In RADAR (RAdio Detection And Ranging), radio pulses are transmitted, in a highly focused beam, by a radar antenna and are reflected off objects and returned to a receiver at the antenna.

The radar's computer determines the direction of the rain from the azimuth (orientation) of the antenna, and the distance to the rain from the time taken for the radar signal to return to the receiver.

The "reflecting power" of the rainfall (which depends on the size of the raindrops and their concentration) is also calculated, thus providing an estimate of rainfall intensity.

In short the display produced by the radar's computer gives a horizontal map of where rain is falling and an estimate of how heavily it is falling.

This particular radar image shows the rate of rainfall in millimeters per hour of the 'echoes' produced.

On the right hand side, underneath the date and time - which is in UTC (known also as GMT) and which is one hour behind Malta Winter Time, one finds a table showing the rate of rainfall. The brightest red means that rain is falling at the rate of 30 mm per hour, and the brightest blue means that the rain is falling at the rate of between 0.2 and 0.4 mm per hour.

One has to bear in mind that clouds move and this means that in one hour 30 mm of precipitation is spread over a very large area. It is only if the cloud stops completely over one area that all the 30mm fall into that area.

PCAPPI ® at the top left hand corner means Pseudo Constant Altitude Plan Position Indicator (Rainfall) - this image is at a constant altitude, 1000 metres above ground – meaning that it shows the rain that is falling at a height of 1 kilometre.
This picture does not show all clouds but it is actually displaying concentrations of rain drops - however, light rain or drizzle from shallow cloud weather systems below 1kilometre are not displayed.

Buildings and hills in the vicinity of the radar produce what is known as "ground clutter" on the radar image. This results from the radio energy that is reflected back to the radar from these targets.

Under highly stable atmospheric conditions, like calm and clear nights or when there is a warm southerly wind blowing, the radar beam can be refracted almost directly into the sea at some distance from the radar, resulting in an area of intense looking echoes. This "anomalous propagation" phenomenon detects "sea return", a phenomenon similar to ground clutter except that the echoes come from the sea surface.

The Malta International Airport’s Weather Radar is a Doppler Radar – which means that it can measure the instantaneous component of motion parallel to the radar beam i.e., toward or away from the radar antenna. This is extremely important to the MIA meteorologists in that it computes the horizontal motion of the rain droplets, and thus the speed and direction of the wind at various levels of the atmosphere.

The radar does not scan directly overhead. Therefore, close to the radar site, data is not available due to the radar's maximum tilt elevation of 31degrees . This area is commonly referred to as the radar's " Cone of Silence". However the Malta International Airport’s Weather Radar has a device to partly compensate for this.

Radar Image Interpretation – some points.

These important points should be taken into consideration when interpreting the radar image.

1. The intensity of echoes tends to decrease with increasing distance from the radar, because:
   · The radar beam widens with distance, therefore the volume of the beam that is filled with rain is decreased - this reduces the echo intensity;
   · The radar beam becomes attenuated, that is it loses power slightly when passing through very heavy rain, thus reducing the echo intensity further out from the radar.
   
   It is essential to keep these limitations in mind when interpreting the outer parts of the radar images.
2. The radar may sometimes detect faint echoes from targets that are not rain, hail or snow, such as aircraft, swarms of insects, flocks of birds or even the ground or the sea surface (see text above).
   
3. A weak echo may not mean that it is raining at the ground because under some certain conditions light rain can evaporate completely before reaching the ground.
   
4. The intensity of very light rain or drizzle may be underestimated because of the absence of large droplets.