SUN, SEA, AND AIR—REVELLE 253 
that absorb infrared, in order to allow a balance between incoming 
and outgoing radiation. In the greenhouse the absorbing material 
is the glass roof. The corresponding materials in the atmosphere 
are three substances present in quite minor amounts: water vapor, 
carbon dioxide, and ozone. 
The temperatures in the upper air do not vary markedly with 
latitude and consequently the amount of back radiation is roughly 
the same all over the globe. But the amount of incoming sunlight 
is greater in the Tropics than in high latitudes. As a consequence, 
air and water warmed in the Tropics must move toward the Poles. 
Part of the energy received from the sun is thus used to carry the 
excess heat absorbed in low latitudes to high latitudes where it can 
be re-radiated. The amount of heat transported across the parallels 
of 30° is 10 to 20 percent of the total incoming radiation, but the 
mechanical work involved is less than 1 percent. 
HEAT ENGINES 
The situation can be thought of as if the sea and the atmosphere 
were interlinked heat engines of very low efficiency. These engines 
do mechanical work against friction by carrying the working fluids, 
sea water and air, from the “firebox” of the Tropics to the radiation- 
cooled “condenser” of the polar regions. The circulation of the work- 
ing fluids is manifest in the winds of the air and the currents of the 
sea. In the atmosphere, it takes place through the coupling of rotary 
current patterns of all possible shapes and sizes. These include the 
trade-wind cells of oceanwide dimensions, the large-scale high- and 
low-pressure areas of midlatitudes, the wavelike jet stream, hurri- 
canes, tornadoes, tiny whirls and vortices. In the sea, major units 
of the circulation include the Gulf Stream and the Kuroshio, the fast- 
moving equatorial currents and the sluggish currents of the abyssal 
depths. These circulation patterns are partly unstable, and show 
themselves to those of us who live in midlatitudes as the radical 
changes in weather with which we are all familiar. In low latitudes 
over the ocean the instability produces the terrifying hurricanes of 
the western Pacific and of our own east coast. 
The behavior of the interlinked heat engines of the sea and the at- 
mosphere is profoundly influenced by four facts: First is their 
peculiar shape; they are essentially two thin sheets wrapped around 
a sphere. Second, the sphere is rotating; the lower layers of air 
are dragged along by the rotation, and the movements of both the 
sea and the air are largely determined by the forces generated by 
the rotation (at a height above our heads of several hundred miles 
there is a transition to a zone where the sparse atoms of gas no longer 
move around the earth’s axis). Third, the ocean is not a continuous 
