Parallax can mean different things in different sciences (See, for example, this link that describes how parallax is used to compute distances in astronomy), but in satellite meteorology, parallax is the apparent shift in an object’s position (away from the sub-satellite point) as a result of viewing angle. (Here is an example.) Parallax generally increases as you move away from the sub-satellite point. It is also large for higher clouds. Consider the simplified example below.

It shows a lake (blue surface) viewed obliquely from a distant satellite. So this surface feature is far from the sub-satellite point. If a very tall cloud develops between the surface lake and the observing satellite, the satellite will still interpret the information as coming from the surface — that is, where the lake is. In reality, however, the tall cloud is displaced towards the sub-satellite point. Note also another consequence of the viewing angle: the temperature of the cloud will reflect the temperature of the side of the cloud that the satellite is viewing. The colder cloud top will be in a different pixel.

Parallax in geostationary imagery becomes obvious when cloud imagery is compared with surface-based observations. The example below is from April 2006, and shows strong convection over northern Wisconsin just south of Lake Superior. More modest convection is over southern Wisconsin near Madison.

The enhanced infrared imagery can be used to infer the height of the cloud (cold clouds are usually higher in the atmosphere). Displacement (parallax) is greater for higher clouds. The coldest clouds are associated with the convection just south of Lake Superior.

The radar for the same time shows a line of convection displaced to the south of the convection.

The displacement is difficult to see in the three individual images, but stands out in the fader that can be seen here (Link requires Java).

The bottom line: when you see a very cold cloud top on satellite imagery, and the cloud top is far from the sub-satellite point, it’s very likely that the position of the cloud feature over the surface is closer to the sub-satellite point than is indicated in the image mapping.

(Added, 2013: The examples below show how the satellite navigation can place severe weather events in a location that is not where you might expect it to be. By parallax-correcting the observation, the severe weather report location is more properly aligned with the cloud top as seen in satellite images. The images below provide GOES-13, GOES-14 and GOES-15 views of a large hail event in northwest Wisconsin; imagery courtesy of Bob Rabin and Jim Nelson, CIMSS)

GOES-13 0.63 µm visible image and original and parallax-corrected storm report (click to enlarge)

GOES-14 0.62 µm visible image and original and parallax-corrected storm report (click to enlarge)

GOES-15 0.62 µm visible image and original and parallax-corrected storm report (click to enlarge)

It is also possible to remap the satellite image, rather than the storm report. The GOES-14 satellite image below was first remapped to a Mercator projection, and that image was then parallax corrected (using infrared imagery to estimate the height of the cloud; note that low clouds show very little parallax correction).

GOES-14 0.62 µm visible image remapped to Mercator projection and then parallax-corrected (click to enlarge)

Added, 2017: The National Weather Service Operational Proving Ground produced a YouTube video, below, discussing the causes and effects of parallax.

NWS OPG “Satellite Parallax Causes and Effects” (click to play YouTube video)

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What appeared to be a rather unusual example of a subtropical cyclone was evident over the northern East Pacific Ocean on 01 November (AWIPS surface analysis + buoys) — however, this compact cyclone was likely associated with a deep cold-core cutoff low that had been over that region for a few days (HPC 500 hPa analyses). GOES-11 IR cloud drift winds (above) showed the broad cyclonic flow around the periphery of the surface low (closer view of AWIPS low-level winds | AWIPS upper-level winds). GOES-11 visible channel imagery (QuickTime animation) showed a cloud structure that almost resembled the eye of a tropical cyclone.

A similar signature was also seen on the GOES-11 10.7µm IR window channel imagery (below), with a ring-like feature of cold cloud top temperatures (-50 to -55 C, yellow to orange enhancement) that was very persistent for much of the day (QuickTime animation). Additional satellite imagery can be found by selecting the Central Pacific 91C.INVEST link at the NRL Tropical Cyclones site.

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A strong cyclone centered over the northcentral US was producing heavy snow across much of North Dakota on 30 October; an associated cold frontal boundary was moving rapidly southward across Nebraska, Colorado, and Kansas during the morning and afternoon hours. The southward push of the cold air behind the front can be seen on an animation of GOES imagery from AWIPS (above), evident as an area of lighter gray enhancement on the 10.7 µm IR window and 3.9µm shortwave IR images (above, upper left and lower left panels) — the leading edge of this cold air was well south of the low cloud deck that was covering parts of South Dakota and northern Nebraska.

In addition, if you look closely, you can also see a subtle reflection of this surface-based boundary moving southward across northeastern Colorado on the 6.5µm “water vapor channel” imagery (above, lower right panel), even though this is a channel which normally senses radiation from altitudes higher in the middle troposphere. A plot of the GOES-12 imager water vapor channel’s weighting function at North Platte, Nebraska (below) indicates that the altitude of the peak contribution for that particular air mass had indeed shifted downward to near 500 hPa (~ 18,000 feet in altitude).

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An early season winter storm dumped up to 25 inches of snow across parts of Colorado on 26 October (NWS snowfall reports); as fate would have it, on that particular day Alaska also reported its first below zero temperature of the season (-7ËšF at Bettles), making for two ominous signs that winter is fast approaching. Aqua MODIS imagery from one day after the storm (above) shows the extent of the resulting snow cover, both in the mountains and also in the eastern Plains of the state; the false-color composite using MODIS channels 2 and 7 (above, right) displays the snow cover as dark red features on the image. Note how the snow cover is distributed both north and south of the Palmer Divide (a west-to-east oriented ridge of higher terrain across eastern Colorado) — upslope flow played an important role in focusing heavy snowfall during different stages of the storm.
A closer view of the Denver area using a MODIS 500-meter resolution true color image (below) shows that many of the small lakes are still unfrozen, and stand out against the surrounding snow covered land surfaces.

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