How does a weather radar work?

Without weather radar, it would not be possible to predict rainfall! Thanks to the data and images from the radar maps, your national meteorological service can anticipate what the rainfall will be in the next few hours… or days (with less reliability). So how does a weather radar work? How can it be used to forecast rainfall?

Weather radar tracks rain and snow

Also known as precipitation radar, weather radar is one of the main measuring devices for national meteorological services and other meteorological organisations.

Displaying real-time precipitation on a map of an entire territory, the weather radar is formidable for rain and snow forecasts.
A monostatic radar (the most common weather radar) consists of a transmitter, from which a radio wave is emitted. This wave is then fed to the antenna employing a waveguide. The duplexer is responsible for directing the wave to the antenna when transmitting or the return signal from the antenna to the receiver when receiving.
As you have seen, there is only one antenna on the weather radar and it has two functions: firstly, it scatters the electromagnetic wave through the atmosphere, and secondly, it picks up the same wave backscattered by the target (raindrops or snow, more precisely hydrometeors). The radar must be carefully calibrated to withstand the megawatt power that the receiver receives.

When the receiver receives the incoming signal, it is its job to process it. It is the processing of the raw signal by the weather radar that provides the data required by the operator: precipitation mapping and monitoring, extraction of meteorological or oceanographic parameters, rain or snow forecasts, etc.
It is the electronic system of the weather radar that controls all this directly and displays this data to the users. Turning a wave into digestible rain and snow forecast information is not easy at first sight!

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Weather radar function for rain and snow forecasts

Before we go any further and begin to understand how weather radar can predict precipitation, let's look at the meaning of radar: "RAdio Detection And Ranging"
It is therefore a question of sending and receiving waves. But what else?

Let's get into detail: how does a weather radar work?
Unlike a rain gauge, a weather radar does not calculate or predict the level of precipitation in water measured in mm/h. They derive radar reflectivity from the backscatter power measurement.
Radar measurement works according to a well-known physical principle: the reflection of electromagnetic waves by certain objects, the targets (in this case, the hydrometeors of precipitation). In the case of weather radar, the radar measures the power that is backscattered by a set of raindrops, and in this way, it can determine exactly where the precipitation is and how it is moving.

Then, to measure the intensity of the precipitation (because the weather forecast announces this data, it must be obtained at some point), it must be deduced using a reflectivity-intensity conversion, according to the Z-R law. Beware, this is getting technical! Who said that weather radar was simple to operate?
We could go into much more detail about how weather radars work, but we'll leave it at that for now. Unless you are a meteorologist, you are unlikely to need more information… and if you are, you already know exactly how a weather radar works!

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How is data from weather radars used?

If you have the following available on your smartphone, on your weather station. If you have rain or snow forecasts on your smartphone, TV, or elsewhere, you are most likely using a weather radar without knowing it. It is very difficult to forecast precipitation other than with this device and it is often directly from your national meteorological services that the information comes.

Knowing the rain forecast or just the weather forecast, in general, makes it easier to do your gardening, organise special hikes or know what to wear this afternoon. Just take a look at your connected devices or an interactive precipitation map on the internet: updated every 15 minutes, they are also available in the form of pages you may know: "Will it rain within the hour?" is a good example.

Knowing the risk of precipitation is very useful and sometimes necessary in some areas. A weather radar is also used to better understand thunderstorms and other storms: analysing them is the key to limiting their impact. This is also one of the major missions of national meteorological services.

The measurement made by these radars is the reflectivity, i.e. the proportion of the wave emitted by the radar that is backscattered by the hydrometeors. To simplify this principle of operation, you can just note that the waves emitted by the weather radars hit the raindrops and are reflected to the radar. This is how their position and forecasts are established.