Carry out background research into your project either taking a synoptical look at all aspects of rainfall and associated precipitation or focusing more specifically on certain areas of rainfall, such as precipitation in your hometown. Show that your experiment is well-rounded and researched by using a selection of sources, including library books, articles and journals and well as articles or information from academic or government department websites, such as the U.S. Geological Survey (USGS). Remember that the reading you carry out ahead of your experiment helps you determine what your experiment will specifically study as well as writing the text that you post on a science fair display board, for example.
Draft a hypothesis --- one or two sentences explaining your prediction for your science experiment, such as you think that there will be a greater level of rainfall in your hometown in February than in March, for example. Submit a research proposal to your educator to gain their consent for your research including your hypothesis; your teacher will be able to help you improve your experiment based on your proposal and hypothesis, including assessing any health and safety issues, such as your leaving the classroom to distribute and collect rain gauges with supervision.
Fashion as many rain gauges as you require for your experiment. Make rain gauges by cutting the top 2 inches from a 2-liter bottle before adding a handful of pebbles inside the bottle and taping the part of the bottle you cut off back inside the bottle upside down --- this acts as a funnel for rain. Pour water into your bottle so the water level is flat, eliminating any discrepancies in the shape of the bottom of the bottle. Tape a ruler to the side of the bottle or use a ruler to mark inches on your bottle's side with a marker pen so the scale starts at the water level. Place your rain gauge in the chosen location or several gauges in certain locations for the same length of time before gathering bottles back in to measure the rainfall level.
Your experiment will leave you with a set of results that can be easily displayed on a graph. Use a line graph with rainfall on the vertical axis and time in one-hour increments on the horizontal axis during an experiment where you gauge rain once each hour in a set location, for example. Make the results of your rainfall experiment clear to all viewers by taking photographs of the experiment setup, including construction of rain gauges and the rain-collecting process, as well as enlarged graphs showing all your quantifiable research outcomes.