Climate can be thought of as long-term average weather. In Boston, any given day in March may be warm or cold, dry or rainy, or even snowy. Records compiled over decades, however, show that, on average, March days in Boston vary from about 42°F (5°C) at the beginning of the month to 48°F (9°C) at its end. March brings an average of 4.3 inches (110 mm) of precipitation, at least some of it typically snow. Montreal is colder and snowier in March; Phoenix much warmer and drier. What controls this broadly predictable distribution of average weather across the planet? Climate in Boston, and around the world, is determined by solar radiation, global patterns of wind and ocean circulation, and Earth’s varying topography, as we discuss in the next section.