Key points of post
Warmest month in Penang is April
Wettest month in Penang is October
The warm season is now much longer than it used to be
Living in Penang, a tropical island off the northwest coast of peninsular Malaysia it often feels as if the weather is pretty much the same all year round; ie. hot, sweaty and sticky. Not true, however.
The data Jason and I
have been analyzing suggest a different story with both air temperature and
rainfall cycling substantially each year.
The box and whisker plots
below show monthly rainfall and air temperatures at Bayan Weather Station,
Penang Airport, between 1935 and 2013.
Clearly one can expect a deluge at any time of the year. Such events, however, are most likely in
September and October and least likely in January and February. Air temperatures, perhaps unsurprisingly, are
coolest around the time when rainfall is highest, and hottest when rainfall is
lowest
There’s a clear
trade-off for the visitor. If you like
it hot, come between February and May, but if you prefer cooler temperatures
come between September and November when you should expect some heavy
downpours.
These box and whisker
plots, however, best summarize the average
air temperatures and precipitation over the years 1934 to 2013 and must be
interpreted with some caution. The boxes
themselves, together with the whiskers, do give some indication of variability. In some years, for example, temperatures can
be higher in October which is typically the coolest month than temperatures in April,
often the hottest month. The problem with the box and whisker plots is that
there is no way to tell which years were unusually hot (or cool or wet).
A good way to explore
how seasonality changes is to plot the time-series data (monthly averages, see
our February 20 2014 post) three-dimensionally, scaling the variable of
interest (e.g. temperature) to different colors. We have done this in the plot
below for monthly average air temperatures between 1934 and 2013. Year is along the x-axis and month along the
y-axis. The graph shows the relatively
recent expansion (2000 to 2013) of higher temperatures throughout the year. In
the 1950s, 1960s, 1970s, and early 1980s the hottest period seemed to be between
February and May but now it extends much further into June, July, and August.
Hence one is much less
likely, nowadays, to experience the relatively cool temperatures (around 26°C between July and
December) that visitors in the past would have enjoyed.
The R code used to produce the box and whisker plots as well as the three-dimension temperature plot depicting the seasonality in Penang are shown below. The code continues from the one used in the previous post describing the monthly average temperatures in Penang.