each forecast is amended according to prescribed criteria, as required.
🌨️ Coded Conditions ❄️
Here is an example of a TAF:
KBOS 041145Z 0412/0518 34015G25KT 5SM -SHSN SCT010 BKN018 TEMPO 1215 1/2SM SHSN VV008 FM 041500 33012G22KT P6SM BKN050
Translation:
Boston Aerodrome Forecast for the 4th day of the month, valid time 12:00 Zulu. Surface wind from 340 degrees at 15 knots with peak gusts to 25 knots; visibility five statute miles; light snow showers; scattered clouds at 1,000 feet above ground level (AGL); ceiling 1,800 feet broken AGL; occasionally, visibility one-half mile in moderate snow showers; indefinite ceiling 800 feet (an indefinite ceiling represents surface-based phenomena obscuring the whole sky). Changes expected on the 4th 15:00 Zulu surface wind from 330 degrees at 12 knots with gusts to 22 knots; visibility greater than six miles; ceiling 5,000 feet broken.
Airman’s Meteorological Information (AIRMET)
An AIRMET is a concise description of the occurrence or expected occurrence of specified en route weather phenomena that may affect the safety of aircraft operations, but at intensities lower than those requiring the issuance of a SIGMET. AIRMETs are intended to inform all pilots, especially those under visual flight rules (VFR) and operators of sensitive aircraft, of potentially hazardous weather phenomena.
Significant Meteorological Information (SIGMET)
A SIGMET is a concise description of the occurrence or expected occurrence of specified en route weather phenomena, which is expected to affect the safety of aircraft operations. SIGMETs are intended for dissemination to all pilots in flight to enhance safety.
Pilot Report (PIREPS)
A PIREP is a report of the actual weather conditions encountered by an aircraft in flight. Traditionally, these reports are transmitted by radio to an appropriate ground station for dissemination, but when necessary, they can be made by telephone after landing. The ground station receiving the PIREP will format and disseminate the information to all concerned parties.
🌫️ Five Things You Should Know About PIREPs:
- A PIREP is a pilot’s report of actual weather conditions encountered while airborne.
- The main purpose is safety — it helps weather forecasters update their data and improve quality of forecast.
- A PIREP file to report good weather is just as important as a PIREP file to report bad weather.
- You can submit them electronically — check out the electronic PIREP submission tool at the National Weather Service’s Aviation Weather Center Digital Data Service (ADDS) website.
- Don’t be overly concerned with strict format or phraseology. The important thing is to relay info beneficial to other pilots.
Radar and Satellite Imagery
Like METARs, radar and satellite imagery are not forecasts. They display a near real-time picture of the current weather at given locations. They are used as tools in producing forecasts, but more importantly, they help to evaluate current conditions — the first place a pilot should start. Radar and satellite imagery provide a great deal of useful information. However, it’s vital aviators know how to use it. Understanding when and how to analyze the different imagery available is key to utilizing these valuable tools for evaluating flight conditions.
Commonly known as Doppler radar, the WSR-88D NEXRAD (Next-generation Radar) provides comprehensive observations that inform about impending weather. It operates in two modes: clear air and precipitation. Clear Air Mode is the most sensitive operational mode, with a slow antenna rotation allowing for extended atmospheric sampling. Images are updated approximately every 10 minutes in this mode. In Precipitation Mode, the radar operates with a faster antenna rotation due to stronger return signals from precipitation targets. This allows images to update faster, approximately every 4–6 minutes. The intensity values in both modes are measured in dBZ (a metric related to precipitation intensity) and are depicted in different colors, based on echo intensities, on the radar image.
Another important factor to understand is the type of radar image you are viewing. Radar can be displayed in Base Reflectivity or Composite Reflectivity. Base Reflectivity is a display of echo intensity (reflectivity), showing the amount of transmitted power returned to the radar receiver at a single elevation. Images are used to detect precipitation, evaluate storm structure, locate atmospheric boundaries, and determine hail potential. Composite Reflectivity displays the maximum echo intensity (reflectivity) from any elevation angle at every angle from the radar. When compared with Base Reflectivity, Composite Reflectivity can reveal important storm structure features and intensity trends of storms.
Satellite imagery is another good tool to better understand current and short-term expected flight conditions and is available in multiple image types. Infrared (IR) imagery senses the surface temperature of an object, like a cloud top, ocean, or ground surface. IR images are independent of visible light and thus available day and night. Looping a series of these images together can offer valuable clues as to whether a system is strengthening or weakening based on whether the cloud tops are cooling or warming. Visible imagery presents clouds using the visible part of the spectrum. The display is similar to what you would see with your naked eye and allows you to distinguish low-lying fog and stratus clouds, which the IR imagery may not detect. Visible imagery is valuable for detecting smoke plumes, dust, and thin layers of volcanic ash, which are usually too warm to be easily seen on IR imagery. However, these images are only available during the day when the sun illuminates the features.
Water vapor imagery highlights the presence of water vapor in the upper atmosphere and provides the ability to identify the presence of jet streams and headwinds aloft and the possibility of mountain wave turbulence even under clear skies. Rivers of atmospheric water vapor will often be visible on the water vapor images even when the IR and visible imagery indicate clear skies.
Current Cautions & Conversions
Winds shown on a METAR, TAF, winds aloft table, or surface analysis chart are represented in true headings, whereas winds represented through an Automatic Terminal Information Service (ATIS), Automated Surface/Weather Observing System (ASOS/AWOS), or PIREP are in magnetic headings. It’s important to be cognizant of wind direction when determining wind components and to make conversions as necessary. Runways are always identified in the magnetic direction, so to accurately figure out all wind components you must convert the wind from true to magnetic. To do this, determine the magnetic variation for the airport, which can be found in the chart supplement. As a reminder, westerly variations are added, easterly are subtracted, from the true direction. A useful adage is that “If you read it, it must be true. If you hear it, it’s magnetic.”
