Global variations of average surface-air temperatures are largely due to latitude, continentality, ocean currents, and prevailing winds. The effect of latitude is evident in the large north-south gradients in average temperature that occur at middle and high latitudes in each winter hemisphere. These gradients are due mainly to the rapid decrease of available solar radiation but also in part to the higher surface reflectivity at high latitudes associated with snow and ice and low solar elevations. A broad area of the tropical ocean, by contrast, shows little temperature variation. Continentality is a measure of the difference between continental and marine ...(100 of 37038 words)