Discover Primary Science and Maths

Measuring Rainfall

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Calculating rainfall at weekends

Most schools leave their rain gauge out on a Friday afternoon and take a measurement on Monday morning. This measurement is the total rainfall for the weekend. It is important to enter this measurement on your wall chart on Monday so it is included when calculating your weekly rainfall.

As an exercise in class you can also work out an average rainfall for each wet day over the weekend. For example, if it only rained on Friday and Sunday, divide the amount of rainfall you measure on Monday morning by two to calculate an average. If it rained on all three days divide the amount of rainfall you measure on Monday morning by three to calculate an average.

Measuring rainfall is one of the most important jobs in Meteorology (the science of the weather). Every day, tens of thousands of weather observers all over the world go to their rain gauges and empty their collection containers into a graduated cylinder to measure how much rain has fallen - just like you can do for this year's Greenwave experiment.

Materials

N.B. For this experiment to be of scientific vaue, you must use the following equipment:

  • 2 litre empty bottle (will become your funnel)
  • 10ml plastic syringe (available from your local chemist) OR 100ml graduated cylinder
  • Metre stick
  • Thread
  • Container to collect the rain (i.e.

Directions

  • Measure 34cm of thread on your metre stick
  • Place the 34cm of thread around the 2lt bottle, cut the top of the bottle at this point, this part will now become your funnel. The funnel should look something like that shown here.
  • Place the funnel into your collecting container and set it up outside in a place where it won't tip over, and where it is not overshadowed. Ideally it should be more than 5 metres from the nearest building / tree or structure. Make sure it is positioned where it cannot be knocked over accidentally by dogs or cats (or children). If you like, place the container in a plastic bucket and pack sand/mud around it.
  • At 9.30am every day, use the plastic syringe or graduated cylinder to measure the amount of rain that has collected in the container. When you are finished measuring, you can throw out the water so that the container is empty and ready to collect rain over the next 24 hours!
  • For the purpose of comparison, you will need to convert this to millimetres by dividing the figure by 9.2. Please round the result to two decimal places. Log your daily readings on your wall chart to see how the rain changes from day to day, and at the end of each week calculate and fill in the total rainfall for the week.