Ocean currents

Heat from the tropics can be transferred to the cold polar regions, by large-scale water movement within the oceans. Each ocean has its own circular pattern of currents. Heat is transferred by warm ocean currents, such as the North Atlantic Drift in the Atlantic Ocean, from low latitudes to high latitudes. Ocean currents are set in motion by the prevailing surface winds associated with the general atmospheric circulation. The direction of water movement is also deflected by the Coriolis force.

High pressure and low pressure

Pressure is measured in millibars. Standard pressure at sea level is 1013 millibars, but large areas of either high or low pressure can occur. Areas of high and low pressure are caused by rising and sinking air. As air warms, it rises, leading to low pressure at the surface. As air cools, it sinks leading to high pressure at the surface.

Cold air sinks. Air flows from high to low pressure. Warm air rises.

On a weather chart, lines joining places with equal sea-level pressures are called isobars. Charts showing isobars are useful because they identify features such as anticyclones (areas of high pressure) and depressions (areas of low pressure).