ForeFlight customers now have access to more winds aloft data for more accurate long term flight planning.
Seven Day Winds Forecast
The winds aloft forecast data in ForeFlight now extends up to seven days in the future (previously 30 hours), giving you more accurate route performance calculations for flight plans within that timeframe, anywhere in the world. The additional high-resolution Global Forecast System (GFS) data is available now for all ForeFlight customers and is automatically incorporated into route performance calculations.
When your flight is more than seven days away, ForeFlight now gives you the advantage of historical winds aloft data. Historical winds are derived from monthly average winds over the past 40 years. This means that when you plan a flight that uses historical winds, ForeFlight assumes the same wind speed, direction, and temperature for every day and time within a given month; move the departure time to a different month and ForeFlight automatically adjusts the value based on the average for that month. You’ll know when ForeFlight is using historical winds by the yellow banner across the top of the Flights view.
It is worth noting here that, unlike the seven day wind forecast, currently only the Flights view uses historical winds. When planning in the Maps view, ForeFlight will assume zero winds if you’re planning a flight more than seven days in advance. However, you can easily send your route plan from the Flight Plan Editor in the Maps view to the Flights view and ForeFlight will automatically re-calculate route performance using historical winds. In addition, ForeFlight on the web uses historical winds for flight planning beyond seven days in the future.
Use the Send To button to move your route plan from Maps to Flights.
Seamlessly integrating the GFS seven day wind forecast and monthly historical winds provides ForeFlight customers with more accurate data for planning a flight, whether it’s next week, next month, or next year.
In ForeFlight Mobile 9.0 we’ve added a high resolution surface wind analysis to the list of map layers you can display through the SiriusXM satellite weather broadcast. This new product includes both windspeed and direction presented as wind barbs similar to the winds aloft layer. Tapping on any wind barb will show the specific details.
The surface wind analysis layer broadcast by SiriusXM will provide an overview of the general circulation of the prevailing wind about 10 meters above the surface. Tapping on any wind barb will display the valid time as well as the windspeed and direction at that location.
Two surface wind layers?
Yes, there will be two surface wind layers when connected to the SXAR1. The layer you have been using in prior releases and the one you can view when connected to the Internet is strictly based on surface observations from the various weather reporting stations around the world (typically airports). This depicts the actual wind reported in the routine observation (METAR) or special observation (SPECI). The surface wind layer is depicted at weather stations as colored wind barbs; at this point in time the wind markers shown include the gust factor.
The surface wind layer that is based on observations is shown as wind barbs color-coded based on the observed wind speed at weather stations.
The new surface wind analysis layer is not observed data from weather stations, but instead is generated by a forecast model, and therefore, completely automated. It’s only available when connected to the SXAR1 and shows an analysis of the prevailing wind at 10 meters above the surface; it does not include the gust factor. Unlike the observed data that is updated when new observations are taken, the surface wind analysis is updated once every hour. When refreshed, this will provide wind data that will be valid at the top of the previous hour.
The surface wind analysis broadcast by SiriusXM shows low level atmospheric circulations very well as seen here as a Nor’easter deepens over the Delmarva Peninsula.
The primary value of this new layer is to show low level circulations at the synoptic scale level. This will point out high (clockwise) and low pressure (counter-clockwise) circulations as well as lines of convergence in the vicinity of strong frontal boundaries. This is difficult to see with the coarse network of observing sites throughout the U.S. But with the high resolution surface wind barb analysis, these circulations and convergence zones show up nicely.
Now in ForeFlight Mobile 8.3, you have a choice between one of two satellite layers on the ForeFlight Map view. The legacy satellite layer was renamed to Enhanced Satellite and the new layer is appropriately named Color IR Satellite. For many, the new satellite layer will look quite familiar. That’s because it was created to generally match the infrared (IR) satellite images located within the ForeFlight Imagery view. Or you may have seen similar color images on aviationweather.gov. While there are some differences, this color IR satellite layer has a rather high glance value to depict the locations of significant adverse weather and help to locate the height of the cloud tops.
The older satellite layer was renamed to Enhanced Satellite with the new layer now called Color IR Satellite.
Why another satellite layer?
Back in November 2014, you may recall that we added color to the global satellite layer. Color was added to enhance or highlight the highest cloud tops that are typically associated with significant large synoptic-scale weather systems and deep, moist convection or thunderstorms. This is especially critical when flying in regions where ground-based radar data is sparse or nonexistent. The new satellite layer takes this a step further by colorizing the entire satellite layer based on a discrete cloud top temperature (in degrees Celsius).
The Color IR Satellite layer should be viewed along with the sky coverage markers. You will notice that many pilot weather reports of icing tend to occur in regions of yellow, green and very light blue.
As I discussed in this earlier blog post high clouds are very cold and emit less infrared radiation than warmer clouds near Earth’s surface. Satellite sensors measure this radiation and meteorologists calibrate this to appropriate temperatures. Knowing the cloud top temperature can help us determine the relative height of the cloud tops and more importantly it can help us understand when supercooled liquid water may dominate the clouds creating a nasty icing threat.
