Satellites, Radar and Air Reports

The two satellite families

Weather satellites come in two orbits with complementary talents:

Geostationary satellites — parked about 36,000 km above the equator, matching the earth's rotation, so they keep the same full-disc view continuously: perfect for rapid image sequences and loops, but with a poor, oblique view of the polar regions.

Polar-orbiting satellites — flying low, around 850 km, in a sun-synchronous orbit that crosses each place at the same two local times every day, one daytime pass and one night-time pass. They scan the whole earth in strips at high resolution, each place roughly twice a day: sharper eyes, slower revisit.

What the pictures are used for

For aviation, the satellite's gift is coverage where nothing else observes:

cloud location and extent over oceans and data-sparse regions;

fronts and depressions, tracked as they move;

tropical revolving storms, kept under continuous surveillance;

fog and low stratus, detected over wide areas;

cumulonimbus tops and anvils, monitored as they grow.

Image loops then turn the still pictures into motion.

Visible, infrared, water vapour

Three kinds of picture, three different physical quantities:

Visible imagery — records reflected sunlight: the brighter, the thicker the cloud. It works only by day.

Infrared imagery — records temperature: the brighter (colder), the higher the cloud tops. It works around the clock.

Water-vapour imagery — sees the moisture of the upper troposphere: bright means moist air, dark means dry, sinking air. Cloud is not required.

From image pairs the processors also derive atmospheric motion vectors: winds computed by tracking cloud and moisture elements between successive pictures.

Reading clouds from above

The textures tell the genera apart:

Stratiform cloud — smooth, uniform sheets, bright in visible if thick. Beware the night trap: low stratus and fog can be almost invisible in infrared, their tops barely colder than the ground; the visible channel finds them again at dawn.
Cumuliform cloud — lumpy, cellular, granular fields.
The cumulonimbus — brilliant in both channels at once, thick and cold, its anvil streaming downwind: exactly the signature the thunderstorms lessons would predict.

Fronts and jets from orbit

Synoptic systems draw themselves. Fronts appear as long, organised cloud bands, and a mature depression as a comma: the spiral head its occlusion, the trailing band its cold front, the air-masses module's plan views photographed from above. Jets need the subtler channels: sharp cirrus and moisture edges in water-vapour imagery mark the jet axis, and atmospheric motion vectors map the wind maxima themselves.

The ground weather radar

Radar adds what satellites cannot see well: where it is raining, and how hard. A ground weather radar transmits pulses and listens for reflections from precipitation: echo strength maps intensity, bearing and timing map position, and Doppler processing adds the winds. Individual radars merge into national and continental composites, looped to show movement. Reading them: cells and lines mean convection, broad uniform echoes mean frontal stratiform rain, and no precipitation means no echo: radar does not see cloud or fog.

The radar in the nose

The airborne weather radar works the same physics from the flight deck. An antenna in the nose paints precipitation-sized droplets ahead, colour-coded from green through yellow to red by intensity. The tilt control aims the beam up or down (the ice-crystal lesson's indispensable habit) and the gain sets the sensitivity.

What the radar cannot see

The airborne radar's limits are exam favourites, because each one has bitten crews:

Precipitation only — it detects precipitation-sized water: not cloud, not fog, not clear-air turbulence, and only poorly ice crystals, dry hail and dry snow (the ice-crystal lesson's weak-echo trap);
attenuation — heavy rain absorbs the beam, so a strong cell can hide whatever lies behind it: a black notch behind a red core is a shadow, not clear air;
beam geometry — the wrong tilt overscans or underscans a cell entirely;
ground clutter — at low tilt the radar paints terrain as weather.

Reading the airborne picture

Interpretation is pattern reading with the avoidance lesson's distances attached: detour around the red cores by the 10 and 20 NM rules; treat hooks, fingers, scalloped edges and steep colour gradients as hail and severe-weather signatures; and treat any suspiciously clear notch directly behind a strong cell as attenuation shadow until proven otherwise, because the beam may simply never have come back.

Air reports

The observing system's last instrument is the aircraft itself, and two kinds of report flow back from it:

Routine air-reports — position and meteorological sections, plus an operational section sent on the operator's request or the commander's judgement, today largely automated by datalink; they feed the forecast models continuously.
Special air-reports (ARS) — the obligatory kind: a pilot must make one whenever the listed phenomena are encountered or observed.

The list to know:

moderate or severe turbulence, and moderate or severe icing;
severe mountain waves;
thunderstorms, with or without hail, that are obscured, embedded, widespread or in squall lines;
heavy duststorms or sandstorms;
volcanic ash cloud, and pre-eruption volcanic activity or a volcanic eruption.

The turbulence lesson said why these reports matter: the aircraft ahead is the freshest observation the system will ever get; the ARS is that fact written into obligation.