Skip to Content

How do you find the average date of the first 90 degree day?

With the hot temperatures that are forecast to occur in the Southeast next week, I’ve been getting some questions about how you determine when the average date of first occurrence of 90 F is for a given location. Here is a way to do it using XMACIS2, an online data search tool from the Regional Climate Centers.  You can find it at https://xmacis.rcc-acis.org/.  It can used to make many other climate data queries as well, so try it out! And it works all over the US, too, not just in the Southeast.

1. From the main menu, pick Single Station and then First/Last Dates.

2. From the Output Selection, pick Table, Max Temp ≥ 90, period beginning Jan 1.

3. From Station Selection, pick your NWS County Warning Area (CWA). For example, in Athens it is FFC for the Atlanta NWS office at Falcon Field in Peachtree City. Then pick your station from within that CWA.

4. When you click Go, it should give you a table of first and last dates of 90 F by year with an average date calculated on the bottom. For Athens GA it is May 18, very close to today!

The complete user guide is at http://xmacis.nrcc.cornell.edu/static/xmACIS_Users_Guide.pdf. This quick fact sheet should also be useful at http://www.glenallenweather.com/upload/richmondclimate/XMACIS_Factsheet.pdf.

Source: Yinan Chen, Commons Wikimedia