Monitoring the weather

Meteorologists have long valued weather radar's ability to depict rainfall location, movement and intensity.

Australian meteorologists can draw on radar imagery from 43 sites. There is particularly thorough coverage on northern coastal areas subject to tropical cyclones.

Radar antennas transmit pulses of radio waves in a highly-focused beam; when these pulses strike a target, reflected energy ('echoes') captured on a receiver depicts the location and intensity of rain, hail, snow and drizzle. Typical displays shown on TV and the Bureau's web site use colour coding to indicate six levels of estimated rain intensity. You can track rain movement on sequences (loops) of images.

Radars can show light rain to a distance of around 150km, and severe thunderstorms to over 400km, limited mainly by the curvature of the earth.

Watch for these characteristic radar features:

Rain bands
Echoes from widespread rain (for instance, a frontal rain band) are usually extensive and fairly uniform in intensity, with ill-defined edges. The estimated rainfall intensity is usually shown as light to medium because rain bands typically produce smaller raindrops.

Cumulus cloud rainfall
Showers from cumulus (tall bubbly clouds) appear as scattered sharp-edged cells. These clouds typically produce high rainfall rates shown as medium to heavy intensities.

Heavy rain from thunderstorms
Radar echoes from thunderstorm rain and hail are very sharp-edged cells with intense cores indicating heavy rainfall. Hailstones produce particularly intense echoes.

Thunderstorm cells may be isolated, or appear in clusters or lines. A cell tends to last for 30 minutes or more.

There are potential indications of severe weather (large hail, damaging winds and/or heavy rain) if you see fast-moving cells, rapidly-growing cells, a bow in the direction of movement of a line of cells, and/or a long-lived cell moving in a markedly different direction to others.

Flash flooding is possible when a cell moves very slowly, or a number of cells pass a particular place.

(Australia's most thunderstorm-prone capitals, Sydney and Darwin, also have specialised Doppler radars which display additional information on the structure of storms.)

Tropical Cyclones
Tropical cyclones produce widespread heavy rain. Radar often ( but not always) captures their distinctive radar signature showing rain bands (often with imbedded thunderstorms) spiralling around the rain-free cyclone 'eye'. This identification gives forecasters valuable information on the centre and movement of a tropical cyclone.

Some cautions on interpreting radar imagery

• The intensity of echoes tends to decrease with distance. (The beam broadens with distance; becomes further from the ground with distance, partly because of Earth's curvature, and may lose power slightly when passing through heavy rain.)
• Mountains within radar range can block or reduce echoes from rainfall beyond the mountains
• Occasionally the radar may detect faint echoes from large fires, swarms of insects, aircraft or even the ground surface, if unusual atmospheric conditions 'bend' the beam.
• Because the radar shows echoes at around 3000 metres altitude, a weak echo may not mean rain at the ground, because sometimes light rain at 3000 metres evaporates before reaching the surface. It is also possible to miss early development of severe thunderstorms if precipitation is held above the radar beam by strong thunderstorm updrafts.
• The intensity of drizzle may be underestimated because of the lack of large droplets to reflect radar energy.