The differential heat input from solar radiation input and loss by infrared radiation is a critical part of maintaining equability (relatively low gradients in temperature from low to high latitudes) on the Earth. The energy balance figures indicate that above about 40 degrees North and South (e.g., the latitude of New York City) of the equator the loss of heat by radiation (infrared), on average, exceeds the input of heat from the sun (visible). What does that imply for our climate? One might think that this should result in permanent snow or ice above this latitude. Right? Indeed, during the last glacial epoch, about 21 thousand years ago, thick continental ice sheets extended to nearly 40 degrees North in North America (just north of I-80). But normally, because of the heat gradient created by the imbalance between solar input and infrared radiation, the atmosphere (and ocean) is set in motion to redistribute heat from low to high latitudes. Otherwise, the tropics would be excessively hot and the high latitudes excessively cold—at all times. Next, we will see how this circulation works.