# How Do We Measure Rainfall? · Frontiers for Young Minds

rain is one of the most shared experiences on earth, and I bet you witness it regularly. sometimes you’re ready with the right clothes and sometimes you’re not! the rain is sometimes long and light, sometimes short and heavy, and sometimes long and heavy. when the rains are long and heavy, they can cause rapid flooding, which is dangerous for the nearby population. rain is also necessary as it provides water for plants and eventually fills rivers. Because rain is both a necessary resource and a threat, it is important to better understand this natural phenomenon.

It is very likely that you have already noticed that rain is variable in time. when you stay in the same place, it doesn’t rain there all the time. Even during a rain event, the strength of the rain can constantly change from very light to very heavy. the heaviest periods of rain are usually quite short. This type of variability in precipitation is also visible on a larger scale, because as you know, there are wetter and drier months or years. there is also variability in where the rains occur. it can rain a lot in one place and not at all, or with a very different intensity, a few kilometers or even a few hundred meters away.

Reading: What does rain gauge measure

Variability is a basic characteristic of precipitation that makes its measurement complex. Meteorologists (people who study the weather) and researchers have developed numerous measuring devices that allow them to study the extreme variability of precipitation. We will explain the operation of the three most used devices. The data we present was collected on the campus of Ecole des Ponts Paristech, where I work.

## how do we measure the amount of rain that falls?

The most common measure of rainfall is the total depth of rainfall during a given period, expressed in millimeters (mm). For example, we might want to know how many millimeters of rain fell over the course of 1 hour, 1 day, 1 month, or 1 year.

You can easily get a rough measure of the depth of the rain at home. just follow these steps: (1) take a bottle with smooth sides, cut off the top and turn it over at the top of the bottle, to create a kind of funnel (see figure 1a). (2) Stick a ruler on the side of the bottle and fill the bottle with water to the zero mark on the ruler, which should be above the bumps on the bottom of the bottle. otherwise, shocks will affect the measurement. (3) Take your rain gauge outside, as far away from buildings and trees as possible. (4) Regularly record the water level (for example, every morning at 8:00 a.m. before going to school) to collect your own data. If you plan to take your measurements during the summer, some of the water inside the bottle will evaporate (up to a few mm per day) and this will affect your measurements. to avoid this, you can add a thin layer of oil to the water. Since it is lighter than water, the oil will float on top of the water and prevent evaporation. The measurements you get from your rain gauge will tell you how much rain has occurred during a certain period of time.

• figure 1
• a. a homemade rain gauge. b. a professional tipping bucket rain gauge. c. an example of tipping bucket rain gauge data, showing how much rain (in mm, y axis) fell over time (x -axis) on June 27, 2017 on the campus of Ecole des Ponts Paristech. the time corresponds to the clock time of that day. the fastest increase, corresponding to the heaviest rain, occurred between 1:00 p.m. and 2:00 p.m.
• Professionals use more complicated devices called Tipping Bucket Rain Gauges and you can see one in Figure 1b. This rain gauge looks like a homemade device, except there are two buckets under the funnel. the water that falls into the rain gauge will be directed to a bucket by the funnel. once the bucket is full, usually after 0.2mm of rain has fallen, it is designed to tip over automatically, meaning the other bucket will now be under the funnel. the process starts all over again with this other bucket, until it fills up and tips over. The rain gauge records the timing of all tips of the bucket, which will give the researcher data on how fast the rain falls over time. Figure 1c shows an example of the data that can be obtained using a tipping bucket rain gauge. These observations were made on June 27, 2017. The depth of rain (in mm) increased rapidly between 13:00 and 14:00, which means that it rained a lot during that period. during a period of light rain, this device is not very accurate. for example, between 05:15 and 13:00, all you can say is that 0.2 mm of rain fell (the tip of a bucket), but you don’t know exactly when that rain fell. if there is a lot of wind, that can also affect the accuracy of the device.

## how do we measure the size of raindrops?

What is rain made of? raindrops, obviously! rain gauges are not sensitive enough to be able to take measurements of individual raindrops. To start collecting data on droplets and their size, you need a device called a disdrometer.

Before I describe a “real” dysdrometer, here’s how you can make one at home (see ref. [1] for a more detailed description). follow these steps: (1) take a plate and put a few millimeters of flour on top. (2) when it rains, go outside with the plate covered, uncover it for a few seconds so that a few drops fall on it and form small craters, and then go back inside. (3) parse the result.

