Learn How to Measure Rainfall
24 Comments


Weather is a really important topic for many
many people. Most of us want to know if it’s going to rain for example, like it is today
or if it’s going to be a hot sunny day, we need to decide whether we need to take an
umbrella like this, or wear a hat or wear warm clothes. So knowing what weather we are
receiving and predicting the future weather for the next day is really important for lots
of people. For farmers it’s particularly important because they are growing crops, for the rest
of us to buy vegetable and fruit and grains and that sort of thing. So they need to know
very accurately how much rain has fallen and whether they will need to irrigate their crops
and that sort of thing. And for people living in towns and cities like most of us do, we
need to know if there are going to be floods, we need to know if the wind is going to get
strong like it is just this minute, whether there is any danger to us, whether we should
be careful because there may be flooded roads that sort of thing, or maybe even our houses
can get flooded. So knowing the amount rain is one of the components of weather that we
need to be able to measure. Now when we’re measuring rain you might think straight away,
well the units we are going to use are units of volume. Because rain is a liquid and we
measure liquids generally using units like “millilitres & litres” because that’s how
we measure liquids. But when it comes to the rain we are not going to measure the volume
simply because there’s a lot of rain falling all over the ground. So I’m standing under
one umbrella, there’s a certain amount of rain falling on the umbrella, but there’s
a lot more rain landing all around me in other areas and to measure the volume we would have
to know how big an area we are measuring the rain over. So instead of that we measure how
deep it is. So just like you might stand in water in a swimming pool and work out where
the water comes to on your body and think whether it’s deep or shallow. We want to do
the same sort of thing with rain and we want to measure how deep the rain is on the
ground. Now of course because most land is not flat, we can’t just put a ruler on the
ground and see where the water comes to, because the water runs away downhill. Just like it
is where I’m standing on my drive way, the rain is running downhill continuously. So
even though we’re getting lots and lots of rain today, it won’t stay here where I’m standing.
So to measure it we need to capture the rain and then measure the depth. So, I’ll just
hold this up, this is an example of what we call a “rain gauge”. So this is for measuring
rainfall, it’s the shape of a cylinder and the rain falls in the top and is collected
inside, and then we can measure inside the cylinder how deep the water is. Now there’s
another part to this, and that is, measuring depths in millimetres is quite difficult because
they’re very small measures and if we want to be truly accurate, it’s going to be difficult
to do that with just a standard ruler in a cylinder. So the makers of rain gauges like
this one have come up with a clever solution -they have a funnel. So if you look at that
you can see it’s wider at the top and there’s a small hole in the bottom. So there’s a nice
big area to catch the rain and then it’s funneled thru the funnel into a small tube inside the
big tube. And so the small tube captures the rain but you can tell that the rain will come
further up the tube than it would if it was just landing in a normal straight cylinder
and that’s because the opening on the funnel is much wider. And then if we get lots of
rain like we are today it goes over that measurement. It will over flow the small tube and go into
the big tube. So I’m going to go and put this back out in the weather -out in the rain,
so I can capture the rain for today and by tomorrow morning I can come out and I will
measure using the markings on the tube how much rain we have received today.

24 thoughts on “Learn How to Measure Rainfall

  1. are the diameters of the tube and the funnel of standard sizes? there must be differences in measurement if their diameters can vary.

  2. So you are not measuring how much comes down per hour or per second, but you are finding out the total amount of rainfall during the entire time it rained?

  3. What's the standard measurement? Is it the volume of rain collected per unit surface area? Different diameter tubes can give different measurements for the depth of the rain (mm or inch) collected. So what is the standard used?

  4. Hello Prof. I am just wondering on how the Meteorology department collect their rainfall data over a big area. Did they take an average or did they just pick one place that has the most rainfall? Your answer is greatly appreciated.

  5. Firstly thanks for the video but i had the exact same question that someone else had in the comment section
    you've said in the comment that the outer funnel has 10x the Area of the small internal cylinder; Is this some kind of standard?
    Because the way i see it having a larger funnel will give a larger reading and since the funnel choice seems arbitrary this could lead to different measurements.

    Thanks a lot Again

  6. Hi Pete, nice Rain video..I liked it ! ( and subscribed to it )
    I've designed and built my own Rain Gauge that measures rainfall by weight, not volume.
    YOu can see it here:
    https://www.youtube.com/watch?v=oseP7alIuAU&t=233s
    And while you're there, Please Subscribe ( I subbed your page )
    That way we can all get to the top, right?
    thanks
    Savanna

  7. So your PHD was in useless information. Why is it so hard to say how much rain is falling in the video as you speak??? Even an approximation is far far more useful than some water in a beaker A picture is worth a thousand words.

  8. What is the different between using a barrel and and cup to measure rainfall. Do you use certain size of cylinder or any size?

  9. SIR; FOR HOW MUCH TIME THAT (CYLINDER LIKE) RAIN GAUGE INSTRUMENT MUST BE KEPT IN THE RAIN FOR MEASUREMENT ?

  10. THE MORE WE KEEP THE RAIN GAUGE IN THE RAIN THE MORE WOULD BE THE RAIN COLLECTED IN THE CYLINDER ; SO IT WOULD BE TIME DEPENDENT ;

  11. I'm trying to understand how the reading is accurate. If the funnel is 5 inches across but the tube inside is 2 inches across the inches is going to be higher. Why use the tube inside. Just measure the rain in the tube.

  12. Hi Sir,
    Thanks for the video . I was thinking for this for long..Much indeed.
    We live in Kerala/India, and in Kerala we are being badly affected these days with hevy rain..
    Many of our cities,villages are flooded.
    Please do pray for us..

    My doubt is, what diameter can we chose for a cylinder, diameter increases depth decreases na ?
    What is the standard diameter size
    Thanks
    Binoj Daniel

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