Cloud tops and icing
In this new color satellite image, purple and darker shades of blue are indicative of tops at high altitudes. At the other end of the spectrum, shades of red and orange are indicative of shallow clouds with tops near the earth’s surface.
Colors such as dark blue and purple on the left side of this scale (in degrees Celsius) represent the coldest (highest) cloud tops whereas colors on the right side of the scale represent the warmest (lowest) cloud tops.
To use the layer to determine the cloud top height over a particular region, zoom in on the area of concern in the Map view and note the temperature using the color scale above. Next, find the MSL altitude that corresponds to that temperature by referencing the local temperature aloft in that region. That gives you the cloud top height. For example, assume you were departing out of Garden City Regional Airport (KGCK) and wanted to know the height of the tops. Zooming in as shown below provides an orange color representing a temperature of approximately 0 degrees Celsius.
The color IR satellite when zoomed in over Garden City shows mostly orange in this area. This corresponds to a temperature of roughly 0 degrees Celsius.
Using the winds/temperatures aloft provided in the Garden City popover, find the altitude that corresponds to that temperature. Perhaps a more accurate approach is to use a tool called a Skew-T log (p) diagram like the one pictured below. Starting from the surface, work your way up the red environmental temperature line and find the first altitude that corresponds to a temperature of 0 degrees Celsius. In this case, that corresponds to an altitude of 4,285 feet as shown on the left. Additionally, the diagram confirms that saturated conditions occur below this altitude representing the presence of clouds with unsaturated conditions above. This kind of analysis will provide the necessary confidence that a climb to 5,000 feet MSL will get you on top of this cloud deck.
A Skew-T log (p) diagram like the one shown here for the Garden City Municipal Airport is an excellent tool to help locate the cloud top height. This depicts a forecast model representation of temperature (red line) and dewpoint temperature (blue line) as a function of height.
The more important colors are perhaps shades of yellow and green and maybe even very light blue. Using the color scale below, clouds with fairly warm subfreezing cloud top temperatures are likely to be dominated by supercooled liquid water and represent a airframe icing threat.
The pale green, yellow and very light blue indicate regions where cloud top temperatures are in the regime where the clouds below are dominated by supercooled liquid water representing an airframe icing hazard.
Don’t become complacent; clouds with colder (higher) tops can and do contain supercooled liquid water and may pack the threat of supercooled large drop (SLD) icing especially within deep, moist convection. However, these colder-topped clouds of darker shades of blue will normally be dominated by ice crystals or more likely be a mixed phase cloud (containing both ice crystals and supercooled liquid water). However, once ice nuclei begin to activate and ice crystals start to form in the cloud, the cloud tends to grow bigger ice crystals at the expense of supercooled liquid water which lessens the icing threat.
Masking out clear skies
As mentioned above, this layer is a close cousin of the static color IR satellite images found in the ForeFlight Imagery view. The static images show not only the temperature of the cloud tops using the same colors, but also the temperature of the surface of the earth. This can make it difficult to know where clouds exist and where the sky is clear. The main improvement is that the new satellite layer attempts to mask out regions where the sky is clear showing the map background in those regions instead of the surface temperature.
Clear regions are masked out to show the underlying map below.
While this masking algorithm works a majority of the time, it can be difficult to get it right every single time simply using temperature alone. For example, anytime there’s a shallow low-topped stratus deck like the one shown below, the tops of the clouds may actually be slightly warmer than the surface of the earth courtesy of a surface-based temperature inversion. So the algorithm may have a difficult time discerning where it is cloudy or clear. So it’s important to always overlay the sky coverage markers to pick up on these issues when they occur.
For some low-topped stratus events, it’s not unusual for the masking algorithm to show clear skies as it did here in the Midwest. The best way to detect this condition is to overlay the cloud coverage markers or during daylight hours check the Enhanced Satellite which operates in the visible spectrum during this time.
So during the late fall, winter and early spring, give this new satellite layer a quick glance. It’ll provide you with a method to determine the tops of most clouds and to reveal where there’s a definite risk of airframe ice.
Now that cold air has infiltrated a good portion of North America, it’s time to review one important aspect of airframe icing, namely, precipitation type. The three basic wintry precipitation types include snow, ice pellets (colloquially known as sleet) and freezing rain (also freezing drizzle). Surface observations (METARs) and forecasts such as TAFs typically report these precipitation types based on what’s reaching or expected to reach the surface. That’s a critical element to understand. If the surface temperature is expected to be even a degree or two above freezing, you may see a forecast for rain (RA) or drizzle (DZ) in the TAF instead of freezing rain (FZRA) or freezing drizzle (FZDZ). However, just 500 feet above the ground a serious icing hazard may be lurking. So let’s take a look at the three primary precipitation types and examine the temperature profile aloft that’s common for each.
Snowflakes are just collections of ice crystals that coalesce as they fall toward the Earth’s surface. For snow (SN) to reach the surface, there needs to be a deep moist layer that is, for the most part, entirely below freezing. More importantly, the key to getting snow is that the top of this moist layer must be sufficiently cold to produce those ice crystals. While there is no definitive temperature, ice crystals begin to dominate when the top of this moist layer is -12 degrees Celsius or colder. Precipitation continues to fall as snow when the temperature remains at or below 0 degrees Celsius from the cloud base to the ground. Wet snow is the result of temperatures slightly above freezing near the surface.
