Monsoon - Process

Process

Monsoons are large-scale sea breezes which occur when the temperature on land is significantly warmer or cooler than the temperature of the ocean. These temperature imbalances are caused because the oceans and land absorb heat in different ways. For oceans, the temperature remains relatively stable for two reasons: water has a relatively high heat capacity (3.9 to 4.2 J g-1 K-1)), and because both conduction and convection will equilibrate a hot or cold surface with deeper water (up to 50 meters). In contrast dirt, sand, and rocks have a lower thermal conductivities (0.19 to 0.35 J g-1 K-1)), and they can only transmit heat into the earth by conduction and not by convection. The result is that bodies of water stay at a more mild temperature, while land temperature are more variable.

During warmer months sunlight heats the surfaces of both the land and the oceans, but the land temperature rises faster. Since the land's surface is warm, the gases expand and an area of low pressure develops. Since the ocean is at a more moderate temperature, it retains a higher pressure than on land. This pressure difference causes sea breezes to blow from the ocean to land, bringing moist air. To complete the cycle, air rises to a higher altitude over land and then flows back toward the ocean. However, upon rising (and while still over land) the air cools, which decreases its ability to hold water, causing precipitation over land. This is the reason why summer monsoons cause a large amount of rain over land.

In colder month, the cycle is reversed. Since the land cools more quickly than the oceans then the air over land has higher pressure, thereby causing sea breezes at the surface which flow from land to the ocean. When humid air rises over the ocean (to complete the cycle), it begins to cool, causing precipitation over the oceans.

Most summer monsoons have a dominant westerly component and a strong tendency to ascend and produce copious amounts of rain (because of the condensation of water vapor in the rising air). The intensity and duration, however, are not uniform from year to year. Winter monsoons, by contrast, have a dominant easterly component and a strong tendency to diverge, subside and cause drought.

Similar rainfall is caused by the moist ocean air being lifted upwards by mountains, surface heating, convergence at the surface, divergence aloft, or from storm-produced outflows at the surface. However the lifting occurs, the air cools due to expansion in lower pressure, which in turn produces condensation. Even more broadly, it is now understood that in the geological past, monsoon systems likely accompanied the formation of supercontinents such as Pangaea, with their extreme continental climates.