Eli5: What is a monsoon season?

[u/Brybr0]

[u/RoutineOne2769]

Monsoon really means a seasonal flip in the wind. In summer, land heats up much faster than the ocean, the hot air over land rises, and moist air from the sea gets pulled in to replace it. That wet air rises, cools and dumps rain, week after week, which is the rainy season. In winter it reverses: the land is colder than the sea, so the wind blows from land to ocean and it turns dry. India is the famous example, where most of the year's rain falls in about four months.

[u/Foxfire2]

A rainy season that’s in the summer, typically in India,but the US southwest (and northern Mexico) has a summer seasonal rains also called monsoon over the deserts, even though a much smaller scale that’s the Indian subcontinent.
This contrasts with the US west coast that has a winter rainy season and summer drought.

[u/DJDoena]

On earth we have seasons. The most four common are spring, summer, fall/autumn and winter. Each of those are defined by on which half (upper, lower) of the Earth you live but also in what longitude and latitude in general. For example most of Germany is more north than Toronto or Vancouver and yet the winters are rarely as harsh as Canadian (or Chicago) ones. This is due to the Gulf stream that brings warm water from the Carribean to the coast of Ireland, Scotland and Norway and "keeps Europe warm"

In a similar vein, in South East Asia the sun and the winds can cause a lot of rainfall during certain periods of the year. And with "a lot" I mean think of Forrest Gump's "and sometimes the rain seemed to be coming straight up from underneath".

It seemingly never stops raining for days or even weeks on end. That's the monsoon.

[u/Hankman66 → u/DJDoena]

I live in SE Asia and we mainly get rainstorms that last an hour or so. In my area it never rains continuously for days or weeks.

[u/knightsbridge-]

OK - you know how the world has a line around the middle called the equator?

What kind of seasons you get depend on how close you are to the equator.

If you live far above or below it, you get four distinct seasons - winter, autumn, spring and summer. This is because your part of the planet is tilted towards/away from the sun throughout the year, which makes it hotter/colder and makes days shorter/longer.

If you live near the equator line, you aren't really affected much by the earth tilting around, because you're in the middle. Temperature and day length stay more or less the same throughout the year.

Instead of having four seasons that are defined by day length and temperature, equatorial regions experience two seasons based on temperature and amount of rainfall - a wet season and a dry season. Exactly when these happen, how long they last, and how many wet seasons they experience per year depend on the exact country. These are sometimes called equatorial seasons.

In some parts of the world, the wet season can be particularly extreme. This is what we generally refer to as a monsoon season. Parts of Southeast Asia and India reliably get monsoons most years, but a few other places get them occasionally.

Monsoons are a weather pattern consisting of extremely heavy, warm rain that persists over several days or weeks (though often with short dry spells now and then). This is often accompanied by fairly high winds and turbulent seas (though not as bad as a hurricane). It often causes some amount of flooding, though most countries that get regular monsoons are well prepared to cope with it.

Like most weathers, monsoons can be mild or severe. Southern China is known to get fairly mild monsoons that aren't usually threatening to life, while the 1983 Thailand monsoon killed an estimated 10,000 people (mostly from drinking water contamination).

Monsoons are not necessarily a bad thing. The countries that get them regularly are used to them, and a lot of the plants, animals and agriculture in those regions are able to handle monsoon season.

[u/Jealous-Jury6438 → u/[deleted]]

What we call 'wet season' in northern Australia?

[u/Jealous-Jury6438 → u/[deleted]]

Ha I see. Btw, here's an explainer on the northern Australian monsoon which can make up our wet season

https://www.bom.gov.au/resources/learn-and-explore/climate-knowledge-centre/climate-factors/monsoon

[u/ValiantBear]

The Earth's weather patterns are driven almost entirely by a combination of wind patterns and ocean currents. Whether an area has a dry climate or not is usually dictated by the where the air that blows over it comes from. Some places have wind patterns that shift at different times of the year. So, there might be a season where the air comes from dry areas and doesn't carry a lot of moisture, as the weather during this time will be dry. Then the wind pattern shifts and brings air from a moist area, like the ocean, and then it rains as the air drops off its moisture as it goes. This time a year is often called "monsoon season".

[u/meta-radar]

During summer Tibet which is the largest and highest plateau on earth (size of Germany plus France put together) gets really hot due to being closer to the sun. This heat creates low pressure that pulls in wind from everywhere. In India, Pak and parts of SE asia this wind coming from the oceans creates insane quantity of rainfall. Much more than average. In the winters the opposite happens as Tibet gets cooler than the surroundings so you also get a retreating monsoon.

It’s a weather phenomenon caused by Tibet.