A typical environmental temperature profile that produces snow. Image courtesy of NOAA National Severe Storms Laboratory.
There are two processes in the atmosphere that can produce freezing rain (FZRA), namely, classical and nonclassical. The classic situation is what most pilots are taught during their primary training. That is, the precipitation starts out high in the cloud as snowflakes. These snowflakes fall through a melting layer that’s warmer than 0 degrees Celsius. If the melting layer is sufficiently warm and/or deep enough, it will melt those snowflakes turning them entirely into raindrops. That rain falls into a subfreezing layer and becomes freezing rain creating a significant airframe icing hazard.
A typical temperature profile that produces classical freezing rain. Image courtesy of NOAA National Severe Storms Laboratory.
The nonclassical case is a bit more complex to explain, but essentially the entire process remains liquid. In other words, the precipitation high in the cloud doesn’t involve snow. This occurs when the weather system isn’t terribly deep and the top of the moist layer is at a temperature warmer than -12 degrees Celsius. Warmer subfreezing temperatures at the tops tend to prefer a liquid process over the production of ice crystals. In the non-classical case, the entire temperature profile aloft may be below freezing or may also have a melting layer. Regardless of the actual profile, the non-classical case is strictly an all-liquid process. In most situations, you’ll see a lot of tiny drops that produce a nasty freezing drizzle environment. Surprisingly, 92 percent of the cases are nonclassical based on a study done by the National Center for Atmospheric Research (NCAR).
Ice pellets (PL) are similar to the classical freezing rain case mentioned above, except that the melting layer is very shallow. This doesn’t entirely melt the snowflake, and the drop retains a slushy inner core. These slushy drops refreeze as they fall through a deep layer of subfreezing air near the surface, and eventually reach the ground as hard little nuggets that bounce on impact.
A typical temperature profile that produces ice pellets. Image courtesy of NOAA National Severe Storms Laboratory.
Keep in mind that ice pellets often indicate the presence of supercooled large drop (SLD) icing aloft. While the frozen pellets will bounce right off of your aircraft while in flight (taking a bit of paint with it), they are often mixed with other forms of freezing precipitation including freezing rain especially at altitudes right below the shallow melting layer.
Here’s a little bit of ice pellet trivia. The abbreviation for ice pellets used to be PE. However, when rain and ice pellets occurred together with rain being the dominant precipitation type, the surface observation includes the term RAPE. This was deemed to be politically incorrect in English speaking countries and the abbreviation for ice pellets was then modified to PL.
So the next time you venture out this cold season, pay attention not only to the precipitation types that are being reported or forecast but also get a sense of the temperature profile aloft.
The radar depictions you see from the SiriusXM broadcast are highly filtered to provide only real precipitation areas. Ground clutter, anomalous propagation, birds, insects and such are carefully removed to provide the cleanest and most representative image. But like any process, there will be times where non-precipitation returns do not get filtered out. More importantly, you may see real areas of precipitation filtered out as well.
While rare, the latter usually occurs in regions where WSI (the weather provider for SiriusXM) implements what is called a manual gross filter. This kind of filter is the most efficient way to eliminate any clutter in large areas that are not expected to see precipitation. But when that filter is left on too long, it’ll be just as efficient at removing real precipitation from the broadcast.
Lightning and a single hail storm attribute marker with no radar depicted.
Here’s one such example depicted above. While connected to the SXAR1 I panned the map over Texas and I saw some lightning and a lone hail attribute marker showing echo tops at 45,000 feet in north-central Texas, but no radar returns. Hmmm?
I verified that I had the Radar Composite turned on (I did) and zoomed the display out as shown below to see that there are plenty of other precipitation areas shown to the northeast and southeast of this area. Given that the area wasn’t cross-hatched with “Radar not available” why wasn’t there any precipitation shown?
Zoomed out to show the presence of other precipitation on the radar composite.
About 15 minutes later I came back to the map to see if there was any change. Notice below that plenty of lightning and storm attributes are being depicted here in north-central Texas; however, there are still no radar returns being rendered. Given this activity, you’d expect there to be some precipitation shown when both lightning and storm tracks are present. This is a classic indication that the real precipitation in this region was being erroneously filtered.
This is a classic signature for a gross filter being left on too long. With the radar composite on, no precipitation is being shown despite the presence of lightning and storm tracks.
Just five minutes later, the gross filter was removed by WSI and the returns suddenly popped into existence as you can see below.
Once the gross filter was removed, the NEXRAD returns associated with these thunderstorms were rendered.
I took a look at the NEXRAD archives and discovered that the first precipitation developed in this region around 12:05 p.m. CDT. The gross filter wasn’t removed until 12:50 p.m. CDT. That’s 45 minutes with no radar for this area of rapidly developing and potentially severe thunderstorms. Moral of the story is to always have lightning ON and be sure the SiriusXM Storm Markers are also set to ON in the Maps Settings menu (the gear button on the Maps view). Having both of these layers on will likely expose these kinds of uncommon events.
