Why does the radar say it's not raining?

Melbourners and those watching day 3 of the Boxing Day Test match between Australia and Pakistan may have noticed some light rain that is not appearing on the radar. So, what is going on?

During the morning hours in Melbourne on Thursday, some light showers can be seen coming in from the southwest with a cooler airmass. However, some suburbs, including the Melbourne Cricket Ground, were also receiving showers with nothing above them on the radar.

Image: Radar and satellite over Melbourne at 10:20am. The MCG is marked with the red drop pin.

While this rainfall makes it seem like the radar isn’t working, its more the case of the rain 'flying under the radar'.

A radar beam is not able to do a full sweep of the sky, and generally only covers the sky about 3° above the horizon of the radar site, with a significant drop-off of beam strength with distance due to the curvature of the Earth. For most precipitation, this is more than sufficient to give a good and useful reading of where rain is and how heavy it its. It does leave one kind of precipitation out though: that coming from low-level clouds.

Thursday's rain clouds in Melbourne were only 500ft (152m) above ground level, so quite close to the ground (normally they are about 2000-3000ft or 600-900m). The nearest radar to Melbourne is located near Laverton, about 19km WSW of the city and at an elevation of 24m. With the limitations of the radar beam, as well as some of the rain being blocked by the skyscrapers in the city, rain was not appearing over the MCG with the drizzle.

Image: A radar beam is good at capturing rain at height and close to the radar, but some showers from low cloud can duck under the beam. Source BoM 

While everything was working as it should, this is an ongoing limitation of weather radar technology. But some helpful questions to ask to see if there will be rain flying under the radar are: 

Are there any buildings, mountains or any other significant terrain between the radar and me? If so, sometimes they will block the signal  Is it the early morning, nighttime, or in the cooler months (May to September)? If so, rain from low cloud is more common during these times.  Is there thick cloud, especially in bands, approaching my location? If so, is there precipitation somewhere else along the rainband? If yes, the chances are good that you will also see some of that rain. 

On both Friday and Saturday mornings, some more drizzle is possible over Melbourne, possibly leading to more delayed starts if play is continuing the 4th and 5th days of the test match. 

You can see the Melbourne radar, and read more about the site itself, at Weatherzone.com.au