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Ocean Surface Topography from Space
The Spherical Shape of the Earth
Because the Earth is a sphere, the surface gets much more intense sunlight, hence heat, at the equator than at the poles. On the equinox, the Sun passes directly overhead at noon on the equator and a square centimeter of ground receives about 1 calorie of heat energy (see solar constant). On the same day, at 60°N, the latitude of Anchorage, Alaska, or Oslo, Norway, or St. Petersburg, Russia, the Sun rises no higher than 30° above the horizon at noon and heats a given parcel of ground with only a half the intensity as at the equator. At the poles, the Sun appears to sit on the horizon for periods upwards of 24 hours, and its rays skim horizontally over the surface.

The 23.5° tilt of the axis means a substantial annual shift of sun angle. The June sun in San Francisco (or Wichita or Norfolk) is as high as the March sun in Guatemala City, while the December sun in San Francisco (or Wichita or Norfolk) is no higher than the March sun in Anchorage, Alaska. During the course of a year, a temperate-zone location can be said to enjoy tropical heat in the summer and to suffer arctic cold in winter.

The distribution of heat around the globe, and through the year, coupled with the physical properties of air, produce a distinctive pattern of climatic zones.

Climatic Zones

6 global circulation cells
The Sun heats the ground or ocean surface most intensely in the tropical zone. The heated air rises, and as it rises, it cools, and as it cools it dumps its moisture as rain. This belt of converging air masses, called the doldrums due to low air and water circulation sometimes causing ships to struggle to escape the region, includes some of the rainiest areas on Earth. The cooled, now drier air is forced by continuously rising air to move out of the way, and so it moves towards the temperate latitudes.

Such air from the tropics meets air moving down from the poles at about 30° N and S, called the horse latitudes, where it settles. Here the sinking air compresses, warms, and absorbs moisture from the surface. This is why the desert belts lie in the horse latitudes. This warm, dry air is displaced by more sinking air and so some of it returns back to the equatorial zone, and some returns to the poles. Such cycling air between low and mid-latitudes defines a Hadley Cell.

Hadley Cell
A similar cell forms between the horse latitudes and the stormy polar fronts at 60° N and 60° S, where warm temperate air moving polewards meets very cold air rolling down from the pole. The lighter warm air is forced to rise over the denser cold air, which chills it and forces precipitation. From this polar front, air returns both equatorwards and polewards. Air immediately over the pole sinks. While it is not warm, it is extremely dry (only centimeters of snow every year). From the poles, air within the polar cap streams back towards the polar front.

Thus, six belt-like Hadley Cells circulate air from pole to pole and establish patterns of climate over the planet. The cells are also characterized by specific patterns of wind flow, a function of the Coriolis force generated by the spin of the Earth. In the temperate zone between the horse latitudes and the polar front, the prevailing westerlies dominate air circulation. In the tropics, the easterly trade winds dominate. Winds around the poles are also easterly.

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