6 Sep 2001
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How to interpret cloud radar images
If you are puzzled as to what cloud types our real-time cloud radar images
correspond to, this page should help.
Meteorological radars measure `radar reflectivity factor' which is
essentially proportional the size of the particles to the power of
six (the scale used in the images below is shown to the right). Hence
radar tends to get a much stronger echo from large particles in the
atmosphere (such as raindrops and ice crystals) than it does from
small particles (such as cloud droplets and dust). We are hoping to
supplement the 94 GHz radar images with real-time images from the
Chilbolton lidar (like a radar but using light instead of microwaves)
in the near future. Because this instrument is sensitive to small
rather than large particles, the combination of the two instruments
enables the cloud type and composition to be identified much more
The images below are all four hour sections. Note that a small signal
is always measured in the lowest few pixels because there is leakage
between the transmitter and the receiver (a feature common to all
If you have any queries or comments, please address them to Robin Hogan.
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 evapourate before they
reach the surface.
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 clouds are composed purely of ice crystals and in radar images
are characterised by their classic `fallstreak' structure.
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) the radar reflectivity at increases sharply
because the dielectric constant of liquid water is higher than that
of ice. The effects of attenuation by rain are also visible as
vertical swaths of anomalously cloud-free air above the regions of
highest rainfall rate, but this is only really a problem for
high-frequency radars such as the 94 GHz Galileo.
When the rainfall is generated by convection the rainfall rate is
much more variable. The melting level is visible at 2 km in this
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
You may have spotted occasions when the radar data looked 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). We are hoping to move to a new data acquisition system
in the next few months which should cure this problem.