HowTo Interpret Radar Images

Approximate Rainfall Rates

The Bureau's radar images show the location of rain in relation to local features such as the coastline, with different colours used to depict rainfall intensity. For example off-white represents light drizzle, while dark red is used to depict very heavy rain (possibly containing hailstones). There are fifteen levels of rainfall intensity shown on the images - each level provides an approximate indication of the rainfall rate in millimetres per hour.

Example of reflectivity

The following values can be used as a general guide but they are not always accurate.

Level Colour Approx. Rainfall Intensity (mm/hr)
0 clear Not visible Under 0.2
1   Off-white 0.5
2   Sky-blue 1.5
3   Light Blue 2.5
4   Blue 4
5   Light Cyan 6
6   Cyan 10
7   Dark Cyan 15
8   Yellow 20
9   Yellow-orange 35
10   Orange 50
11   Orange-red 80
12   Red 120
13   Dark Red 200
14   Maroon 300
15   Dark Brown over 360

Radar Features to Watch For

Rain Bands (Example)

Radar echoes from widespread rain (e.g. from a frontal rain band) are usually extensive and fairly uniform in intensity, with ill-defined edges. The estimated rainfall intensity usually appears as light to medium because of the smaller raindrop size produced in such rain bands.

Showers from Cumulus Clouds (Example)

Radar echoes from showers falling from cumulus (tall bubbly clouds) appear as sharp-edged cells scattered around the radar display. The estimated rainfall intensity can be medium to heavy owing to the high rainfall rates from such clouds.

Heavy Precipitation from Thunderstorms (Example)

Radar echoes from the rain and hail produced in thunderstorms are very sharp-edged cells with intense cores indicating heavy rainfall. Hailstones produce particularly intense echoes because of their large size. Thunderstorm precipitation cells can appear as isolated cells or in clusters or lines. Each cell tends to last for 30 minutes or more. 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 may indicate the potential for severe weather (large hail, damaging winds and/or very heavy rain). Also a very slow moving cell or the repeated passage of a number of cells over a particular location could indicate potential for flash flooding.

Example: Radar Image loop of Melbourne Thunderstorm, 2/12/03

Tropical Cyclones

Tropical cyclones produce widespread heavy rain. The tendency for the rain bands, often with embedded cells, to spiral around the rain-free cyclone 'eye' produces a characteristic radar pattern. Of course, many tropical cyclones are so extensive that they are rarely seen entirely by a single radar. Nevertheless, identification of the 'eye' on radar is very useful in identifying the tropical cyclone centre and its movement.

Example: Radar Image loop of Tropical Cyclone Larry, 19/03/06 (5MB)

Very intense bushfires

Radar images may show areas of smoke from very intense fires. This type of phenomena is called Pyrocumulus. Very large fires may be distinguished by an elongated area of near-stationary echoes emanating from the source of the fire. See example from the 2009 bushfires in Victoria.