Weather science

Monsoon explained

A monsoon is not a storm — it's a seasonal wind shift that reverses air flow across an entire continent. Here is what actually happens.

The definition

A monsoon is a seasonal reversal of atmospheric circulation and precipitation. Winds blow one direction for months, then reverse.

Not synonymous with heavy rain.
Not a specific type of storm.
The rain is a consequence, not the definition.
Word from Arabic mausim meaning season.

How monsoons form

Continental land heats up in summer faster than adjacent ocean.
Rising warm air over land creates low pressure.
Cooler high pressure over ocean pushes air toward land.
This ocean-to-land flow carries moisture.
Moisture releases as rain when air rises over topography.
Winter reverses: land cools faster than ocean, wind flows from land to ocean.
Winter monsoon is dry season.

The major monsoons

South Asian Monsoon

India, Pakistan, Bangladesh, Sri Lanka. Peak June-Sept. Feeds 1.5 billion people.

East Asian Monsoon

China, Korea, Japan. Peak June-Sept. Feeds another billion.

West African Monsoon

Sahel region. June-Sept.

North American Monsoon

Arizona, New Mexico. July-Sept. Weakest major monsoon.

South American Monsoon

Amazon basin. Nov-Mar (Southern Hemisphere summer).

Australian Monsoon

Nov-Mar. Northern Australia.

The South Asian Monsoon

The most economically important monsoon in the world.

Starts May in southern India.
Reaches Bombay in early June.
Reaches Delhi in early July.
Peaks July-August.
Retreat starts September.
Delivers 75% of India's annual rainfall in 4 months.
Feeds crops that sustain 1.5 billion people.
A weak monsoon = famine risk.
A strong monsoon = flooding damage.
Forecast accuracy has direct economic impact.

The North American Monsoon

US chasers know this as the 'summer thunderstorm season.'

Not as intense as Asian monsoons.
Peak: July-September.
Affects: Arizona, New Mexico, western Texas, southern Colorado, southern Utah, southern Nevada, northern Mexico.
Origin: Gulf of California moisture.
Storms typically afternoon/evening pop-ups.
Photogenic — big dry-air CAPE + moisture.
Lightning frequent.
Flash flood risk high.
Landspouts and tornadoes rare but occur.

The precipitation pattern

Not continuous rain — comes in bursts (called active phase vs break phase).
Storm cells drift with the flow.
Cyclones intermittently strengthen.
Some regions in India get 300+ inches per monsoon.
Cherrapunji India: 467 inches in July 1861 — world record for a month.

Monsoon failure

When the monsoon fails (arrives late, ends early, or has an active break period at the wrong time):

Crops fail.
Reservoirs don't fill.
Rural economies collapse.
Migration follows.
Historical famines: 1770 Bengal, 1876-79 Great Famine (5.5M dead), 1943 Bengal (2M dead).
Even modern monsoon failures displace millions.

Climate change and monsoons

Warming air holds more moisture: monsoons may deliver more total water.
But delivery may be less predictable: worse variability.
Timing may shift.
Onset dates shifting slightly later in some studies.
Extreme rainfall events increasing.
Drought years also increasing.
Overall stability decreasing.

Chasing monsoons

Arizona monsoon chase season: July-September.
Big Tucson supercells possible.
Photogenic lightning storms.
Photogenic clean skies before storms.
Rainbows and vivid sunsets.
Chase Arizona monsoon less crowded than Plains.
Kartchner Caverns area especially popular.
Ryan Wunsch and other AZ chasers publish annual outlook.

The Cherrapunji record

Cherrapunji in Meghalaya, India holds the record for highest annual rainfall: 1,042 inches (26,470 mm) in 1861.

The town is 4,500 ft up in the Khasi Hills.
South-facing slope receives monsoon moisture head-on.
Rain compressed into 4 months.
July alone: 366 inches (world record for one month).
Village of Mawsynram is competitive, sometimes wetter than Cherrapunji.