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Meridional flow (or meridional current) means fluid (e.g., a planet's atmosphere) circulation between the equator and poles, parallel to meridians (lines of equal longitude), i.e., north-to-south or south-to-north flow. This type of circulation results in locations experiencing extreme temperature changes. Conversely, east-to-west or west-to-east air flow is termed zonal flow. The terms are also used to describe the details of an instance of convection, e.g., within stars or within geologically active planets.