O

Occluded front

Composite of two fronts, which is formed when a cold front overtakes a warm front or a quasi-stationary front. It forms as a cyclone moves deeper into colder air, separating the air behind the cold front, from air ahead of the warm front.

Ocean buoy

Buoy weather station. A floating or fixed buoy, which carries instruments for measuring different meteorological elements and transmitting the information by radio.

Ocean conveyor belt

Global scale oceanic recirculation system of water masses that determines today’s climate. It is primarily driven by the sinking of cold and salty North Atlantic Deep Water (NADW) which then flows southward and when it reaches the Antarctic Circumpolar Current (ACC), part of it continues into the Indian and Pacific Oceans at depth, enters the Atlantic Ocean through the Drake Passage, going back to the North Atlantic. But most NSAW rises close to the surface in the ACC and then, once refreshed, it enters all three ocean basins, and it is slowly carried by different ocean currents back to the northern waters, toward the Greenland and Labrador Seas to cool and sink again, completing the circuit.

Ocean current

Movement of ocean water, characterized by its regularity. Commonly, a continuous stream flowing along a definable path, but they might also present a cyclic nature.

Orographic uplift / Orographic lifting

Upward air motion, caused by wind flow over sloping terrain, with features such as hills or mountains.

Outlook

Forecast report issued when hazardous weather or hydrological events can occur. It provides valuable information to people who may be at risk and need considerable lead time to take providences.

Overgrazing

Over exposure to grazing (regular consumption of part of one organism by another one, without killing it), either for too long or without giving enough time to recovery. This process results in usefulness reduction of land resources, and finally in erosion and desertification.

Overrunning

Condition given when relatively warm air glides upward, moving over a surface-based colder air mass, resulting usually in low clouds, fog and light precipitation.

Ozone

Triatomic form of oxygen, usually found in the stratosphere.

Ozone Hole

Thinning of the ozone layer over the Antarctica.

Ozonosphere / Ozone layer

Region of the atmosphere, between 6 and 30 miles (10 and 50 km) above surface, which contains large concentrations of ozone, being maximum from about 13 to 16 miles (20 to 25 km). Due to the absorption of UV rays by ozone molecules, this layer filters UV radiations, limiting its penetration to Earth’s surface.