On Tuesday, January 19th, at around 6:17 am, Phoenix Sky Harbor recorded its first measurable rain of 2021! The average first measurable rain for a year, using the 30-year average (1980-2010), is January 12th.

There have been 11 years since 1896 when measurable rain was recorded on January 1st, with the latest-ever first measurable rain in a year being June 7th set back in 1973. The second latest is May 29th set in 1937, and the third latest jumps to March 18th, recorded in 2012.

So has the first day of measurable rain in a year changed over time?

Of course, after all, you are dealing with the weather, which feeds directly into the climate. The graph below shows the first day of measurable rain (in Julian days) and the 30-year rolling average. As can be seen, the latest 30-year average was January 21st in the late 1930s, which was mainly influenced by 1937. The earliest average came in on January 10th in the early 2000s, and since that time, we have been on a slow increase, with the average (including this year) sitting at January 17th.

Something of note, there is no correlation between the first day of measurable rain in a year and the yearly rain total.