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Skywatchweather.in
Three satellite channels answer three different questions. Knowing which to consult, and when, is the difference between looking at cloud and understanding it.
Monalisa DuttaSatellite imagery is the most widely available meteorological product and among the most frequently misread, largely because the three principal channels are treated as though they were variations of the same picture. They are not. Each measures a distinct physical quantity and answers a distinct question.
The visible channel measures reflected sunlight, in essentially the manner of a photograph taken from orbit.
What it shows well. Cloud texture and structure with excellent clarity. Thick cloud reflects strongly and appears bright white; thin cloud reflects weakly and appears grey. The channel resolves the sharp, granular texture of vigorous convective cloud, the smooth uniformity of stratus, and the fibrous appearance of cirrus. Low cloud and fog are generally distinguishable from the surface beneath.
Its limitation. It requires sunlight and is therefore unavailable at night, which is a considerable constraint given that convective activity over much of India frequently peaks during the late afternoon and continues after dark.
The visible channel provides the clearest indication of cloud thickness, and is particularly useful for identifying the sharp-edged, bubbling appearance of developing convection.
The infrared channel measures thermal radiation emitted by cloud tops and by the surface. It does not depend upon sunlight and is therefore available continuously.
Because temperature in the troposphere decreases with height, the measured temperature serves as a proxy for altitude. Cold cloud tops are high cloud tops.
Imagery is conventionally displayed with an inverted scale, so that cold appears bright and warm appears dark. Enhanced colour tables are frequently applied to the coldest temperatures, allowing the most vigorous convection to be identified immediately.
Its limitation. The channel measures only the temperature of the highest surface it can see. It cannot distinguish between thin cirrus and thick cumulonimbus if both have similarly cold tops, and it cannot see beneath an upper layer. Low cloud and fog, being close to surface temperature, are frequently difficult to identify at all.
This is the channel least familiar to general readers and, in my assessment, the most interesting.
The water vapour channel measures radiation in a band strongly absorbed by water vapour, revealing moisture content in the middle and upper troposphere — broadly between 3 and 8 kilometres. It functions independently of cloud, and therefore shows atmospheric structure in regions which appear entirely clear in other channels.
What it reveals. Bright areas indicate moist mid-level air; dark areas indicate dry air. Because the moisture is carried by the flow, the imagery effectively traces the circulation. Jet streams appear as sharp boundaries between moist and dry regions. Troughs, ridges and upper-level circulations become visible. Dry intrusions descending from upper levels — frequently associated with convective development or suppression — can be tracked directly.
For understanding why weather is developing rather than merely observing that it has, the water vapour channel is often the most informative single product available.
Forecasters rarely rely upon a single channel. The characteristic combinations are these.
| Appearance | Interpretation |
|---|---|
| Bright in visible, cold in infrared | Deep, thick convective cloud — an active storm |
| Dim in visible, cold in infrared | Thin high cirrus — frequently anvil outflow rather than active convection |
| Bright in visible, warm in infrared | Low cloud, fog or shallow stratus |
| Dark in water vapour beside active convection | Dry intrusion, often associated with intensification or with suppression at the edge |
The overshooting top. A small, very cold dome protruding above the general anvil surface indicates an updraught vigorous enough to penetrate the tropopause. It is a reliable indication of a severe storm.
The anvil and its shadow. In visible imagery near sunrise or sunset, the shadow cast by an anvil upon lower cloud confirms substantial vertical development and helps distinguish an active storm from decaying cirrus.
The cold-U or enclosed warm area signature. A U-shaped or V-shaped pattern of very cold tops surrounding a slightly warmer region downwind is associated with particularly intense convection.
The cyclone eye. In a mature tropical cyclone, the eye appears as a warm, clear region in infrared imagery, surrounded by the very cold tops of the eyewall. Eye clarity and symmetry are among the visual indicators used in intensity estimation.
Monsoon cloud organisation. During an active monsoon phase, extensive organised cloud accompanies the monsoon trough. During a break, the deep convection shifts north towards the Himalayan foothills or retreats towards the equator, leaving central India comparatively clear. The transition is frequently visible in satellite imagery before it becomes apparent in rainfall figures.
The two systems are complementary rather than alternative. Satellite observes cloud from above and covers vast areas including oceans where no radar exists. Radar observes precipitation from below, at high resolution, but only within a few hundred kilometres of each site.
A developing system over the Bay of Bengal will be seen by satellite long before it approaches radar coverage. Once within range, radar provides the detail of intensity and internal structure which satellite cannot resolve.
Both are available together on our Live Weather Dashboard, and the India Meteorological Department publishes INSAT imagery across all channels.
Meteorology consultant and former Squadron Leader in the Indian Air Force, with over eleven years of experience in operational aviation meteorology, weather forecasting and flight safety support. She now advises technology organisations on AI-driven weather forecasting and decision-support systems. Consultancy services →