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Precipitation

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Ask the residents of Southport NC about Hurricane Floyd in September 1999 and they won't tell you about wind. They will tell you about nearly 25 inches (64 centimeters) of rainfall that fell in just over two days! Same for Hurricane Harvey in Houston in 2017. Could this be a sign of things to come?

Warm air holds more moisture than cooler air. What does a moister atmosphere mean for precipitation? It means two things — more precipitation over the globe and a higher frequency of extreme precipitation events. The spatial pattern of precipitation is complex — far more so than for temperature — and measuring the frequency of extreme events is a challenging statistical problem. But some progress has been made on these questions, at least for certain regions of the globe.

First, let's take a quick look at the precipitation pattern of the globe, which is now being measured in great detail by NASA's Aquarius satellite.

Map of rainfall averages in terms of mm/day for the month of August, 2012
Rainfall averages in terms of mm/day for the month of August 2012.
Credit: NASA
Map of rainfall anomaly in terms of mm/day for the month of August, 2012
Rainfall anomaly for the month of August 2012
Credit: NASA

The two images above show the rainfall averages in terms of mm/day for the month of August 2012, above, and the rainfall anomaly for the same month, below.

Earlier satellites did not have the same resolution, but the record goes back to the late 1970s, allowing us to get a picture of the longer-term mean precipitation patterns, which can be seen in the video below. Watch this movie a few times through to see the annual patterns of precipitation, and focus especially on the region around India, where the summer monsoons show up beautifully.

Video: Average Long Term Monthly Rate of Precipitation (00:03) This video is not narrated.

This movie shows the mean precipitation rate for each month of the year over the time period from 1981 to 2010.
Data from NASA, movie created by David Bice.

On a global scale, there is no clear sign that the amount of precipitation is increasing, as can be seen in this next figure, which plots the global average precipitation rate for each month.

Graph of globally averaged precipitation history from satellites
Monthly average precipitation rates, averaged over the globe from 1979 to 2012.
Credit: David Bice

The figure above shows the history of the monthly average precipitation rates, averaged over the globe — kind of a difficult thing to swallow at first. Think of the values plotted along the y-axis as being the precipitation rate, averaged over the whole globe for a given month. As you can perhaps see, there is a strong seasonal cycle (with peaks in the fall each year) — I've removed the seasonal variation from the raw data to give the thicker blue line, showing variability that is not related to the seasonal cycle. One thing that is clear is that there is no general upward or downward trend over this period, although there are ups and downs, some of which correspond to the El Niño oscillations.

So, global precipitation, on the whole, is not going up, as far as the data reveal. What this means is that although the atmosphere is getting moister, the new addition of moisture is not coming out as precipitation — the atmosphere is retaining this extra water vapor.

But because the trend suggests there will be extra water vapor in the atmosphere in the future, when the conditions are right for a big precipitation event, the event might be bigger than today or in the past. In addition, predictions suggest there might be more storm events that exceed a certain threshold so that they are classified as extreme precipitation events.

More regionally, the El Nino cycle produces a dramatic change in precipitation patterns in parts of the globe with Southeastern Australia becoming dry in summer and more prone to bushfires. In the US, El Nino corresponds to heavy winter rainfall in California as the so-called "Pineapple Express" picks up and transports heat and moisture from the tropical Pacific. Folks in California are desperately hoping that the currently building El Nino will put an end to the drought. More on that in Module 8. El Nino is generally not a good time to visit Florida in the winter as the same pattern extends across the southern US.

Graphic illustrations of El Nino and La Nina impact on precipitation in U.S.
Impact of El Nino-La Nina on US Precipitation
Credit: Wikipedia