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Watching the weather
Watching the weather

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3.2 Precipitation radar

The water droplets from which clouds are formed are typically in the range of a thousandth of a millimetre to a tenth of a millimetre in diameter and are widely dispersed. Raindrops on the other hand are much larger, typically 1-2 mm across. Regions of the atmosphere through which rain is falling contain markedly more water than an equivalent volume of cloud. As a consequence, a region of falling rain presents a more significant obstacle than a cloud to anything passing through it.

The word 'radar' is an acronym for 'RAdio Detection And Ranging'. Radar works by sending out a pulse of electromagnetic radiation and timing how long it takes for a portion of the signal to return as a result of reflections from objects encountered along the path. The delay time between the main pulse and its echo determines the distance between the target object and the radar station. The delay time is three-millionths of a second per kilometre and, although that sounds like a short time, it is easily possible to determine the distance between the emitting source and the reflecting object to within one metre.

The strength of a radar echo depends on two factors - how far away the target is and what it is made of. Strong radar echoes are created by dense, nearby objects whereas echoes from very small remote objects are easily 'lost in the noise'. If you were spotting distant aircraft by radar, you might try turning up the receiver only to be frustrated by finding that faint signals from distant objects get lost in what is known technically as 'clutter'. This is the radar equivalent of the hiss you hear on audio systems. Among the various contributions to clutter is rainfall.

Most developed countries operate radar networks to detect precipitation. By comparing the strength of an echo with the delay time (which effectively gives the distance) precipitation radar will provide estimates of the rates of rainfall; the larger the raindrops detected, the stronger the return signal. This is especially valuable in warning of imminent flooding as the radar images give a clear indication of the intensity of the rain.

A single radar station can be expected to detect the presence of rain within a radius of 100 km or so. The network is planned to give appropriate coverage with overlapping areas. A complete scan of the skies around each station takes a few minutes, so typically data are updated every 10-30 minutes. The precipitation radar coverage of the stations operated over part of the USA is shown in Figure 14.

Figure 14 The composite radar reflection image from a large network of weather radar stations across some of the central states of the USA at 1148 UTC on 2 May 2008. The colours show the intensity of the reflected radar echo, with red indicating a much stronger reflected signal than blue, and white showing regions of no reflection. The reflected intensities are related to the density of water drops in each cubic metre of air; when this density is large the reflected intensity is also large. Typically, the bright green colour corresponds to regions of light rain. The red band of extremely heavy rainfall is prominent and occurs along the line of a cold front, with yellow areas indicating more moderate rain further north and west (i.e. behind the surface edge of the front). The front reaches the southern tip of Texas, where it is much weaker and there is only light rain.

The curvature of the Earth over the range of precipitation radar is significant: because a radar beam is straight, the beam effectively climbs away from the Earth's surface with increasing distance from the radar station. The geometry is illustrated in Figure 15. At a distance of 50 km the radar simply cannot 'see' lower than about 0.5 km and by 200 km from the radar station the minimum height for detection by radar is almost 5 km. At 200 km range, a great deal of rain will therefore go undetected below the beam.

Figure 15 Owing to the curvature of the Earth, radar cannot detect low-level precipitation at long distances. The curvature of the Earth is greatly exaggerated for clarity.

Determining the lateral speed of movement of a patch of rain can be done in two ways. The simplest is to track the rainfall and to compare its position at successive times, a few minutes apart. An even more immediate measure can be obtained from technology that works like the radar speed traps designed to identify speeding motorists. In this kind of device, the exact time difference between the reflections of successive radar pulses is measured electronically, and from this the speed of the reflecting object can be calculated. Thus rainfall radar gives a means of assessing wind speed at the cloud base, from the ground, without the use of radiosondes. Better still, thanks to the ingenuity of the instrument engineers, meteorologists can pick out radar reflections from patches of turbulence and dust in the air, so air movements can be 'read' from the ground even when it isn't raining! For this reason, radar is especially valuable for detecting.