This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
A monsoon is, at its simplest, a wind system that reverses direction with the seasons, and the reversal is driven by the fact that land and ocean do not hold onto heat the same way. Land heats up and cools down quickly; the ocean beside it heats up and cools down slowly. In summer, a large continental landmass grows far warmer than the ocean next to it, the warm air over the land rises, and a region of low atmospheric pressure forms in its place, drawing in the cooler, moisture-laden air sitting over the ocean to replace it. That inflowing ocean air is what produces a monsoon's defining feature, weeks or months of heavy, reliable rainfall, as the moisture is lifted, cools, condenses and falls. In winter, the pattern inverts: the landmass cools faster than the ocean, a region of high pressure builds over the now-colder land, and the wind reverses to blow from land to sea, carrying dry continental air outward and largely ending the rains until the next summer.
The best studied version of this system, the South and Southeast Asian monsoon, is intensified by the sheer scale of the geography involved: the Himalayan plateau heats to an unusually high summer temperature for its size, strengthening the low-pressure pull, while cross-equatorial wind patterns, most notably what meteorologists call the Somali jet, funnel enormous volumes of moisture-laden air off the Indian Ocean directly into the subcontinent each June. The system's timing is not perfectly fixed from year to year; onset and withdrawal dates can vary by weeks, and one of the strongest known modulators of that variability is El Nino-Southern Oscillation, itself a phenomenon of shifting ocean temperatures and trade winds in a different ocean basin entirely, the tropical Pacific. An El Nino year tends to weaken and delay the Indian monsoon, sometimes substantially, because the same large-scale shift in tropical atmospheric circulation that defines El Nino also disrupts the pressure gradients the monsoon depends on, one of the clearest examples of two ocean basins thousands of miles apart behaving as parts of one connected system rather than as independent local weather.