On a larger scale, such as a continent surrounded by oceans, heat build up on land over time will result in lower density air masses, or areas of low pressure. Conversely, denser air associated with high pressure dominates ocean surfaces. Wind and ocean currents that result from air flowing from high to low pressure mix areas of warmer and colder air and water, contributing to the global energy balance. This exchange is evident at different levels of the atmosphere. Air converging into a low pressure center at the surface rises, leading to moisture condensation and the subsequent release of heat into the upper atmosphere. Diverging air at the surface in a high pressure center is associated with subsiding air from the upper atmosphere and evaporation, a mechanism for energy storage.
Just as energy imbalances develop between land and water surfaces, the variation in space and time of solar heating due to the earth's tilt create seasonal hemispheric energy imbalances. The hemisphere receiving the most direct radiation (during the summer months) experiences a net radiative heating (more energy is gained from the sun than is lost to space). The winter hemisphere is at the same time experiencing net radiative cooling. As part of a global compensation, heat is transported from warmer to cooler areas by ocean and wind currents. Since the areas of heat surplus and deficit change throughout the year, as in the sea breeze example, the direction of transport must change as well. Figure 3 shows low latitude surface wind directions averaged over the summer and winter seasons. As noted earlier, climates dominated by monsoons experience the most pronounced seasonal wind shifts, indicative of a pronounced land-sea effect. In the South Asian example, the rainy season, typically beginning in June, is preceded by nearly two months of scorching temperatures, cooled only with the commencement of the summer rains brought by the southwesterlies. January is the peak of the dry season, which is marked by cool, dry northeasterly flow over most of the region.
Nice explanation of why it rains in through the cracks of my windows.
