RADAR stands for RAdio Detecting And Ranging and as indicated by the name, it is based on the use of radio waves. Radars send out electromagnetic waves similar to wireless computer networks and mobile phones. The signals are sent out as short pulses which may be reflected by objects in their path, in part reflecting back to the radar. When these pulses intercept precipitation, part of the energy is scattered back to the radar. This concept is similar to hearing an echo. For example, when you shout into a well, the sound waves of your shout reflect off the water and back up to you. In that same way, the pulse reflects off precipitation and sends a signal back to the radar. From this information the radar is able to tell where the precipitation is occurring and how much precipitation exists.
Radars in their basic form have four main components:
- A transmitter, which creates the energy pulse.
- A transmit/receive switch that tells the antenna when to transmit and when to receive the pulses.
- An antenna to send these pulses out into the atmosphere and receive the reflected pulse back.
- A receiver, which detects, amplifies and transforms the received signals into video format.
The received signals are displayed on a display system.
Radar output generally comes in two forms: reflectivity and velocity. Reflectivity is a measure of how much precipitation exists in a particular area. Velocity is a measure of the speed and direction of the precipitation toward or away from the radar. Most radars can measure reflectivity but you need a Doppler radar to measure velocity.
The physics behind radar has its roots in wave theory. The German Heinrich Hertz discovered the behaviour of radio waves in 1887. He showed that the invisible electromagnetic waves radiated by suitable electrical circuits travel with the speed of light, and that they are reflected in a similar way. In the following decades these properties were used to determine the height of the reflecting layers in the upper atmosphere. This is why data received from the radar is called reflectivity.
Some 40 years earlier in 1842 the Austrian physicist Christian Doppler had discovered what is now called the Doppler effect. This is the theory that sound waves will change in pitch when there is a shift in the frequency. An example of this would be an ambulance siren, which has a higher pitch when it is approaching, but a lower pitch if it is travelling away. With Doppler's theory you can calculate how fast the ambulance is moving based on the shift in the siren's frequency. This theory is used by Doppler weather radar to determine the speed of precipitation in the atmosphere, toward or away from the radar. Since precipitation as it falls generally moves with the wind, you can determine the wind velocity with Doppler technology.
Although already invented, radar was further developed during World War II, with work on the technology stimulated by the threat of air attacks. Radar had many uses during the war - it was used for locating enemy ships and aircraft, to direct gunfire, and to aid ship and aircraft navigation.
Though the military continues to use radar, the technology was released to the public after World War II and was quickly used by many other industries. Radars are now used to help navigate ships in fog and airplanes in bad weather. Radar can detect a speeding car and track a satellite. Most importantly for meteorologists, radars can detect all sorts of atmospheric phenomena.
Weather radar images are generally a map view of reflected particles for a specified area surrounding the radar. Depending on the intensity of the precipitation, different colours will appear on the map. Each colour on the radar display will correspond to a different level of energy pulse reflected from precipitation.
The strength of the pulse returned to the radar depends on the size of the particles, how many particles there are, what state they are in (solid-hail, liquid-rain) and what shape they are. After making many assumptions about these factors and others, the approximate rain rate at the ground can be estimated. In fact, the most reflective precipitation particles in the atmosphere are large and usually have a liquid surface (water-coated hailstones).
Radar images will not always accurately reflect what is occurring in the atmosphere and not everything you see on the radar will be precipitation. For example, the radar sometimes detects precipitation that occurs higher in the atmosphere but doesn't reach the ground. That's why the radar may appear to show rain when rain isn't occurring. This is called virga.
If the radar is close to the coast and the beam is broad enough, it may reflect off the sea and return strong reflectivity that is really just sea "clutter". At some wavelengths the radar beam is not fully reflected when passing through very heavy rain or hail, thus reducing or obscuring the echo intensity further out from the radar. 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. This is considered "ground clutter" and can also be produced by buildings and trees. Occasionally birds, planes, ships and even a dense enough swarm of insects can be detected by weather radar. This is even more common with Doppler radars due to their higher sensitivity.
As you move further away from the radar, the returned echo becomes weaker. This occurs because as the radar beam broadens with distance, the proportion of the beam that is filled with rain lessens and reduces the echo intensity. The radar beam is also 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. For example, 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. Yet rain that is 200 kilometres away from the radar will be detected at a height of 3 kilometres.
The Japanese squadron that bombed Pearl harbour was detected by a prototype Hawaiian Radar before the air raid but no alert was sent out, as nobody believed the inexperienced radar operators!
Bats have a Doppler radar, of sorts. Their noses are able to send out a short 'cry' which reflects off objects in the distance and sends back an echo received by their ears. From this the bat is able to tell if an animal is in its vicinity and if that animal is moving towards or away from it.
Some wind changes can be seen on the radar as very thin slow moving lines. This is because insects usually congregate around wind changes and if there are enough of them, the radar beam will be reflected. Similarly when a swarm of bats take off at dusk they can sometimes be tracked on the radar.