You will see something similar to what is shown in figure 2a, and you will notice that not all the drops are the same size, some are very small and some are very large! The craters are actually larger than the droplets because the water spreads out a bit after hitting the plate, but they still allow you to directly visualize the wide range of droplet sizes, even for a very short time.

• figure 2
• a. measurements obtained with a homemade dysdrometer, made of flour on a plate. b. an optical dysdrometer. It consists of an emitter that generates a sheet of light towards the receiver. when a drop falls through the device, the receiver is shadowed (see ref. [1, 2] for a scientific paper using this device). c. disdrometer data showing the number of drops, according to droplet size classes, measured during a rain event that occurred on June 27, 2017 on the campus of Ecole des Ponts Paristech. on the x axis, you can see the size of the diameter classes and on the y axis, the number of drops for that class. As you can see, the width of the various classes is not always the same. they are smaller for the small droplets that are more numerous d. Rain rate in mm/h (y axis) over time in h (x axis), during the same rain event shown in c . this graph shows that there were three peaks of rain during that day.
• As you can imagine, meteorologists and researchers wanted a more automatic and precise device than the flour plate. now they mainly use optical disdrometers, which work as shown in figure 2b. This type of dysdrometer consists of two parts: a transmitter and a receiver. the emitter generates a sheet of light a few mm high. the receiver is aligned with the emitter, so when it’s not raining, the receiver receives all the light. however, when a drop passes through the sheet of light, guess what happens? the amount of light received is less, because part of it is blocked by the drop. if the drop falls very quickly, the duration of the decrease in received light will be short. this is how the downward velocity (velocity) of the falling droplet is estimated. if the dip is large, the signal measured by the receiver will decrease more than with a smaller dip. this is how the droplet size is estimated. In this way, the size and speed of each drop that passes between the emitter and the receiver are measured.

raindrops can be 5 to 6 mm in size. the largest drops break apart during their fall. in fact, at this size they are not strong enough to resist the power of the wind they feel when they fall rapidly. the speed at which drops fall increases with their size: 1mm drops fall at 3m/s while 5mm (very large) drops fall at 8m/s. Figure 2C shows the number of droplets of each size that fell during a storm that occurred on June 27, 2017 in the Paris area. small droplets are much more numerous than large ones. but don’t forget that a 1mm drop has a volume 125 times smaller than a 5mm drop! this means that, although not numerous, large drops account for much of the depth of the rain. let us now consider successive time steps of 30 s. Then, by adding the volume of all the drops that passed through the disdrometer during a 30-s time step, you can estimate the amount of rain depth that fell during each 30-s time step. this estimate will give you the rainfall rate, and is usually expressed in mm/hr. Rain rate gives you an idea of ​​the strength of the rain. the rain rate corresponds to the depth of rain that would accumulate during 1 h, if the rain rate remained constant during this hour (which actually never happens in real life). Figure 2d shows the rainfall rate (in mm/h), with 30-s time steps, during the same event on June 27, 2017. The strong variability in the rainfall rate can be easily seen in the graph.

## how do we make rain maps?

So far, we have only discussed devices that can provide rainfall measurements at a precise location. Both the pluviometers and the disdrometers only give an idea of ​​the rain that fell on them, but not on the surrounding areas or 20 km away. to create rain maps, which are maps showing the amount of rain that has fallen in a given period of time (for example, 5 min or 1 h) in multiple locations, we need to rely on weather radar .

The operation of the weather radar is summarized in Figure 3a. First, the radar equipment transmits an electromagnetic wave in one direction, which transfers some energy through the atmosphere. when this energy reaches a drop of water in a cloud, a small part of that energy is sent back to the radar equipment. the team then measures this tiny amount of energy received from all the droplets. using special computer programs, it is possible to convert the amount of energy received into the amount of rain. It is important to remember that a radar does not directly measure the amount of rain, but instead measures the amount of energy returned by the drops. this conversion of energy into amount of rain turns out to be complicated and people are still researching to improve it [3]. for example, droplet size distribution and droplet location within a radar pixel are currently assumed to be homogeneous. it is an oversimplification of reality that can affect measurements [4]. the computer program allows the radar equipment to estimate the amount of rain in places far from it. the radar equipment can rotate and can also change its angle, so it can estimate the rate of rain in its entire environment.

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