The images below are all four hour sections. There is always a small signal measured in the lowest few pixels of each images - this is due to leakage between the transmitter and receiver (a feature common to all radars).
Stratocumulus
This is the most common cloud type in Britain and is composed entirely of liquid water droplets. Because the droplets are very small (around 10-20 microns in diameter), they tend to give a fairly low radar signal.
Stratocumulus with drizzle
If stratocumulus is more than a few hundred metres thick, then larger `drizzle' drops (around 100-200 microns in diameter) can grow. These give a larger return to the radar, but usually evaporate before they reach the surface.
Altocumulus
This mid-level cloud occurs below freezing and is typically composed of both ice crystals and supercooled liquid water droplets. The radar signal is usually dominated by the larger ice crystals, which tend to be several hundred microns across.
Cirrus
Cirrus clouds are composed purely of ice crystals and in radar images are characterised by their classic `fallstreak' structure.
Rain
Frontal or `stratiform' rain looks like this. Ice crystals nucleate high in the atmosphere and grow as they fall. At the melting level (2.4 km in this example), where the falling ice becomes rain, the radar reflectivity increases sharply. This shows up as a sudden colour change at 2.4 km. The reflectivity changes at this point because the dielectric constant of liquid water is higher than that of ice. The rain also attenuates or fades the measuring radar signal. Vertical swaths of air above the regions of highest rainfall appear to be cloud free - these areas are not really empty of cloud, they just appear to be because the radar signal has been faded as it travels through the intense rain. This is only really a problem for high-frequency radars such as the 94 GHz Galileo.
Showers
When rainfall is generated by convection the rainfall rate is much more variable. The melting level is visible at 2 km in this example.
Insects
On hot summer days insects are often visible in the lowest 1 or 2 km. They can often be seen carried up from the surface in convective plumes, and in this example have formed a layer at around 800 metres (lidar observations confirmed that no cloud was present on this occasion). In the tropics insects are much more of a problem for cloud radar.
Glitched data
Very occasionally you may spot occasions when the radar data looks like this - it's not a real cloud at 9 km, but occasionally the radar detector gets out of synch with the transmitter, and the transmit pulse appears at 9 km (as well as cirrus echos `folding' and appearing at low levels).