Now that ForeFlight Mobile 7.7 introduced a second radar layer to the app, what are the practical advantages of each? As I mentioned in my earlier blog post, the composite reflectivity and lowest tilt radar layers both provide a high glance value to the pilot to highlight the location and movement of the truly nasty adverse weather. But I think you’ll find that these two layers are more often similar than they are different.
Go to any pilot gathering discussing weather and you’ll likely discover a majority of pilots genuinely swear by the composite reflectivity mosaic. In fact, you may even hear a few so-called “experts” stand up in front of an audience and attempt to convince them that you should only ever use composite reflectivity. Depending on your particular flying habits and aircraft capabilities, you may find that the base reflectivity from the lowest tilt is actually more useful and accurate. However, before we get into the pertinent differences, let’s examine how each mosaic is built.
The nuts and bolts of NEXRAD
Every NEXRAD radar site throughout the U.S. scans the sky with multiple 360-degree sweeps at increasing elevation angles. It starts the process (called a volume coverage pattern) at 0.5 degrees and finishes at 19.5 degrees assuming the radar is in precipitation mode. The base reflectivity from the lowest elevation angle (called the lowest tilt) is most representative of precipitation, if any, that is falling out of the base of the cloud and reaching the surface. So the lowest tilt is what interests most of the general public so that’s what you are likely to see on various websites that depict weather radar.
The composite reflectivity, on the other hand, includes the base reflectivity from every elevation scan. Depending on the scanning strategy of the particular radar site, this could be up to 14 different elevations. The highest base reflectivity value from each of these elevations is what’s included in the composite reflectivity mosaic. Consequently, you don’t know if the reflectivity depicted is near the base of the cloud, somewhere in the middle or near the top simply by looking at the mosaic.
A cross section of this mesoscale convective system (MCS) provides a better indication of the altitude of the highest reflectivity in the storm. In this case the precipitation core is below 6 km or 20,000 feet.
More is not always better
One of the chief issues with the composite reflectivity mosaic is that it often has a very large footprint when compared to the lowest tilt. It tends to exaggerate the areal impact of the precipitation event making it challenging to determine where it’s safe to fly. Shown below is a two image animation over the southeastern Florida peninsula that toggles between the composite reflectivity and lowest tilt. Notice on the composite reflectivity mosaic at least one-half of the area depicts returns that are not likely to be actual precipitation falling from the sky. Most of the green contours to the northeast of Lake Okeechobee are low dBZ returns from ice crystals in the thunderstorm’s anvil and are not likely a threat to pilots flying at lower altitudes 10 or more miles from the storm, but below the anvil.
An animation comparing the composite reflectivity and base reflectivity from the lowest elevation angle (lowest tilt).
High ice water content
If you fly a turbojet aircraft in the upper flight levels, the composite reflectivity mosaic can be quite important to examine. The thunderstorm anvil like the one shown above can contain a high enough concentration of ice crystals (called high ice water content) to be a problem. These ice crystals can be ingested into jet engines causing power-loss or damage within the engine core. Engine instability such as surge, stall, flameout, rollback and damage of compressor blades due to ice shedding have been reported in these conditions. So if you are a pilot circumnavigating deep, moist convection in a turbojet aircraft, the composite reflectivity mosaic provides some indication of where the high ice water content may be located.
Down low and below
During the warm season when thunderstorms are the most common, the lowest tilt depiction is one that is useful to pilots that like to fly down in the bumpy air below the cloud deck. Typically the footprint of the areas of precipitation will be less giving pilots a cleaner image leaving behind just the cellular structure that’s most important when flying within a convective environment. Even so, it’s still important to keep your distance. Bear in mind that nasty convective wind shear often occurs below building convection or when flying near mature thunderstorms. Gust fronts from thunderstorm outflow as well as microbursts are the biggest threats especially with high-base convection.
What about the radar from my Stratus?
At the moment, the base reflectivity from the lowest elevation angle isn’t part of the ADS-B broadcast. So while en route you will only have the regional and national composite reflectivity mosaic available. The current provider of ADS-B radar does a good job removing most non-precipitation returns, however, they don’t broadcast any returns below 20 dBZ which is typically what you’d see in areas with a thunderstorm anvil.
Here is the ForeFlight mapping of colors to dBZ levels found in the Pilot’s Guide. Notice that the first shade of green under ADS-B doesn’t start until 20 dBZ whereas the Internet scheme starts as low as 5 dBZ.
In the end, when both depictions are available as they are in ForeFlight Mobile, each radar should be given its due time during your preflight analysis.
ForeFlight 7.7 introduces the ability to share Logbook draft entries with other pilots, as well as a new radar layer, a new rate of descent instrument, Stratus ESG support, and lots of work under the hood to improve general map performance.
Flight Logging Just Got Easier with Flight Sharing
The new Flight Sharing feature in ForeFlight Logbook makes it easy for pilots to send and receive draft flight entries. Pilots can share a flight with one or more people right from the ForeFlight app. The receiving party then modifies and accepts the entry into their own Logbook. Flight Sharing makes it convenient when practicing approaches with a friend– use one iPad to collect the flight details, then simply share the entry. It is also a time-saver for corporate flight crews who can now share a logbook entry between the Captain and First Officer, reducing time spent on the administrative aspect of a flight.
