Glossary
Obliquity of the Ecliptic
the tilt of Earth’s axis relative to a line perpendicular to Earth’s orbital plane (the "surface" along which the planet orbits the sun). It is not a fixed quantity. The present value is from 23°2’ to 24°27’.
Obscuring Phenomena
any atmospheric phenomenon, except clouds, that restricts vertical visibility (e.g., dust, rain, snow, etc.).
Occluded Front
the front formed by a cold front overtaking a warm or stationary front and lifting the warm air above the earth’s surface
Occluded Mesocyclone
a mesocyclone in which air from the rear-flank downdraft has completely surrounded the circulation at low levels, cutting off the inflow of warm unstable low-level air
Ocean Prediction Center (OPC)
one of nine NCEP centers. Issues weather warnings and forecasts out to 5 days for the Atlantic and Pacific Oceans north of 30 degrees North. It is located in Camp Springs, Maryland.
Orographic Lift
the lifting of air caused by its passage up and over mountains or other sloping terrain
Oscillation
the repeated variation of some measure around a central value. A swinging pendulum is an example of oscillation. In meteorology, an oscillation is a shift in position of high and low pressure systems. One example is the AO (Arctic Oscillation).
Outflow Boundary
a storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air
Outgoing Longwave Radiation (OLR)
a polar orbiting satellite (POES)-derived measurement of the radiative character of energy radiated from the warmer earth surface to cooler space. This measurement gives information on cloud-top temperature, which can be used to estimate tropical precipitation amounts (which is important in forecasting weather and climate).
Overhang
a radar term indicating a region of high reflectivity at middle and upper levels above an area of weak reflectivity at low levels. The overhang is found on the inflow side of a thunderstorm (normally the south or southeast side).
Overrunning
when an air mass aloft is in motion relative to another air mass of greater density at the surface. This term is usually applied in the case of warm air ascending the surface of a warm front.
Overshooting Top (or Penetrating Top)
a dome-like protrusion above a thunderstorm anvil; it represents a very strong updraft and, hence, a higher potential for severe weather with that storm. A persistent and/or large overshooting top often is present on a supercell.
Oxygen
in its free form, a colorless, tasteless, and odorless gas in the atmosphere. Oxygen makes up approximately 21% of the atmosphere.
Ozone
a triatomic form of oxygen that is a nearly colorless gas of pungent odor; it is formed naturally in the upper atmosphere, generally between the heights of about 10 and 50 km and it results in the absorption of a large part of the sun’s most intense incoming radiation
Ozone Hole
a severe depletion of stratospheric ozone over Antarctica that occurs each spring. The depletion is caused by a chemical reaction involving ozone and chlorine, primarily from human produced sources, cloud particles, and low temperatures.
Ozone Layer
an atmospheric layer that contains a high proportion of oxygen that exists as ozone. It acts as a filtering mechanism against incoming ultraviolet radiation. It is located between the troposphere and the stratopause, around 9.5 to 12.5 miles above the earth’s surface.
