IR satellite limits

You've heard the old saying that a picture speaks a thousand words? Well, it might, but it also may trigger a thousand questions. For example, if you look at the observed ceilings based on the latest METARs (EZDeparture Advisor must be moved all the way to the left to see observations), you'll notice many stations reporting MVFR, IFR and LIFR ceilings. But to the northwest, there are no ceiling reports at all. What's going on? Missing data? App dropped the ball?

Actually, this ceiling layer shown above on the EZMap in EZWxBrief only adds a marker to the map if there's a ceiling being reported (or forecast). If the sky cover is reported (or forecast) as clear (or clear below 12,000 feet), scattered or few, then no marker is placed on the map since this does not constitute a ceiling. Ceilings can only be reported for a sky coverage of broken, overcast or an indefinite ceiling (vertical visibility). So this leaves a huge void of markers in the northwestern section of this area (on purpose).

Apps such as Garmin Pilot may also at times provide some confusing imagery. Above is the Internet IR color-enhanced IR satellite image also with a ceiling marker overlay that is valid around the same time as the EZMap display in EZWxBrief. At first glance it would appear that there are some very obvious inconsistencies. Where you see the color contours, the satellite depiction is suggesting there are clouds. Areas without contours should be clear?

Well, that's certainly not the case here. In fact, in many places where there is a MVFR, IFR or LIFR ceiling where the IR satellite image shows no clouds. But where there are cloud contours (blue and green) to the northwest of Charlotte, the ceilings markers mostly say UNL for unlimited. It is likely the case there are clouds here, but those clouds have bases that are above 12,000 feet AGL (or could be scattered or few in coverage) and are being reported by automated station. There are two stations in that area that have been augmented by a human observer (shown by the red arrows) that suggest the ceilings are pretty high at 20,000 ft and 25,000 feet.

Why is it missing the clouds where you see the lower IFR ceilings? It's pretty simple; these are surface-based stratiform clouds. Therefore, the tops of the clouds in this area are pretty shallow (possibly ground fog). The IR satellite image actually depicts cloud top temperatures as a height above mean sea level. That is, every object that has a temperature above absolute zero emits radiation at a specific frequency. The satellite determines that temperature and assigns it a height based on atmospheric conditions in that area.

However, when the tops are low to the ground (especially in the morning with a nocturnal temperature inversion present), often the surface temperature and the cloud top temperature are nearly equal. This means that the product can't tell the difference between the ground temperature and the temperature of the top of the cloud. This shows you how important it is to always show the ceiling markers when viewing any cloud tops satellite imagery.

The cloud tops image from the SiriusXM broadcast actually captures the cloud cover much better than the Garmin Pilot Internet IR satellite. The darker shades of gray are lower (warmer) tops with the lighter shades of gray representing higher (colder) tops. Higher tops that are in the southeaster area of this image are indicative of deep, moist convection. Even so, it's still important to always overlay the latest surface observations when viewing any satellite imagery. These two layers coupled together make a great resource to know the relative depth of LIFR conditions and the location of deep convection.

Most pilots are weatherwise, but some are otherwise™

Dr. Scott Dennstaedt

Weather Systems Engineer

Founder, EZWxBrief™

CFI & former NWS meteorologist

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