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If you follow the weather – who doesn’t? – from time to time you will hear about El Niño and La Niña.

Weather forecasters will talk, for example, about how a developing El Nino could bring a wetter or perhaps drier winter. Or they’ll tell you how an established La Niña makes for a more active hurricane season.

But sometimes there is no El Niño-La Niña conversation at all.

A basic guide to help you understand what they are talking about or not talking about.

What exactly are El Niño and La Niña?

Both are intermittent climatic events that originate in the equatorial Pacific Ocean but can have far-reaching effects on weather anywhere in the world.

The two are related: they are opposite phases of what is called the El Nino-Southern Oscillation or ENSO. Thus, they can never occur simultaneously. And there are many times when neither is.

What is ENSO?

ENSO describes the fluctuation of two things in the equatorial Pacific: the surface temperature of the ocean and the pressure of the air above it.

The temperature component is pretty simple, and most news reports focus on it. El Niño can develop when sea surface temperatures are about 1 degree Fahrenheit or more above average. When temperatures are below average, La Niña can occur. When temperatures are at or near average – this is called ENSO-neutral – neither will thrive.

The air pressure part is a little more complicated. It refers to the air pressure difference between the western and eastern parts of the equatorial Pacific. Scientists use readings from Darwin on Australia’s north-central coast and Tahiti, more than 5,000 miles to the east.

When pressure is lower than normal in Tahiti and higher than normal in Darwin, conditions favor the development of El Nino. When the opposite happens, La Niña can develop.

The two components are strongly related, and the conditions for both must be true for an El Niño or La Niña to occur. For example, if sea surface temperatures support El Niño but air pressure conditions are not favorable, El Niño will not develop.

Why does one or the other occur?

Scientists aren’t sure what exactly starts the process. But from time to time, air pressure conditions in the equatorial Pacific change, affecting trade winds that normally blow from east to west. Winds move on the surface of the water heated by the sun, pushing it.

If the trade winds get stronger, as during La Niña, more warm water is pushed westward. And in the cold of the eastern Pacific, deep waters rise to take its place.

If the trade winds weaken, such as during an El Niño, less water moves westward and the central and eastern Pacific warms more than normal.

A large body of warm water in the ocean transfers a lot of heat to the atmosphere via convection, the warm, moist air that rises from the sea surface and forms storms. Heat affects atmospheric circulation in both north-south and east-west directions.

The location of all this convection is important. In El Niño, because the warm water stays in the eastern Pacific, convection takes place there. In La Niña, the eastern Pacific stays cooler and convection takes place much further to the west.

What are the effects?

Changes in atmospheric circulation can cause changes in weather conditions in various parts of the world, which meteorologists call teleconnection. Much of this has to do with the location of the jet stream, which are high-altitude winds that sweep the planet from west to east.

In El Niño, the jet stream tends to shift south. This could bring wetter, cooler conditions to much of the Southern United States and warmer conditions to parts of the North. Elsewhere, El Niño can create hot and dry conditions in Asia, Australia and the Indian subcontinent. Parts of Africa and South America may also be affected.

In La Niña, the jet stream shifts northward. This can lead to hot and dry conditions in the Southern United States and cooler and wetter weather in parts of the North, particularly the Pacific Northwest. Parts of Australia and Asia may be wetter than normal.

La Niña can also cause more hurricanes in the North Atlantic because typically less wind shear can disrupt the structure of cyclonic storms while changes in wind speed and direction occur.

It is important to note that these are just typical effects. El Niño and La Niña sometimes do not follow the expected patterns.

And strength matters: For example, a strong El Niño (as measured by how above normal sea surface temperatures are) can have greater effects than a weaker one.

How often do they occur and how long do they last?

Both El Niño and La Niña occur on average every two to seven years, and El Niño occurs slightly more frequently than La Niña.

They can last for the better part of a year, although sometimes they take longer. La Niñas sometimes “double drop” – one occurs, ending when sea surface temperatures rise to ENSO-neutral conditions, and then the second occurs when temperatures drop again.

Where do the names come from?

El Niño got its name from South American fishermen in the 17th century. Usually around Christmas, they noticed warmer waters from the shore at times. Thus El Niño – the “little boy” or, in the context of Christmas, the Christ child.

La Niña was an afterthought. It became La Niña, the “little girl,” as it was more or less the opposite of El Niño.

What about climate change?

Scientists aren’t sure how El Niño and La Niña might change as the world continues to warm from greenhouse gas emissions. Some research suggests that extremely powerful attacks will occur more often than they do now. However, it is unclear how this will affect wet or dry patterns in the United States and elsewhere.