In a comment on a recent post, it was asked how one goes about getting monthly precipitation values. This post will offer a tutorial on one method of obtaining precipitation data from the National Climatic Data Center (NCDC). Note that this will only work for locations in the United States.

The first step is knowing where to find NCDC on the Internet. Then you need to locate a station using the Station Locator - this is a link the left panel on their home page.

This page will look like this:

For this example, we want to find data for Kauai, Hawaii. There are two options here, the first is to select the ‘State’ button and enter ‘HI’ into the box and press ‘Search’. The other is to click the link titled ‘State/Division/County/City’.

Then select ‘Hawaii’ from the drop-down list. The click ‘Select County’. Pick Kauai and then ‘Locate Stations’.

You’ll be looking at a screen that looks something like this:

Now you need to pick which station you want data from. Precipitation values for the Hawaiian islands will be different depending on where you are. Looking at the Google Earth image, notice that the eastern half of the island is green and wet, while the western half of the island is brown and dry.

For this example I picked a station called “Moloaa”, and it’s located at the pointer ‘A’ in the image above.

After clicking on the name of the station, a page like this will appear:

Scroll down to where it says “Data Inventories”. There will be a line that looks like so

DAT INV DOC SOD -Daily Surface Data (TD3200/3210 combined)

Click on the part that says ‘DAT’. Now it’s going to ask if you want the simple options or advanced options. Use the advanced options.

This is where we get to choose which data we want from this station. Choose precipitation - the 6th line down. On the top-right, choose which dates you want - January 2000 through December 2007. You can mess with the output format if you like. I chose comma delimited without data flags. This just means that the values of precipitation will be separated by commas, and there won’t be a long of extra flags to tell me if there is something wrong with the data. Click continue.

You’ll be asked to check an inventory file to make sure they actually have data for the dates you want. Look for a column titled “# OF MONTHS (DATA ROWS) AVAILABLE”. If you see lots of zeros, you probably don’t want to submit it for processing. I see lots of 12s. Which mean they have all the data I want. Enter your email address and they will notify you when everything is processed. This small example took less than a minute.

The next screen will give links to the data file you want. Try clicking, if it doesn’t work just wait for the email.

The data file will be an ASCII file that looks something like this:

If this looks like it’s written in code, then you probably need to find someone to help you use it. This file contains the daily precipitation values for the dates of interest. Each line represents one month of data. There are two values associated with each day of the month. The first tells you which day it is, and the other the precipitation amount. If you know how to use Microsoft Excel (or other spreadsheet program) you can just tally up each of the daily precipitation values to get the monthly values. Watch out for the 99999 and -99999 values which indicate bad data. Note that the values are in need to be divided by 100 to convert them into inches.

I’ve done that and uploaded them here: Kauai Monthly Precipitation.

Finally, we can plot the data to see what it looks like.

Clearly there was a big spike in February and March of 2006. I don’t know what that was from, or even if it’s real. But it’s in the station data, I checked. That big value skews the monthly plot for those months. The other months are similar to values found on the Internet.

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