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.
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).
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.
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.
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.
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.
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.
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.
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.