Hamilton Family Vacation, August 2023
Now let’s look at how Hamilton used some of these tools for his family’s trip in the summer of 2023. Keep in mind that because there was a scheduled boat ride, timing was important. Since the destination airport (Moultonborough) is VFR only, he needed an alternate airport. Laconia Airport (LCI) with an instrument landing system (ILS) approach and weather reporting, is just across the lake. It’s a long drive around the lake from Laconia to the boat dock though. Laconia lacks a TAF, so he needed an official alternate, which was Manchester, NH (MHT). These factors add complexity to weather planning. Air and ground transport had to be coordinated over four outcomes:
- Laconia reports VFR — fly to Moultonborough.
- Laconia reports marginal VFR — shoot an ILS to get below the clouds and continue to Moultonborough.
- Laconia is IFR — land there, change shuttle pickup location, and allow extra time.
- Laconia is below minimums — go to a distant airport (MHT) and arrange transportation.
Weather conditions significantly impact aircraft flight time, performance, and safety.
Before the Flight
No weather forecast is good at 10 days out. However, Hamilton’s more than 30 years of experience let him reasonably estimate the probability of each outcome. The Aviation Weather Center Prog Charts give a look at the highs, lows, and fronts a week out. The Area Forecast Discussion predicts four days out and discusses flight conditions, winds, and storms. This is when Hamilton started thinking about his flight. The nearest aviation TFR forecast site, Concord, N.H., issues a 24-hour forecast. That was a key point in his planning. Summertime is thunderstorm time, typically driven by midday heating of moist air. Takeoff is 7 a.m., and storms usually are a factor only when there is frontal activity. The same goes for adverse winds. Morning fog, however, is common in the New Hampshire mountains.
Hamilton used the grid winds in his electronic flight bag to determine how long it would take and planned a departure time that would put him and his family at the destination in time for the scheduled van pickup.
The initial forecasts indicated IFR conditions with possible thunderstorms. Four days before departure, all indications pointed to no thunderstorms but possible sub-VFR conditions. The convective forecasts agreed. This was better than the forecast for the day before the planned departure. When the TAFs came out, they looked good enough to continue with the original plan to land at the destination at 10 a.m. Hamilton went to bed and slept comfortably. He had plans for all contingencies and the next day looked good.
Right Before Takeoff
At this point, TAFs are very reliable and NEXRAD and METARS are highly relevant. There’s now enough data to make a go/no-go decision. Hamilton needed to decide: Is it safe to depart? Will he need an arrival alternate or a departure alternate? Should he tell the van company to pick him and his family up at Laconia instead? Does he change his departure time based upon the weather?
There was no major precipitation on NEXRAD. The first half of the trip was marginal VFR with spots of IFR conditions. The second half was marginal VFR to VFR. Laconia was in IFR in low clouds and fog. The Concord forecast called for it to lift by an hour after the 7 a.m. takeoff and be in VFR by arrival time. No forecast is perfectly reliable, but Laconia was definite, and Moultonborough was highly probable. So far, conditions were substantially as forecast the night before.
Pilots cannot make good decisions based on incomplete or missing information.
Enroute
Here is where datalink weather really pays off. Hamilton uses XM satellite weather. It provides several weather products, and it works even when the airplane is on the ground. (ADS-B weather gives an adequate suite of products. It can only receive data when the airplane is high enough to pick up a station. This is a few hundred feet at his airport.) NEXRAD weather is one of the best products that XM delivers. Hamilton recommends using NEXRAD with caution though because there can be a lag of 5–10 minutes or more, but it is still his best strategic planning tool. He backs up the NEXRAD with his Stormscope, which detects lightning in real time.
Hamilton and his family departed their home airport in marginal VFR conditions with plenty of cloud clearance. Air traffic control (ATC) cleared him on course and up to 7,000 feet. The winds were a bit less favorable than planned, but he was above the clouds into sunglasses weather.
“One of the good things about being a pilot is that we see a good deal more sunshine than nonpilots,” states Hamilton. “The cloud tops on this day were brilliant in the morning sun.”
An hour out, passing Atlantic City, N.J., the undercast started to break up. Passing New York City, Hamilton had a broken deck. Best of all, the METAR at Laconia now showed good VFR conditions. Not only was he not going to Laconia, but he also didn’t need to do an instrument approach.
Arrival
The Laconia METAR indicated south winds, so Hamilton anticipated an approach to Runway 20. That’s a steep descent on final to a short runway, so he prepared for it mentally
“It was good VFR the rest of the way and we spotted the mountains surrounding the lake when they were 15 miles away,” he said.
There was some haze as he approached the lake. At 3,000 feet overhead Laconia, he canceled his IFR flight plan and descended to cross the lake 1,000 feet above the water. He and his family made it to the boat with time to spare.
Sunset
Whether you are new to flying or a seasoned aviator like Hamilton, weather planning is essential. The evaluation products we discussed are just a few of the products available to support an exhaustive weather briefing and inform good judgment for all aviators. Make sure to familiarize yourself with all the tools available and understand how each product fits into your flying timeline to develop a total awareness of the atmospheric conditions.
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Nicole Hartman and Rebekah Waters are FAA Safety Briefing associate editors and technical writer-editors in the FAA’s Flight Standards Service.
