anomaly: A brief and unexpected difference; something that is unusual and especially outside the normal range of what occurs.

Atlantic: One of the world’s five oceans, it is second in size only to the Pacific. It separates Europe and Africa to the east from North and South America to the west.

atmosphere: The envelope of gases surrounding Earth, another planet or a moon.

average: (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.

climate change: Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.

computer model: A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.

consensus: An opinion or conclusion shared by most, if not all, of a specific group.

develop: To emerge or to make come into being, either naturally or through human intervention, such as by manufacturing.

drought: An extended period of abnormally low rainfall; a shortage of water resulting from this.

dynamic: An adjective that signifies something is active, changing or moving. (noun) The change or range of variability seen or measured within something.

El Niño: Extended periods when the surface water around the equator in the eastern and central Pacific warms. Scientists declare the arrival of an El Niño when that water warms by at least 0.4 degree Celsius (0.72 degree Fahrenheit) above average for five or more months in a row. El Niños can bring heavy rainfall and flooding to the West Coast of South America. Meanwhile, Australia and Southeast Asia may face a drought and high risk of wildfires. In North America, scientists have linked the arrival of El Niños to unusual weather events — including ice storms, droughts and mudslides.

ENSO: (short for El Niño-Southern Oscillation) A natural cycle of changing temperatures in the ocean and atmosphere, near the equator in the Pacific Ocean. During ENSO events, atmospheric pressure also changes in affected areas. (see also El Niño)

equator: An imaginary line around Earth that divides Earth into the Northern and Southern Hemispheres.

equatorial: (adj.) Having to do with Earth’s equator or its general vicinity.

fluid dynamics: The study of liquids and gases in motion.

global warming: The gradual increase in the overall temperature of Earth’s atmosphere due to the greenhouse effect. This effect is caused by increased levels of carbon dioxide, chlorofluorocarbons and other gases in the air, many of them released by human activity.

hurricane: A tropical cyclone that occurs in the Atlantic Ocean and has winds of 119 kilometers (74 miles) per hour or greater. When such a storm occurs in the Pacific Ocean, people refer to it as a typhoon.

La Niña: Extended periods when the surface water around the equator in the eastern Pacific cools for long stretches of time. Scientists will announce the arrival of a La Niña when the average temperature there drops by at least 0.4° C (0.72° degree F). Impacts on global weather during a La Niña tend to be the reverse of those triggered by an El Niño: Now, Central and South America may face severe droughts while Australia floods.

mid-latitudes: That part of the globe that lies midway between Earth’s tropical and polar regions. Most people live in these temperate regions and most of the world’s food is produced here.

model: A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes. Or an individual that is meant to display how something would work in or look on others.

monsoon: A system of winds that influences the climate of a large area. In southern Asia, monsoons blow from the southwest in the summer and usually bring heavy rains.

National Oceanic and Atmospheric Administration: (or NOAA) A science agency of the U.S. Department of Commerce. Initially established in 1807 under another name (The Survey of the Coast), this agency focuses on understanding and preserving ocean resources, including fisheries, protecting marine mammals (from seals to whales), studying the seafloor and probing the upper atmosphere.

Pacific: The largest of the world’s five oceans. It separates Asia and Australia to the west from North and South America to the east. The term can also refer to island nations that sit within the Pacific Ocean.

sea: An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.

transition: The boundary where one thing (paragraphs, ecosystems, life stage, state of matter) changes or converts into another. Some transitions are sharp or abrupt. Others slowly or gradually morph from one condition or environment to another.

tropics: The region near Earth’s equator. Temperatures here are generally warm to hot, year-round.

wave: A disturbance or variation that travels through space and matter in a regular, oscillating fashion.

weather: Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.