Shared entries are just like drafts from Track Logs – review and edit the information, then tap Approve to add it to your Logbook.
Track Medical Currency in Logbook
Don’t let your medical expiration sneak up on you. When you add your medical certificate to the Logbook Qualifications section, you can now also add it to your currency summary view. This keeps the time remaining until your medical certificate expires front and center.
Stratus ESG Support and Firmware Upgrade for Stratus 1S/2S
Bundled with ForeFlight 7.7 is a firmware upgrade for Stratus 1S and 2S receivers. This upgrade adds support for the Stratus ESG, Appareo’s new all-in-one ADS-B Out solution. Stratus 1S and 2S devices can connect to the Stratus ESG via a USB cable to take advantage of its auxiliary power for continual charging, as well as the transponder’s externally mounted WAAS GPS and ADS-B receivers for maximum reception.
The upgrade also adds new features to the Stratus 2S. The built-in Flight Data Recorder now has automatic flight leg detection which automatically stops a Stratus Track Log and starts a new one when a full landing is detected. In addition, customers now have the option to save AHRS calibration settings between uses – this is especially helpful for taildragger aircraft pilots who set ‘straight and level’ attitude while inflight.
New ‘Lowest Tilt’ Radar Layer
For more informed preflight planning, you can now choose between the existing NEXRAD composite reflectivity layer and a new NEXRAD base reflectivity from the lowest elevation angle, or Lowest Tilt, layer. The current radar layer — renamed ‘Radar (Composite)’ — does what its name implies: it shows a composite view of multiple angles of radar scans. The new Radar (Lowest Tilt) layer shows only the lowest angle scan, generally providing a more accurate picture of where precipitation is actually reaching the ground.
The composite radar image at left shows precipitation over Atlanta, but the lowest-tilt scan on the right reveals that precipitation is only reaching the ground well west of the city.
In addition, you can display either radar layer in the low resolution, 4-color scheme defined by the Radio Technical Commission for Aeronautics as the standard for airborne radar coloring. This option is available in the Map Settings menu as “Four-color Radar”.
Finally, the Lightning layer no longer declutters groups of lightning strikes, allowing you to see all the strikes in a given area to get a better sense of where dangerous convection is occurring in a storm.
Find Your Rate of Descent to Destination
Also on the Maps view is a new option in the instrument panel: Descent to Dest. This instrument uses your GPS ground speed, GPS altitude, and distance to destination to compute the required rate of descent in feet per minute to be at your destination elevation upon arrival.
The Descent to Dest instrument shows the rate of descent required to be at destination elevation upon arrival.
Military Flight Bag Gets new Data Features
Military Flight Bag (or MFB), our dedicated subscription plan for military customers, now allows charts and data to be loaded onto an iPad over a wired computer connection — a process termed “sideloading”. While most of us have ready access to high-speed Wi-Fi and cellular connections, many of our military customers operate in areas of the world with slow or no internet – imagine trying to download a 2GB chart update over dial-up. Sideloading allows these updates to be delivered to multiple devices by connecting them to a central computer with the data already on it, giving military customers added flexibility in how they operate around the world.
In the pilot world there is a ubiquitous debate that continues to thrive over what ground-based radar product is better to use – NEXRAD composite reflectivity or NEXRAD base reflectivity from the lowest elevation angle. Without question, both of these radar mosaics provide a high glance value to the pilot to highlight the location and movement of the truly nasty adverse weather along your proposed route assuming you understand each of their inherent limitations. Now in ForeFlight Mobile 7.7, you’ll have the opportunity to wrangle over which is best since we’ve added a high resolution base reflectivity layer from the lowest elevation angle to complement the current composite reflectivity layer within the app.
But wait…there’s more! In addition to this new layer, we now offer two new low resolution NEXRAD mosaics, namely, a composite reflectivity and lowest elevation angle base reflectivity layer. These two four-color ground-based radar mosaics comply with the dBZ-to-color mapping standards defined by the Radio Technical Commission for Aeronautics (RTCA) documented in Table 3.2 of DO-267A. More on these later.
You can now select from one of two radar mosaic depictions in ForeFlight Mobile. The selections include Composite reflectivity and reflectivity from the lowest elevation angle or Lowest Tilt.
Base does NOT equal lowest
First, let’s squash a misnomer about base reflectivity. Many pilots (and even weather professionals) may use the term “base” in base reflectivity to imply lowest. That’s not what it means. In fact, every elevation angle generated by the WSR-88D NEXRAD Doppler radars has a base reflectivity product. The amount of energy directed back to the radar is measured and recorded in a logarithmic scale called decibels of Z (abbreviated dBZ), where Z is the reflectivity parameter. Next, these basedata returns are processed by a radar product generator (RPG) to produce hundreds of meteorological and hydrological products including a few near and dear to pilots such as reflectivity.
