Atmospheric conditions

Radar signals can be adversely impacted by poor atmospheric conditions.

Atmospheric conditionsThe radar beam normally travels in a straight line; however, certain atmospheric conditions may cause the beam to bend upwards or downwards. The effect of this condition is known as anomalous propagation.Anomalous propagation

1. Sub-refractionSub-refraction — Sub-refraction occurs under unstable atmospheric conditions and causes the radar beam to bend upwards. Sub-refraction causes the radar beam to overshoot distant targets that would have been detected in standard atmospheric conditions. Sub-refraction results in a reduced operational range of the radar.

2. Standard refraction — Standard radar beam refraction occurs under normal atmospheric conditions.

3. Super-refractionSuper-refraction — Super-refraction occurs under stable atmospheric conditions and causes the radar beam to bend downwards. Super-refraction causes the radar beam to follow the earth’s surface and improves the operational range of the radar, enabling detection of targets over the horizon.

Ducting

Ducting is a special type of super-refraction occurring when the radar beam, which is bent downwards, reflects off the earth’s surface back into the atmosphere. In this scenario, the beam is trapped by a layer of dense air which causes the beam to be reflected back to the earth’s surface. This action may occur a number of times, allowing targets to be detected over far greater distances than the radar’s usual operation range. However, these echoes may return several signals later and are shown at false ranges. Transmission 'jitter' techniques are applied to minimize these false echoes or second-time-round returns.

Examples of atmospheric conditions:

• Fog and mistFog and mist — Fog and mist may cause some signal attenuation, resulting in a small reduction in radar range.

• Dust stormsDust storms — In some locations, dust storms can produce difficult conditions, appearing similar to clutter on screen.

• Hail, Snow and IceHail, Snow and Ice — Hail and snow produce effects similar to that of rain clutter. Dense snow has a greater effect than lighter flurries which, owing to the small reflecting surface, have minimal effect.

  The echoes from ice depend on the form and shape of the ice. In general, the effects produced by various forms of ice are as follows:

  • Smooth flat ice: Most of the radar beam is reflected at the angle of incidence, providing little or no return signal. Sometimes an advantage is gained by setting up the controls to obtain sea clutter right up to the edge of the ice. Patches of water in a smooth ice field are often revealed by clutter returns when sufficient wind disturbs the surface of the water.

  • Pack ice: Strong multiple echoes are obtained from pack ice, producing a pattern on screen similar to excessive sea clutter. The ice left in the wake of a vessel passing through an ice field may be distinguished clearly on the screen.

  • Ice walls: Strong echo returns are obtained depending on the angle at which the walls are with respect to the sea surface, to scatter the reflected beam.

  • Icebergs: As the angle of iceberg faces is rarely normal to the surface of the sea, much of the reflected signal from the radar beam does not reach the antenna, providing a weak signal return. Also, the surrounding dense air produces a higher than usual atmospheric attenuation.

  • Growlers: The detection of growlers by radar is uncertain due to the small surface area above water and the mass that is submerged.