Observing the weather for purposes of forecasting is a worldwide enterprise. Routine measurements are made by national agencies in every country of the globe. These observations and their distribution are handled by international agreement and the World Meteorological Organization, a specialized agency of the United Nations. It has become obvious that events in one part of the world may have a great effect on weather experienced in distant regions. ‘Worldwide effects and forecasting’ considers these long-distance relationships between various atmospheric conditions—teleconnections—the most famous being El Niño. It also looks at the potential solar influence on Earth’s weather, as well as how chaos theory is used in ensemble forecasting.