If you surveyed a group of IFR pilots, tops are likely one of the most requested features. Now, the wait is over. With ForeFlight Mobile 8.3, you can view both echo tops and cloud tops when connected to the SXAR1 SiriusXM satellite receiver. These two sought-after weather products are now included with the current ForeFlight Mobile SiriusXM pricing tier at no extra cost.
You can find the echo tops and cloud tops selections in between the radar and satellite layers in the ForeFlight Mobile app.
Cloud top height
First and foremost, the cloud tops depiction from SiriusXM is not a satellite image per se. Instead it depicts the height of the cloud tops in reference to mean sea level (MSL). Second, the cloud tops overlay does not infer the depth of the cloud layer. Consequently, a high overcast cirrus deck at 30,000 feet may mask one or more cloud layers below. Third, not all cloud layers may be shown, especially when there are regions of low-topped stratus or scattered to broken fields of fair weather cumulus clouds. So it’s important to always overlay the sky coverage markers to augment the cloud tops layer.
Here’s a common limitation during a low-topped stratus event. Notice that the sky coverage markers around Houston, Texas indicate the presence of overcast skies, however, the cloud tops layer shows the sky as clear.
The cloud tops layer is always valid in the recent past since it’s based on observed data. It is typically updated with a new image once or twice an hour. Tops above 25,000 ft MSL are color-coded using blue, orange and red to visually enhance the highest tops. Tops below 25,000 ft are shown as simple shades of gray.
The echo tops layer (left) may appear to look like a radar depiction (right) from a color perspective, However, it has a much lower spatial resolution than the composite or lowest tilt radar mosaic.
Echo top height
Like cloud tops, echo tops depict a height above mean sea level so it’s not a radar depiction per se. Simply put, echo top height is based on the highest elevation angles at which greater than 18 dBZ reflectivities are detected. Keep in mind that echo tops are primarily used by meteorologists to identify more significant storms by locating the highest tops. So it’s important know that echo tops are not the same as cloud tops. The actual top of the cloud is always higher than the echo top.
In this vertical cross-section of a thunderstorm, reflectivity is shown using colors similar to what you would see on a NEXRAD mosaic. Dark blue represents a reflectivity of 15-20 dBZ. So, the echo tops are likely found near the top of the dark blue regions on this image.
Filtering by altitude
On the ForeFlight Map view, both the echo tops and cloud tops can be filtered by altitude. When selecting either one of these layers, an altitude selector similar to the one that appears with the winds aloft layer is shown. This provides a quick way to determine tops that are above a selected MSL altitude. Initially, the altitude selector will be positioned at the lowest setting, namely, 0 feet MSL. This is the selection that will show all cloud or echo tops. Setting the cloud tops altitude selector to 10,000 feet, for example, will remove any clouds with tops below this altitude leaving only clouds with tops above 10,000 feet. Therefore regions without tops data are regions without clouds or tops that are below the selected altitude.
The altitude selector allows you to filter all of the cloud tops (or echo tops) below a specific altitude. In this example, all cloud tops below FL300 are removed leaving only those tops above that altitude. For convective tops, it’s also a good idea to overlay the lightning layer.
Echo top clutter
Echo tops received through SiriusXM do not go through a rigorous filter like you may see with the two radar layers. Therefore, it is normal to see echo top clutter around and near the various NWS radar sites as shown below. Typically these are not associated with real areas of precipitation and often occur during fair weather. Simply moving the altitude selector up to the next rung at 5,000 feet will remove many of these annoying areas of clutter.
Echo tops clutter showing tops below 5,000 feet will often occur around the various NWS radar sites. Here you can see clutter around the NEXRAD sites at Charleston, W. Va., Sterling, Va., Dover, De., and Mount Holly, N.J.