Time of rain events each month in 2014

When it rains…

When does rain fall in Cape Town? What time of day is it safe to leave home without an umbrella, during which months? Am I right in thinking it always pours with rain between 6.00 and 7.00 in the morning when I have to think about getting up?

We haven’t been collecting our weather station data for long enough, but I can tell you what time of day it most often rained during 2014, in each month. I showed you a slice of one of these circular charts last month, to visualise what the wind speed is at different times of day, by month.

Here’s a full circular chart showing the frequency of rain events (i.e. rainfall in a particular hour) in 2014, throughout the day, divided by month. February, April, October and December 2014 were particularly dry months – there are hardly any blue stripes on those segments. The darkest blue stripe you can see, in May, tells us that during the hour from 4pm to 5pm in the month of May, there were six or more days on which it rained (“rain events”).

Time of rain events each month in 2014
Time of rain events each month in 2014

What can we conclude from this circular plot? Looking at the rainier months (May to August), a pattern of rain during the night (outside edge of the circle) can be discerned, as well as in the early hours of the morning (near the centre of the circle). The morning – specially in May-July – seems to be a time when the rain is less frequent during this time.

Once we have a few more years of data, these circular plots will take on more conviction as we are able to smooth out annual fluctuations in rainfall and see broader patterns. March 2014 looks surprisingly (blue) wet, or April was surprisingly (pale) dry. Another year’s data will allow us to figure out which was the case. Watch this space.

A note on the graph

This chart is after Jason and Doug’s chart on their Penang weather blog. The piece of pretty R that did the plotting is here, if that floats your boat:

ggplot(df_summarised, aes(x=month, y=hour, fill=events)) +
  geom_tile(colour="grey70") +
  scale_fill_gradientn(colours = col, name="Rain\nevents\n")+
  scale_y_continuous(breaks = seq(0,23),
                     labels=c("12.00am","1:00am","2:00am","3:00am","4:00am","5:00am","6:00am","7:00am","8:00am","9:00am","10:00am","11:00am","12:00pm",
                              "1:00pm","2:00pm","3:00pm","4:00pm","5:00pm","6:00pm","7:00pm","8:00pm","9:00pm","10:00pm","11:00pm")) +
  coord_polar(theta="x") +
  ylab("HOUR OF DAY")+
  xlab("")+
  ggtitle("Number of rain events by month and time of day")+
  theme(panel.background=element_blank(),
        axis.title.y=element_text(size=10,hjust=0.75,colour="grey20"),
        axis.title.x=element_text(size=7,colour="grey20"),
        panel.grid=element_blank(),
        axis.ticks=element_blank(),
        axis.text.y=element_text(size=5,colour="grey20"),
        axis.text.x=element_text(size=10,colour="grey20",face="bold"),
        plot.title = element_text(lineheight=1.2, face="bold",size = 14, colour = "grey20"),
        plot.margin = unit(c(-0.25,0.1,-1,0.25), "in"),
        legend.key.width=unit(c(0.2,0.2),"in"))

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Clare

Lapsed mathematician, creator of order, formulator of hypotheses. Lover of the ocean, being outdoors, the bush, reading, photography, travelling (especially in Africa) and road trips.

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