Weather Radar and Met Satellites

Meteorology · Chapter 22 · DGCA CPL and ATPL

How radar sees rain inside clouds and how satellites watch the whole sky, and how to read what they show without being fooled.

Remember

Radar: pulse delay = range, dish = bearing. Up to 500 km, best within 250 km (rain attenuation, Earth curvature). PPI = plan, RHI = height (CB tops).
Bands: K ~1, X 3, C 5, S 10, L 20 cm. K/X cloud, X storms and airborne, S rain and cyclones (cyclone detection radar). Quantitative 10 cm; ultra-sensitive sees convection, CAT, waves.
DWR: frequency shift = motion (shear, microbursts, tornadoes, cyclone winds). Echo ∝ diameter⁶: drizzle missed. Melting snow: bright band below 0 °C, a ring on the PPI.
Echoes: diffuse + bright band = stratiform (weak); sharp = convective; top to −40 °C = CB; blurring = decay. Hook/"6", dry hole, slant columns, converging, growing, large, fast = severe. Spiral bands = cyclone.
Airborne: X band, 3° beam, ±15° tilt, ±70° screen. Green 1-4, yellow 4-12 (avoid), red 12+ and magenta (always avoid); red flashes every ½ s.
Polar 650-1500 km (NOAA), each place every 12 h. Geostationary ~36 000 km over the equator; five cover the globe. INSAT-3D 82°E (2013), 3DR 74°E (2016): every 15 min; 3-hourly from 0300 UTC, hourly in cyclones.
Imagery: thick and water cloud brightest; jet = long cirrus streaks; dust tan, haze pale, smoke brown-grey, ash brown. IR: coldest whitest.

How does radar see rain you cannot?

Seeing inside clouds

Four cards with real photographs. Swipe or tap Next.

A spinning dish in a ball A weather radar's rotating antenna sits inside a protective dome. It sends out short radio pulses; raindrops, snow and hail scatter some back. The time delay gives the range, the antenna direction gives the bearing. IMD runs S-band cyclone radars along the coast and X-band and Doppler Weather Radars inland. Range reaches about 500 km, but it is most reliable within 250 km. Since 1975 IMD has also used X-band Multi Met radars with radiosondes to find upper winds; the older radars are being replaced by Doppler Weather Radars.
From above and from the side The PPI (plan-position indicator) shows the echoes as a map, with bearing and range. The RHI (range-height indicator) slices the storm vertically, so the forecaster can read the height of a CB's top.
A radar slice through a tropical storm A satellite-borne cloud radar cut this vertical slice through a tropical storm. The colours show echo strength: towering cores of heavy rain rise to the top of the troposphere. Doppler weather radar also measures how fast the rain moves towards or away from it, revealing wind shear, gust fronts, microbursts and the spin of tornadoes. Weather radars have measured rain inside CU and NS since the 1950s. Most modern ones use a pulse-Doppler technique (rain rate, drop motion, radial wind); newer dual-polarised radars also tell rain, snow and hail apart and size the drops more accurately.
The cyclone's signature On radar and satellite images a tropical cyclone shows long spiral bands of storms wrapping into its centre. They last a long time and are one of the clearest signatures in all of meteorology.

Takeaway: Radar sends out pulses and listens for echoes from raindrops and hail. The stronger the echo, the bigger and more numerous the drops, and the worse the weather.

How does a radar know where the rain is?

Pulse, range and scopes

The basics behind every weather radar. Tap Next.

Pulse and echo RADAR stands for radio detection and ranging. The dish sends out a short radio pulse. Raindrops, snow and hail send a little of it back as an echo. The time the echo takes to return gives the range; the way the dish is pointing gives the bearing. So radar finds rain clouds, especially CU and CB, and shows where they move, how fast, and how tall they are.
Why range is limited Two things limit a radar. Heavy rain soaks up the beam (attenuation), so storms behind it look weaker. And the beam goes straight while the Earth curves away below it, so far-off low cloud is missed. A weather radar can see up to about 500 km, but the picture is reliable within about 250 km. Displays normally show echoes within 25 NM, or out to 75 NM for longer range.
PPI: the map view The plan-position indicator (PPI) turns with the dish. It paints every echo at its bearing and range, so you get a map of the rain around the station.
RHI: the side view The range-height indicator (RHI) nods the dish up and down along one bearing. This slices the storm from top to bottom, so you can read the height of a CB's top straight off the screen.

Memory hook: Up to 500 km, best within 250 km. PPI = plan (bearing, range). RHI = height (CB tops).

Takeaway: A radar sends short radio pulses and times the echoes from rain and hail. The delay gives the range; the way the dish points gives the bearing. It reaches about 500 km, but the picture is best within 250 km: heavy rain soaks up the beam and the Earth curves away below it. The PPI shows a map view; the RHI shows a side view, so you can read how high a CB goes.