Perceptions playing tricks..

Measurements vs Perceptions

Climate is one of the areas that most strikingly illustrates that our perceptions can play tricks on us. I took one example, that may resonate with those somewhat familiar with South African geography.

Consider the following cities: Bloemfontein, Cape Town, Durban and Johannesburg.Few that claim to know a little about these cities would disagree that Bloem is known for biting cold winters (relative to the others), Joburg for generally a pleasant climate, Durban for a climate supporting beach holidays (if a bit uncomfortable in summer), and Cape Town for long wet winters.

I just took a few selected “public facts”, of course each of these cities have other peculiarities that they are known for (Joburg thunder storms, Cape Town wind etc).

Now some questions

  • Which one of these for cities is the driest (lowest average annual rainfall)?
  • Only one of these cities fail to have any month in which the average temperature exceeds 20°C. Which one?
  • Which of these cities has the highest temperature in the middle of summer?
  • Which one of these cities has the least amount of annual sunshine?
  • The answers

    • The answer to the first question is Cape Town. The city known for it’s long rainy winters (which is very uncharacteristically for Africa), is also the driest city in the group.
    • The second question’s answer is Johannesburg. The city that is known for a pleasant human friendly climate fails to reach an average of 20°C, even in January, it’s warmest month. Many people consider 20°C the hallmark of summer.
    • As for the third question, Bloemfontein has the highest maximum temperatures in summer.
    • The answer number four shows is that sunny Durban is the cloudiest city in the group.

    Assessment and Explaination

    So how did you do? Perhaps you deliberately made counter intuitive guesses because you recognised where this post was going.

    The thing is, we confuse measures. We confuse likelyhood of rainy weather in daytime with amount of rain, we confuse warmth with sunshine. We assume that places that are cold in winter would have cooler summers. We confuse rain and cloud cover. We confuse dreariness with cold and rain, and heat-humidity discomfort with high temperatures.


    It depends what we want to measure. Our perception of Cape Town is on the money if we assess necessity for umbrellas (due to the nature of the rainfall), but if it is provision for storm water draineage, than the info conveyed by our perception is wrong.

    Our perception of heat discomfort is a function of a few things, of which temperature is but one. An important one, but not the only one. So for an operation that require us to make a decision based on temperature alone, it would be wrong to infer that from heat discomfort.

    Our perception of sunshine is based on a few things, including the nature and seasonality of cloud cover, and the effect on cold induced discomfort.

    If we want to determine a site to suitable for solar power, we should also know that amount of sunshine (which is roughly the amount of hours when a shadow can be distinguished on the ground) is not the same as the amount of solar radiation a place receives. Not all sunshine hours are equal, let alone all cloudy hours.


    I’ve used a subject which we can all relate to. In Business many of the same lessons, albeit it with different concepts apply. In order to make optimal decisions, we must know the difference between Revenue and Margin, Sales and Quantity.

    We must also be honest with ourselves as to

    • which measures are useless
    • which combination of measures do not show a purpose being shown alongside each other, even if they are valuable in their own right (that is another topic).
    • what the context is (what does it mean when we say Durban is not particularly sunny)

    when are we busy with costly hair splitting (also another topic)

    I am not discounting perceptions as having no role in decision making. The words “perceptions”, “stereotyping” and “generalization”, in spite of their negative press, are valuable tools we use in our minds to reduce noise in data, to weigh the role of factors, for sanity testing of systems that claim sophistication, and to use in in the absence of the availability proper quantification. What is more is that we can sharpen and hone those skills if we frequently calibrate them with well presented facts. You would be right to pack warm clothes for your winter trip to Bloem, and your umbrella for Cape Town, but wrong to assume that Cape Town receives higher volumes of rain than Durban (about twice that of Cape Town), Johannesburg (about 1.5x times that of Cape Town) or Bloemfontein (about the same as Cape Town and London, UK).

    Another point illustrated here is that these cities (that are reasonable choices for evenly spaced cultural / economic centres, though one could argue for cities like Port Elizabeth to be included) are not representative to say much about South Africa’s climate. For example, a Martian may not find it worth mentioning that South Africa receives any winter rain at all, due to the small area to which it applies, so he may dispose the advice that in general, South Africa does not receive any winter rain. But that approach would ignore the fact that a significant cultural and tourist centre, Cape Town, is situated slap bang in the heart of that area.

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