Exploring Changes in Rainfall in Palo Alto Over the Past 35 Years

Insurance companies, farmers and development agencies need highly accurate information to identify climate trends in specific areas. In this post I use the Planet OS API to explore over thirty years of historical precipitation in Palo Alto, California.

Did you know that at least 30 years of high-quality data is required to determine an environmental trend? Accurately identifying precipitation trends is extremely important for industries exposed to flood and drought risk, such as agriculture and insurance. Unfortunately there are very few datasets that provide the consistency and temporal coverage required for these analyses.

I am excited to share that this week we added one of them to the Planet OS Datahub, Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS). CHIRPS is a 30+ year quasi-global rainfall dataset that enables comparisons between current rainfall patterns and historical averages, providing very accurate results.

Even though the data is freely accessible to the public, working with large-scale datasets like CHIRPS can be difficult and time-consuming. That is what motivated us to build the Planet OS Datahub, which provides an easy to use API for accessing CHIRPS and many other environmental datasets.

CHIRPS data is widely used by the insurance and agriculture industries as well as development agencies for drought and flood prediction. Meteorologist Eric Holthaus described the importance of CHIRPS in his article for Ensia:

“Nothing like CHIRPS has existed before. The new data set has about 100 times the detail as previous attempts to merge satellite and ground-level precipitation data and provides unprecedented coverage across years and continents.”

Below is an example that uses CHIRPS data to quickly evaluate historical precipitation in Palo Alto, California. Palo Alto has a Mediterranean climate with cool, relatively wet winters and warm, dry summers. However, because the city is located next to the Santa Cruz mountains, which blocks the passage of rain-producing weather systems, there is a rain shadow in Palo Alto that causes very low average annual rainfall.

Exploring Changes in Rainfall in Palo Alto Since 1981

Here are some of the things I discovered using CHIRPS data in Palo Alto:

  • 90.3% of the days since 1981 have been completely dry in Palo Alto.
  • 1982–1983 and 1997–1998, the strongest El Niño years, were also the wettest for Palo Alto.
  • Curiously, during another strong wave of El Niño in 2015–2016, it rained significantly less. It’s almost like El Niño had the opposite effect.

To learn more about these findings, as well as how to use the Planet OS API to work with CHIRPS data, check out my historical rainfall analysis Jupyter Notebook on GitHub.


The Planet OS Datahub provides free access to high-quality earth science data, and is routinely updated with new datasets. If you’d like to be notified when new data becomes available, follow Planet OS on Medium or subscribe to our email newsletter to receive future updates in your inbox.