After examining the year’s worth of temperature and rainfall data from our home weather station, we get to the fun stuff: wind. To visualise wind data, we use a construct called a wind rose. You can think of it like a compass, with north at the top and south at the bottom. We use the compass to display two pieces of information about the wind: its speed, and the direction from which it blew.
We represent the speed of the wind by a colour scale. In these wind roses, the strongest winds are dark blue to dark grey, and the lightest wind is yellow and pale green. The direction that the wind came from is plotted directly onto the compass: the more the wind blew from a particular compass heading, the longer the segment on the chart. The compass is divided into twelve segments of thirty degrees each (12 x 30 = 360 degrees).
In the wind rose above, you can see that the single most common direction from which the wind blew at our house between December 2013 and November 2014 is from headings between 75 and 105 degrees: easterly. The next most common wind direction was from headings between 285 and 315 degrees: a north wester. Next most common were south easterly winds (105-135 degrees), and then south south easterly winds (135-165 degrees). This summarises the wind regime in Cape Town: south easterly winds in summer, and north westerlies in winter. We can blame the South Atlantic high pressure for this.
If I split the wind data by season, using 15 May as the start of winter, 15 August as the start of spring, 15 November as the start of summer, and 15 February as the start of autumn, we get the following picture:
It is clear from the seasonal division of the data that the clearest wind regimes occur during the summer (east to south south easterly winds) and winter (north westerly winds, some of which are warm berg winds preceding cold fronts). During spring and autumn, we get wind from all directions, but a lot of easterly winds in both those seasons. Unfortunately an easterly wind makes diving on the western side of False Bay very unpleasant – those are the days we stay at home and work in the pool!
One thing that’s important to keep in mind when looking at these charts is that the wind differs quite a lot at a given point in time, all around Cape Town. Both direction and strength can be modified by mountains, of which we have many, and data collected in Sea Point, Newlands or Kommetjie over the same time period would possibly look quite different. We’ve figured out what it means for False Bay when our weather station registers a particular wind strength or direction, but these readings aren’t exactly what you’d get if you went out in the bay. The wind on the bay is stronger than it is at our house during summer (no mountains to obstruct the south easter), and less forceful in winter (it has to blow over the mountains to get to False Bay).
We’ll refer back to these weather station posts in another series of posts I want to do when I get a chance, about understanding False Bay’s weather patterns and their effects on the sea state. Stay tuned.
