The artefact that paved way for accurate rainfall detection

A weather radar is one of the technologies that changed the way we observe rainy systems. In simple words, radar is an instrument that transmits energy in the form of a wave. The wave travels through some distance until it comes in contact with an object which reflects the wave back towards the source. By measuring the time taken by the wave to hit the object and return to its source, one can infer the distance of the object from the detector. In the early days, this technology was widely used to detect and destroy enemy planes during war-time.

It is impossible to make anything fool-proof. There will always be some glitches in the technology that limits its applications. Radar technology for the detection of enemy's planes is no exception to this. Whenever it rained, the radar sent an alert suggesting that some objects are detected within the scan range. This lead to a series of false-alarms to an extent that the army stopped altogether to rely on radar signal during rainy conditions.

Marshall was the first to recognise that the artefacts that obscure the detection of war-planes and ships comes from rainfall and snow and the same can be used to quantify rainfall. In other words, the radar which was being used for the detection of warplanes, ships, and other objects has great potential to monitor and observe the weather.

Arthur Bent, a researcher working from the Massachusetts Institute of Technology conducted a detailed study on the possibility of using radar for the detection of rainfall. Bent in his published work [1] explained how radar echoes that are observed can be used in the detection of localised and general precipitation. This was perhaps one of the earliest work that explains the fundamental operation of weather radar.

[1] Arthur E Bent, 1946, Radar Detection of Precipitation, Journal of Meteorology, Vol. 3, pp. 78-84.

How many drops make the rain?

"Little drops of water... make the mighty ocean", wrote Julia Carney. We have a name for those "little drops of water". It is called rain or raindrops. The question that intrigued many great minds for ages is what is the size of the raindrop? And when it rains, how many drops are there in it?

In 1948, John Stewart Marshall and his doctoral student Walter Palmer performed a fundamental experiment to make the measurement of raindrops [1] for correlation with radar echoes. There wasn't any sophistication involved in their work. The duo collected raindrops on dyed filter papers which they then used to measure the size and get the number distribution of rain. 

Marshall and Palmer published their findings as very short communication. The paper, which was only one and a quarter page in length, became a game-changer. More than 70 years have passed. We have developed many new and innovative technologies to measure the raindrop size more accurately. But the distribution developed by Marshall and Palmer in 1948 with only the basic tools from their time is still being used for the estimation of rainfall with remarkable accuracy!

[1] J.S. Marshall and W. McK. Palmer, 1948, The distribution of raindrops with size, Journal of Meteorology, Vol. 5, pp.165-166.