Last Updated: May 10, 2023
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In March of 1991 I started a daily record of rainfall. We were in a drought at the time and on water restrictions, so the garden was suffering somewhat!
I have recently been asked why I collect the data, in a similar manner as if the person were asking me why I was wearing a fried egg on my head. Yes, I suppose I am mad, but I am also a keen gardener, and rain, or rather, the lack thereof, affects me greatly! :-)
It is somewhat difficult to intelligently present such a large body of data, but obviously the first thing is to establish a season, since calendar years are meaningless in the Summer rainfall areas! Hence the 2000 season runs from July 1999 to June 2000, much like SARS really :-) (I guess that now needs explaining: not Severe Acute Respiratory Syndrome, but the South African Revenue Service!!)
In all the years of recording, the split at July has let me down only a few times.
It is hard to make a LOT of sense out of the data. We supposedly have a 7 or so year dry cycle and a 7 or so year wet cycle. It is difficult to conclude that from my data. The cause of the cycle is apparently the “El-niño” effect in the Pacific. It certainly has produced widespread drought, but the trouble is: South-West is where all the “weather” comes from, and this is interrupted by it.
If this pattern were to have continued, we would be in the hands of El-niño! The Tropical Complex from the North, however descends from the North, via Botswana, and has assisted us in Egoli for many years. As a direct result, the rainfall has been quite good when, for example, the northern VryStaat has been terrible. The “Usual” pattern would have catered for the Vrystaat before us!!
The ONI (Oceanic Niño Index) is published in raw data form at https://origin.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ONI_v5.php. Further discussion (focussed on USA effects) is at https://www.climate.gov/news-features/understanding-climate/climate-variability-oceanic-nino-index.
A positive value of ONI corresponds to a warming El Niño, which generally implies drought for us, and a negative ONI corresponds to a cooling La Niña, implying rain. Note that the ONI is expressed as ∘C above or below a 30-year average. (Sort of kills the "its only 2 degrees" argument…)
In above figure, it can be seen that there is at least some correlation. In 1990-2, there is a strong El Niño, producing significant drought. 1996-7 was a moderate La Niña, with very wet conditions. So the “strength” of the ONI does not necessarily correlate. 2010, of course was rather damp, in the presence of a very strong El Niño…2015 onwards is a bit of a mixed bag, with 2016 particularly uncorrelated. But 2022 is once again, damp.
Lastly, most publications state that the average rainfall in JoBurg is 500–750mm. JaNee.
Simply a plot of the annual total seasonal rainfall. Note that in 1996 we moved from Crosby to Kensington, ie west JHB to east JHB, but there shouldn’t be much of a difference :-) <— See note above, I am changing my mind :-) The average values shown do not include the effects of the current (incomplete) season, as that would artificially affect them.
Notice that in very dry years, less rain falls per day (obviously for far fewer days too) and on very wet years, more falls per day. But on other years which are not extreme, but do vary quite a lot in total rainfall, the average amount per rainy day is pretty constant!! I did not really expect the consistency.
Bottom right is the overall average.
It is difficult to present the monthly data, as there are now too many years of it, and we are running out of graph space :-)
It is to be noted that the really wet months are a tad random!! And yes, Feb 2000 was a reasonably damp month. Note too that May ’97 in the “dry” months was rather special, leading to the pretty good seasonal total, even so late in the year.
Another way of looking at this is the min/mean/max type of graph per month. I have also added the First Quartile, Median, Third Quartile. I think that the month of February shows beautifully the classic question so often asked by newcomers to statistics: “What is the difference between a mean and a median”. The difference between min/max and 1st and 3rd quartiles is even more marked. The difference is caused by a once-off 30mm in 1992, and a once-off 401.5mm in 2000.
Also of interest is a similar graph of the number of rainy days by month.
For those oddballs that really like detailed stuff, here is the data on a yearly and a monthly basis.
The cumulative graphs show very nicely “when” the season “kicks-in”. Some “good” seasons are awful to begin with, some start off with a bang, but fizzle dismally.
Note the scale changes: “Wet” and “Dry” months have different maxima on the yranges, but are consistent within those classifications for comparison purposes.
The online version is "http://ytdp.ee.wits.ac.za/rain.html"
This document was translated from LATEX by HEVEA.