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The patterns of temperatures around the Earth include familiar trends of hot tropical conditions near the equator, and chilly boreal conditions to the far north and south. Empirical measurements of temperature of course support this general trend, and traveling northward for 550 km from a starting point of 35° latitude reduces the average annual temperature by 2.5 °C when 40° latitude is reached (Figure 2.12). But why do temperatures follow this pattern? The air at any given latitude does not sense its location, so latitude is a covariate with temperature, not a direct driver. A more causal explanation can be plotted with temperature in response to the annual amount of incoming solar energy. The statistical fit for the data is the same, whether we use latitude or solar radiation on the X axis. This is because the amount of solar radiation relates strongly to latitude because of the geometry of a round planet with a tilted axis moving around the sun. The key difference between these two “explanations” of average temperatures is that one provides a good predictive answer, and the other provides both a predictive answer and a good explanation for why.

We can be very confident in the general trend of temperature in relation to latitude or incoming solar energy, as the 95% confidence intervals around the average trends are relatively tight. The actual temperatures for some locations fall substantially outside this confidence band, because the confidence band relates to the average trend, not to the dispersion of sites around the trend. What might explain why one site falls above the average trend, and another falls below? Temperatures also tend to be colder at higher elevations, and the third graph in Figure 2.12 shows that adding information on elevation can improve the prediction of temperature compared to latitude alone. Other factors are important too, including distances from oceans (which tend to moderate temperatures) and mountains (which limit the ability of oceans to affect temperatures).

FIGURE 2.12 Average annual temperatures for sites around the world decline with increasing latitude (distance N or S from the Equator (top left). Latitude is a good predictor of temperature, but this is only a correlation not a process‐based explanation. The annual amount of incoming solar radiation (top right) provides a strong explanation, and has the benefit of relating directly to a process influencing temperature. The variation of individual sites around the general trend can explained in part by considering other factors, such as elevation (bottom).