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Answers to the most common 'new radar viewer' questions are linked below.

Interpreting Radar Images - Frequently Asked Questions

 

It is raining where I am but no rain is showing on the radar.

  1. Radar curvature
    The effects of the curvature of the earth on weather radar.
    The intensity of echoes tends to decrease with increasing distance from the radar. This is because:
    • the radar beam broadens with distance, thus decreasing the proportion of the beam which is filled with rain, which reduces the echo intensity;
    • the radar beam becomes further from the ground with distance (partly because of the Earth's curvature, and partly because the beam is angled upwards by a fraction of a degree), thereby missing the lower parts of the rain. A horizontal radar beam detects raindrops at a height of 1 kilometre above the Earth's surface from rain that is 100 kilometres away from the radar. It detects raindrops at a height of 3 kilometres from rain that is 200 kilometres away;
    • the beam can lose power slightly when passing through very heavy rain, thus reducing the echo intensity further out from the radar;

    Thus precipitation that is occurring some distance away from the radar might not show at all, or may show with a reduced intensity. The presence of significant echoes at large range probably indicates the presence of large amounts of rain at high levels above the ground (e.g. a thunderstorm). At these distances the radar echoes are likely to be reflections caused by ice rather than rain drops, where the relationship between reflectivity and rainfall rate is different.

  2. The presence of mountains within the range of the radar can block part or whole of the radar beam, thus significantly reducing the echo intensity from rain on the other side of the mountains.

  3. Because of the variations in rain closer to the radar site and in the refractive index of the air, attempts to correct for these limitations have not been very successful. Consequently the estimation of rainfall rates using the radar imagery should be used only as a very rough guide.

I measured 20 mm of rain between 7 pm and 8 pm, but the radar imagery only showed a rainfall rate of 2 mm per hour. Why the difference?

  1. The radar reflectivity is strongly dependent on the diameter of raindrops in the cloud not the amount of rain drops and therefore rainfall rates. Tropical maritime rainfall consists of very many moderate sized raindrops so that the reflectivity is much less than for similar rainfall rates in continental area rain clouds. The later rain clouds typically consist of very large raindrops but much less in number.

The radar shows that it is raining , but it is dry where I am.

  1. The radar may sometimes detect faint echoes from non-precipitation targets such as aircraft, areas of smoke/ash from large fires, swarms of insects, flocks of birds or even the surface (when unusual atmospheric conditions bend the radar beam back down to the surface!). Permanent echoes are created when the radar beam reflects off ground features and buildings, normally within about twenty kilometres of the radar site, but mountain ranges further away can sometimes generate permanent echoes.

  2. Remember that due to the Earth's curvature, the radar beam becomes higher above the ground the further it travels from the radar. Thus a weak echo may not mean that it is raining at the ground because under some circumstances light rain aloft can evaporate completely before reaching the surface.

  3. The intensity of drizzle may be underestimated because of the lack of large droplets.

My area of interest is covered by two (or more) radars. However, the echoes from the different radars show quite different shapes and rainfall intensities.

  1. The main reasons for differences include the different distances and angles from the radar transmitters, the presence of topography and differences in the frequency and angle of the radar beam. Also the collection period of the two radars may well have been different.

The radar is showing some echoes that don't look like rain.

  1. Sometimes the refractive index of the air is such that the radar beam becomes "bent" and reflects the ground or ocean surface some distance away from the radar. This is known as Anomalous Propagation and occurs usually when there are strong temperature inversions present. Electronic processing of the returned signal usually detects the steadier reflections from ground clutter, but reflections from the waves on water are more likely to appear like true rain echoes.

  2. Near sunrise and sunset the radar antenna momentarily scans the sun. On occasions this can be seen as a pencil line radiating out from the centre of the image in the direction of the sun.