A more accurate description would be to prefix the product with the elevation angle such as “0.5 degree base reflectivity.” Nevertheless, you may see labels like “Composite Reflectivity” and “Base Reflectivity” on various public and subscription-based websites including those from NOAA. It’s likely that the base reflectivity is from the lowest elevation angle (or lowest tilt) of NEXRAD radar. That’s because the lowest elevation sweep is most representative of precipitation that is reaching the surface which is helpful to the average person on the street including hikers, golfers, boaters and anyone else who wants to know if they need to take the umbrella to work. Unfortunately, the elevation angle is usually dropped (likely due to ignorance or brevity) from these labels.
This is an animated comparison of the composite reflectivity and lowest elevation angle for convection in Florida. Notice the composite reflectivity provides a larger footprint since it picks up on the ice crystals that make up the cirrus anvil.
You might be surprised to learn that in many locations across the U.S., the composite reflectivity image you study before or during a flight is largely made up of only three or four of the lowest 14 elevation scans of the radar. So in these areas the composite reflectivity and base reflectivity from the lowest elevation angle are not all that different. These areas include regions where the NEXRAD coverage is sparse. Which surprisingly doesn’t only occur in the western U.S. Places such as my home town of Charlotte, North Carolina have distinct gaps in radar coverage.
Radar to the max
Each NEXRAD radar makes multiple 360° azimuthal sweeps at increasing elevation angles from 0.5° to 19.5° depending on the current mode of operation. The number of elevation angles (or tilts) depends on the scanning strategy or Volume Coverage Pattern (VCP) of the individual radar which is set by the radar operator that is located at the local weather forecast office that monitors and manages that particular radar site. A composite reflectivity image considers the base reflectivity from all of the most recent sweeps at each elevation angle and shows only the maximum reflected energy in the vertical column above each location within the radar’s effective coverage area.
It’s all about range
With respect to ground-based radar, range or distance is the key. Even though the lowest elevation angle is only 0.5°, at 124 nautical miles away the center of the radar beam is already nearly 17,000 feet above the surface due to the curvature of the earth. So it is easy to see how the higher elevation angles may easily overshoot precipitation that is not in the immediate vicinity of a radar site. Moreover, even if the beam is low enough to see the storm, it may still overshoot the precipitation core. Let’s take a look at an example.
Below is a two-image animation from the NEXRAD located at the Greenville-Spartanburg Weather Forecast Office in Greer, South Carolina. This shows the returns received from the lowest elevation angle or lowest tilt of the radar which is 0.5° and the fourth elevation angle which is only 1.7° (remember that 19.5° is the maximum elevation). Notice the radar at the lowest elevation has identified an area of weather over Fayetteville, North Carolina (seen on the far right). This cell is approximately 150 miles away from the radar site in Greer (on the far left). However, given it’s distance from the radar, the 1.7° elevation scan completely overshoots this area of precipitation. That means the composite reflectivity image in the Fayetteville area is likely made up of only the lowest three elevation angles of the radar. The remaining higher 11 elevation angles overshoot the precipitation in this region.
This two-image animation from WDT’s RadarScope app shows the base reflectivity from the 0.5 degree and 1.7 degree elevations. The NEXRAD radar producing this image is located in Greer, SC on the far left. Notice that some returns farther from the radar completely disappear as the radar beam overshoots the weather entirely.
Now it’s true that other adjacent radars such as the one from Raleigh Durham, North Carolina might be able to see this area of weather at higher elevation angles. However, due to the curvature of the earth, the radar beam from the highest elevation angles often overshoots much of the precipitation out there unless it is close to the radar site. This means that locations where there is little overlap between adjacent radars, expect the composite reflectivity image to be very similar to the base reflectivity image for the lowest elevation angle in these gaps.
The four-color radar
If you are flying with airborne radar, you may want to look at the new low resolution four-color NEXRAD mosaic now available in ForeFlight Mobile. The colors depicted in this radar mosaic match the standard color-to-dBZ mapping defined by the RTCA as documented in Section 3.8.2 (Table 3-2) of RTCA DO-267A (shown below). This standard is also used for airborne radar displays.
This is Table 3.2 of DO-267A that defines the color-to-dBZ mapping for airborne radar.
To see the four-color radar depiction, simply select one of the two radar layers on the Map view. Then tap the gear button next to the Map mode button and scroll down the Settings window until you see the setting switch labeled Four-color Radar just above the Radar Opacity slider. Tapping on the right side of this switch will change the radar depiction from the high resolution radar mosaic to the four-color mosaic. You can also find this four-color switch in the general Map View settings.
The four-color radar switch is located in the general Map View settings or can be found under the gear button at the top of the Map view.
If you use the Stratus (FIS-B) to receive weather while in flight, you won’t find the capability to select the lowest tilt, but you will find the four-color radar will also be available for the composite reflectivity mosaic. As you can see below, the four-color radar mosaic (second image) provides a much more ominous depiction of the weather as compared to its higher resolution counterpart (first image).
Normal resolution radar mosaic from FIS-B (Stratus).
Four-color radar mosaic from FIS-B (Stratus).
