The word "monsoon" comes from the Arabic mawsim, meaning "season." In meteorological terms, it refers to the seasonal reversal of wind patterns that drives massive shifts in precipitation — turning dry regions wet and wet regions dry on a predictable annual cycle. Monsoons are not just heavy rain; they are an entire atmospheric circulation pattern that shapes the lives, agriculture, and economies of billions of people.

How Monsoons Work

The driver of monsoon circulation is differential heating between land and ocean. As summer approaches in the Northern Hemisphere, the land heats up much faster than the adjacent ocean. This creates a large low-pressure zone over the heated land mass, and the cooler, moister air over the ocean is drawn in to fill it. As this moist ocean air rises over the land, it cools, and the moisture condenses and falls as rain.

In winter, the process reverses: the land cools faster than the ocean, creating high pressure over land that drives winds outward toward the sea — the dry monsoon, or northeast monsoon.

The Indian Subcontinent: The World's Most Consequential Monsoon

The South Asian monsoon — affecting India, Bangladesh, Pakistan, Nepal, Sri Lanka, and Myanmar — is the most impactful monsoon system on Earth. It sustains agriculture for approximately 1.5 billion people. The monsoon typically arrives on the southwest coast of India (Kerala) around June 1st each year, then sweeps northward across the subcontinent over the following weeks, reaching Northern India and Pakistan by late June to mid-July.

Mumbai receives its heaviest rainfall during July and August (600–700mm per month), while Cherrapunji in Meghalaya, India — one of the wettest places on Earth — receives over 2,500mm in July alone. The 2024 monsoon season brought record rainfall to several Indian states.

Southeast Asia: Two Monsoon Systems

Southeast Asia experiences two monsoon seasons driven by different wind patterns:

  • Southwest Monsoon (May–October): Brings the "wet season" to most of mainland Southeast Asia — Thailand, Vietnam, Cambodia, Myanmar, and peninsular Malaysia. Bangkok's wet season peaks July–September with over 200mm of rain per month.
  • Northeast Monsoon (November–March): Brings heavy rain to eastern-facing coasts like Vietnam's central coast and the eastern side of peninsular Malaysia (including Kuala Lumpur's wettest season).

Singapore, sitting almost on the equator, experiences both monsoon seasons and generally has no true dry season — rainfall is distributed throughout the year with slightly wetter periods during each monsoon transition.

East Asia: The Mei-Yu Front

China, Japan, and South Korea experience a monsoon-like system called the Mei-Yu (China) or Baiu (Japan) — a persistent rain band that forms as tropical and polar air masses collide in late spring and early summer. The Mei-Yu front typically brings weeks of persistent rain to the Yangtze River valley, eastern China, and Japan's Honshu island during June and early July.

West Africa: The Sahel Monsoon

The West African monsoon brings most of the annual rainfall to the Sahel region — countries including Senegal, Mali, Burkina Faso, Niger, and northern Nigeria. The monsoon front moves northward from the Gulf of Guinea from May through August, then retreats southward in September and October. Lagos, Nigeria receives over 300mm of rain in July and again in October (a double rainfall peak).

Track current conditions in monsoon cities — Mumbai, Jakarta, Bangkok, Lagos — on the SunorSnow dashboard.