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Home » Cartography, Environmental Geography, North America, Northern California

Short GeoCurrents Break, But First a Seemingly Impossible Rainfall Map

Submitted by on July 29, 2015 – 7:03 pm 2 Comments |  
(Note to readers: GeoCurrents will soon be taking a short summer break. Regular posting will resume in mid-August. But before the pause begins, I have one more post, which discusses the possibility of a seemingly impossible map. )

California July 2015 RainfallThe map posted to the left appears to be bogus, as it depicts patterns that would seemingly not be found in nature. It shows precipitation over a specific 30-day period (from late June to late July, 2015) in California, mapped as a percentage of average precipitation, recorded over many years, during the same one-month span. Over a large swath of the central part of the state, colored dark red, rainfall is shown as having been extremely meager, less than five percent of what would normally be expected. Yet just to the northwest, northeast, and south of this region, large areas are depicted as having been extremely wet during this same period, with more than 600 percent of average rainfall. (This map, and the next three maps in this post, are found here.)

At first glance, such a pattern might appear highly unlikely but not actually impossible. What seems truly absurd is rather the lack of intermediate gradations. Along the “A-B” line that I have inserted only one intermediate color is encountered between the dark red of “less than five percent” and the hot pink of “more than 600 percent.” Such mapping seems to contradict the continuous nature of natural phenomena such as rainfall, and therefore appears impossible. Would not one have to pass through all of the color categories on the map when moving from a place with the highest value to a place with the lowest value? Yet as it so happens, that is not the case in this instance.

The most basic problem here is one of scale. Intermediate categories are present but cover areas too small to appear on the map. They would be present, in other words, if the map had been made at a much larger scale. Such a map, however, would be much too large to depict all of California on a single computer screen.

More interesting issues are encountered when we consider the specific features of California’s climate along with the unusual weather that the state experienced in mid July 2015 that led to such an bizarre map.

California July Rainfall 2015 Map 3Over the lowlands and hills of central and southern California, July precipitation is essentially nil, as can be seen on the next map. Only over the highest mountains does average rainfall in this period reach 0.5 inches (12 mm). In the lowlands, most years see no rainfall during this time of the year. But every decade or so a rare rain event occurs, generating average rainfall totals of a couple hundredths of an inch. In Sacramento, for example, July precipitation averages 0.04 inches, but in most years the actual figure is absolutely nothing. As a result, the dark-red areas on the map that appear abnormally dry during this period actually experienced typical conditions.

California’s extreme summer drought can make an area that received a California July 2015 Rainfall Map 2meager shower seem quite wet on a map such as this. The location near Sacramento labeled “C “is shown as receiving more than 200 percent of average rainfall on the fist map, but as the next map shows, all that it got was around 0.1 of an inch (2.5 mm).

The most striking feature of the initial map is the vast area that received more than 600 percent of average precipitation. Due to the hyper-aridity of California’s summer, some of these areas received only about a quarter of an inch. Other places, however, got more than two inches (50 mm), and one small area recorded over five inches (127 mm), staggeringly high figures for mid-summer in California. Most of this rain resulted from an unusual influx of moisture in the middle of the month from the so-called Arizona monsoon. The rainfall associated with this event pushed northward about halfway up the state and then abruptly terminated, giving rise to the extraordinarily sharp gradient along the “A-B” line in the first map. (In northwestern California, the relatively high precipitation figures for this period also reflect the unusual passage of an upper-level low-pressure center earlier in the month.)

California 2015 Rainfall MapThe rain that did fall in California this summer did very little to alleviate the state’s on-going, multi-year drought. As can be seen in the map posted to the left, almost all parts of the state are well below average for the period extending back to October 1, 2014. Oddly, however, there are a few specks of green and even of blue on this map, indicating higher than average rainfall. Palo Alto, where I live, is on the edge of one of these small zones, as it received unusually heavy rain during two December 2014 storms.

Climate Prediction MapDrought-weary Californians are hoping that the El Niño conditions that have emerged over the eastern Pacific presage a wet winter. Climate prediction maps, however, indicate a good chance of a heavier than normal precipitation only in the southern two-thirds of the state. California’s vast water delivery system, however, relies mostly on precipitation that falls in the northern third of the state.



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  • We are having a second rain this week in Santa Clara county, and a thunderstorm at that…

  • Martin this post interests me, as that is the sort of conundrum that launched me into GIS in the first place – I had aeromagnetic geophysical maps of the US and CDN mid-continent that I tried to use to extend my geological work in the Great Lakes region in the mid-eighties. I couldn’t match the scales cross-boundary, and even had I been able to, the data made little sense in the geological context (gravity & aeromagnetic data help delineate ancient continents in the Precambrian crust). So I started using GIS as a means to collect, calibrate and coordinate data… which I’ve been doing for 30 yrs now OMG

    But along the way I learned a few things, which I think stymie your study here. First is a question of scale, your ‘seemingly nonsensical’ data most likely reflect the way it’s posted, as you say you need larger scale maps to depict details not visible at smaller scales… zooming in&out of data needs digital data not just map sheets as you point out. Secondly you’re posting time-sequence data, and that’s the knottiest problem we have found in GIS… to the effect that Esri is just getting a handle on it 45 yrs after _they_ started! Thirdly is understanding the science behind the data – you’re a linguist if I recall correctly and climate data are not for the faint of heart – although it sometimes takes a fresh pair of eyes to see something ‘old hands’ may have missed – I for ex. post shipping data that turned out to be the best if not only climate data offshore 150-350 yrs ago, but while it got the Met Office and ACRE’s attention , I leave climate modelling to them and stick to compiling and presenting complex &/or vast datasets.

    My aim is to show, and then perhaps convince, experts that a few simple yet generic data base and computer mapping workflows go a long way to help elucidate geodata relationships, in ways that may have proven hardly possible by other means. You’re in the best place to show exactly that with ethno-linguistic data, and I love you maps, but I see so much more to be done if the subject experts, be they academia or professional, cooperated with the computer mapping and data basing community.