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Raining On Your Data Parade

Science Snack
Raining On Your Data Parade
What do you do if something looks odd in your data?
Raining On Your Data Parade
What do you do if something looks odd in your data?

Sometimes when you’re looking at data, you come across something surprising—a measurement that’s so unusual, you wonder if it’s correct. Take a look at some actual data from a rain gauge, then investigate what is likely to be accurate, and why.

Tools and Materials

None needed.

To Do and Notice

Examine the graph labeled December 2014 Precipitation from the Exploratorium’s Environmental Field Station. What is being measured? Where were the measurements taken? What time period do they cover? What patterns do you see in the data? Do you notice anything unusual about any of the measurements?

The graph shows that more than 20 inches (51 cm) of rain fell in one day, on December 31, 2014. Does this seem likely?

What do scientists do when they find data that seem unusual? Ideally, they repeat the experiment, but that’s not possible in this case, because rainfall is a natural event. However, we can look at other local data to help verify or refute the data from Pier 15.

Look at the NOAA California Climate Station Precipitation Summary and find the average annual precipitation for San Francisco (the far right-hand column).

Look at the NOAA Monthly Precipitation Averages for Bay Area Cities and find the average monthly precipitation data for “San Francisco City.”

The National Weather Service collects precipitation data at a weather station not far from the Exploratorium, so we can compare their data to ours. Look at the information you obtained from the NOAA National Weather Service Forecast Center website. What do you notice? Which measurement for December 31 seems more likely—more than 20 inches (51 cm) in one day or 0 inches (0 cm) in one day?

What's Going On?

In evaluating environmental data, scientists compare their findings with other data and use common sense to help determine whether or not their data are likely to be accurate. San Francisco’s average annual precipitation is 23.65 inches (60 cm)—that’s how much rain falls in one year, on average. On December 31, 2014 the National Weather Service recorded 0 inches (0 cm) of rain. They also recorded 11.7 inches (30 cm) of rain for the entire month of December, 2014. San Francisco gets an average of 3.2 inches (8 cm) of rain in December. So the Exploratorium’s measurement of more than 20 inches (51 cm) of rain in one day seems extremely unlikely.

After comparing our data with National Weather Service rain measurements for the same day, we decided that the National Weather Service data were more likely to be correct.

We also tried to guess what might have caused our rain gauge to record such a high measurement on December 31, 2014. The rain gauge is located on our roof, and is very difficult to access, so it’s unlikely that someone dumped a bucket of water into it. What do you think might have happened?


The Community Collaborative Rain, Hail & Snow Network is a national network of volunteers measuring precipitation.