The reason for this may not be obvious. The data broadcast for FIS-B radar does not specifically include the raw dBZ values. Instead it uses intensity encoded values or “bins” that map to dBZranges as shown in the table below. Notice the wide 10 dBZ ranges for intensity encoded values of 2 and 3. Based on the RTCA standard defined in the table above, these are mapped in the ForeFlight four-color radar to green and yellow, respectively. Red is mapped to intensity encoded values of 4 and 5 with magenta mapped to 6 and 7. Because of the wide ranges as they map to the RTCA standards, the four-color radar depiction from FIS-B will use much “warmer” colors than the standard depiction.
This table from RTCA DO-358 defines the intensity-to-dBZ mapping for FIS-B radar broadcasts. The intensity encoded values of 0 and 1 are considered background and are not displayed as a color. ForeFlight chose to use magenta for intensity encoded values of 7.
Keep in mind that the four-color radar mosaic is a low resolution depiction and will not emphasize storm characteristics like you may see with the Internet radar. This is especially true for the initial evolution of convective cells.
Pilot weather reports are the eyes of the skies. They are not only consumed by pilots, but they are critical data for meteorologists as discussed in this earlier blog post. For example, SIGMETs for turbulence and icing often live and die by pilot reports. It’s rare to see a SIGMET issued for severe or extreme turbulence until pilots begin to report those conditions. As such they are an important part of any preflight briefing and are even more valuable as they trickle in over ADS-B while en route. That’s why we’ve given pilot report symbols used in ForeFlight a much needed facelift.
The new ForeFlight pilot weather report symbols help to quickly identify adverse weather along your proposed route of flight.
The hunt is over
In ForeFlight Mobile 7.5.2, we’ve significantly enhanced the way you see pilot weather reports displayed in the Map view. Prior to this release, pilot reports were loosely organized into three types, namely, turbulence, icing and sky & weather – each represented by a single pilot report symbol (chevron, snowflake and eyeball, respectively). However, this required you to tap on each and every PIREP marker to see important details such as altitude and intensity. Moreover, routine (UA) and urgent (UUA) pilot reports looked exactly the same. Now, standard pilot report symbology used in this release makes it clear as to the type of report, intensity, altitude (when known) and whether or not it’s an urgent pilot report without the need to tap on the pilot report symbol. So the hunt is over; with the added glance value, the truly nasty weather conditions reported by pilots jumps right out of the glass.
The good, the bad and the ugly
Pilots can include all sorts of things in a report, like seeing a flock of geese or even critters camping out on the runway. But reports of adverse weather (or lack thereof) of turbulence and icing are typically made through a subjective estimate of intensity. In order to enhance the glance value and minimize taps to get information, ForeFlight now uses standard pilot report symbols for turbulence and icing reports. Reports that do not contain turbulence or icing details are defaulted to use the legacy sky & weather “eyeball” symbol. These may contain reports of precipitation, cloud bases and cloud tops as well as outside air temperature and winds aloft (speed and direction).
Each icing and turbulence pilot weather report is shown in the ForeFlight Map view with one of the symbols above that depict the reported intensity. From left to right, the top row includes icing intensities of null (negative), light, moderate and severe. Also from left to right, the bottom row includes turbulence intensities of null (negative), light, moderate, severe and extreme.
Some intensity reports are “rounded up” to minimize the overall number of icons to remember. For example, you may notice in the symbols above that ForeFlight doesn’t use the official symbol for trace icing. Consequently, a report of trace icing is rounded up to use the light icing symbol. Similarly, we’re not providing a symbol for reports that straddle two intensities such as “moderate to severe.” Therefore, a “light to moderate” turbulence report will be rounded up to use the moderate turbulence symbol; a report of “moderate to severe” turbulence will be rounded up to use the severe turbulence symbol and so on.
All urgent pilot reports and reports of a severe nature will be tagged with a red badge to add increased glance value to those reports. For example, shown here is an urgent pilot weather report for severe turbulence at 8,000 ft MSL in the Florida Panhandle.
Above and beyond the different turbulence and icing symbols and to further attract your attention, urgent pilot reports in ForeFlight contain a red badge in the upper-right corner like the turbulence report shown above. These badges will typically be included on a turbulence or icing symbol for a report for severe or extreme turbulence and/or severe icing, respectively.
However, you may also see a red badge included with a weather & sky report like the one shown below. This is typically an urgent pilot report for low-level wind shear (LLWS) or mountain wave activity that did not also include any turbulence or icing details. Also, reports of hail, tornadoes, waterspouts or funnel clouds will be classified and tagged as urgent.
A red badge on a sky & weather (eyeball symbol) pilot report means that the report was tagged as urgent even though no icing or turbulence details were provided. Most of the time this means that low-level wind shear or mountain wave activity was reported by the pilot.
Altitude at a glance
If the pilot report contains a flight level (MSL altitude), this flight level is displayed below the symbol using three digits. For example, from the icing pilot report shown below, 057 is added below the symbol which identifies the reported altitude of 5,700 feet MSL.
A light icing pilot weather report at 5,700 feet MSL (FL057).
