Rain as an unwanted echo
Radar was developed in the 1930s as a way to detect aircraft. Britain, the United States, Germany, the Soviet Union, Japan and other countries all worked on versions of it, and the British Chain Home stations that watched for approaching aircraft are among the best known. The basic idea is the same for all of them: transmit a pulse of radio waves, wait for echoes from objects in the path, and measure the delay to find distance.
Operators soon noticed that echoes did not come only from aircraft and ships. Rain, snow and hail also reflected the signal, producing patches of unwanted clutter on the screen that could hide real targets. For military users this was an annoyance. Some operators and scientists realized, however, that the patches told them something about the weather. The details of who noticed first are not well documented, and various wartime reports in several countries describe the effect. That is why naming a single first weather radar is a difficult exercise. The best that can be said is that the use of radar to see precipitation emerged in the early 1940s from wartime systems, with similar observations made in several places. The basic technology of wartime radar detection is described in our article on radar detection.
The effect of wavelength
A key technical point is that the strength of the echo from raindrops depends strongly on the radar’s wavelength. Radars using shorter wavelengths, in the centimeter range, produce stronger echoes from raindrops than the longer-wavelength sets of the early war. The development of the cavity magnetron in Britain in 1940, discussed in our article on the radar magnetron, made compact centimeter-wavelength radars practical, and these were widely used by the Allies. The same advance that improved detection of aircraft and submarines also made rainfall easier to see.
This is also why high-frequency radars attenuate in heavy rain: some of the energy is absorbed or scattered, so a strong storm can hide the weather behind it. Radar meteorologists later chose wavelengths for different purposes, depending on whether the aim was to see farther or to see small drops.
Postwar research
When the war ended, large numbers of radar sets and trained operators became available. Meteorologists in several countries started studying what these sets showed.
- In Canada, researchers at McGill University, in a group led by J. Stewart Marshall, studied the connection between radar echoes and rainfall, and later work by Marshall and Walter Palmer on the size of raindrops became well known in the field.
- In the United States, the Thunderstorm Project of 1946 and 1947 used radar and aircraft to study how storm cells develop and decay.
- In Britain and elsewhere, researchers studied rainfall measurement and storm structure.
These studies showed that radar could estimate where rain was falling and how heavy it was, although the conversion from echo strength to rainfall depended on assumptions that could produce large errors. That limitation remains true of radar rainfall estimates today, which are usually checked against rain gauges.
Building networks
From the late 1940s, national weather services began installing radars for forecasting. In the United States, adapted military sets were first used and then replaced by purpose-built designs. A widely used model was the WSR-57, introduced in 1957, which became the backbone of a network of radars along the coasts and across the country. It was used extensively for hurricane tracking, and thunderstorm warnings improved as the networks grew.
Other countries built their own networks, and radar maps showing rain became a familiar feature of weather broadcasts. These networks also supplemented the forecasting methods discussed in our article on forecasting instruments.
The Doppler step
The next major advance was Doppler radar, which measures the frequency shift of echoes caused by the motion of raindrops toward or away from the radar. This gives information about wind inside a storm, including rotation, which can indicate a tornado is forming. Research in the 1960s and 1970s in the United States, including at the National Severe Storms Laboratory, led to a national network of Doppler radars, installed in the 1990s and known as NEXRAD. Many countries followed with their own Doppler networks.
Later upgrades added dual-polarization, which sends pulses of two orientations and helps distinguish rain from hail, snow and non-weather echoes such as birds and insects. These upgrades are still sometimes described as improvements to the same basic wartime idea.
Legacy
Weather radar shares its roots with other civilian uses of radar technology, discussed in our article on civilian radar. It filled a gap that neither ground stations nor satellites could, by showing precipitation in detail and in near real time. It also helped prepare the ground for later satellite observations, which complement radar by watching the whole globe. Forecasts remain uncertain, and radar is best seen as a tool for monitoring storms rather than a guarantee of what will happen next.