On the other hand, when the flight level is unknown (FLUNKN) as it is in the icing pilot report below, we will just show the appropriate symbol (turbulence, icing or sky & weather) without an altitude. Even so, there may be specific altitudes reported, but you’ll have to tap on the pilot report marker to examine the raw report for those details. In this case, light rime ice was reported between 6,000 and 4,500 feet MSL, for example.
Flight level in this light icing report is unknown (FLUNKN). Tapping on the report reveals more details.
I see double
If the pilot reported both icing and turbulence in the same report, you will see a pair of symbols side by side like the ones shown below with the center of the symbol pair representing the actual location of the report. This pair of report symbols indicates light icing and light turbulence at 16,000 feet MSL.
A pair of reports means that both icing and turbulence details were provided for the altitude shown in the marker.
Spreading the wealth
To keep everything consistent you will also see these standard symbols show up when tapping on the Map with the AIR/SIGMET/CWAs layer displayed. AIRMETs for turbulence and icing are displayed with their respective moderate symbol and SIGMETs for turbulence and icing will be displayed with their respective severe symbol. For example, in the list below, it’s very simple now to see that the last item in the popover is a SIGMET for turbulence.
Standard symbology is also used in the display of AIRMETs and SIGMETs for icing and turbulence.
Even though there’s now more information available at first glance, you will still want to examine the details of any relevant pilot reports by tapping on the specific markers. Like anything new, it may take a little while to get used to the new pilot report icons. But we feel that the use of standard symbology is critical for flight safety and these changes will provide less taps and a much higher glance value for determining the location and altitude of the most nasty weather being reported by pilots. Lastly, keep those pilot weather reports coming; they are important for all stakeholders in aviation safety.
While not rare, it is a pleasant surprise to see a fairly quiet radar mosaic stretching from coast to coast. Unless you are specifically looking for nasty weather, a tranquil radar usually means decent flying weather, outside of cold clouds, in most locations that are not reporting low ceilings and reduced visibility due to a radiation fog event. This also means you may not see some of the other familiar markers you’d normally expect to be displayed on the Map with the radar layer on. One of these markers that is often missing is the echo top heights.
Overall, a fairly benign radar with the most significant returns in southern California.
First, let’s get one thing out of the way; echo tops are not the same as cloud tops. Cloud tops are always higher. Second, echo tops represent the mean sea level (MSL) height of the highest radar echo of 18 dBZ or greater. Third, echo tops heights are added to the NEXRAD mosaic in ForeFlight only when the echo tops consistently exceed 20,000 feet MSL. In other words, you won’t see an echo tops report of 15,000 feet, for example. So it’s understandable for customers to believe echo tops may be “missing” from the radar mosaic when the radar is fairly benign. Moreover, there may be some intense-looking echoes in various locations, even some with storm tracks and mesoscale circulations shown, but no echo top heights anywhere to be found. Let’s take a look at a recent example.
In the image above, notice that most of the U.S. is enjoying an early evening free of any significant weather. A few light echoes in southeast Arizona, some light snow in Montana and Idaho, showery precipitation in western Washington and probably the most intense area of weather in southern California. Zooming in on that area below, there are some areas with reflectivity values greater than 40 dBZ (yellow and orange) indicating moderate precipitation. But there’s not a single echo top height displayed even though there are several storm tracks identified. The storm tracks are there since the cellular structure and the relative high reflectivity of the echoes has triggered the NEXRAD algorithms to generate one. However, this algorithm is completely independent of the echo top height.
Cellular returns indicate showery precipitation. A few cells have storm tracks defined, but despite their intensity, no echo tops are shown.
Despite the intensity of these cells in southern California, the echo top heights are likely below 20,000 feet. Since cloud tops are higher than echo tops, let’s examine the cloud top height in this area. The best way to determine the height of cloud tops is to examine the satellite imagery in ForeFlight like the color-enhanced infrared satellite image shown below. This satellite image shows the cloud top temperature. Notice the pale green colors within the black circle where the most significant returns are located. Using the color bar at the top of the image, these solid pale green colors equate to a cloud top temperature of about -20 degrees Celsius.
The color-enhanced infrared satellite image shows the temperature of the surface of the earth or temperature of the cloud tops. In this case, clouds in southern California have cloud top temperatures of -20 degrees Celsius.
Once the cloud top temperatures are known, it’s a simple process to compare this cloud top temperature against the temperatures aloft using ForeFlight. Below are the Winds and Temperatures aloft for Bakersfield near one of the more intense cells at this same time. This clearly shows at 18,000 feet MSL the temperatures were -6 degrees Fahrenheit or -21 degrees Celsius. So cloud tops in this region were definitely below 20,000 feet.
The ForeFlight Winds and Temperatures aloft show a temperature of -6 degrees Fahrenheit (-21 degrees Celsius) at 18,000 feet MSL over Bakersfield.
If you were paying close attention to the radar loop, you may have noticed that one lone echo top height marker appears (pointed to by the red arrow below) of 201 indicating an echo top height of 20,100 feet in this cell. So when you see a lack of echo tops reported, it just may be that those tops are below 20,000 feet.
A single echo top height of 20,100 feet MSL did pop up on the radar loop bolstering the idea that most echo tops were below 20,000 feet.