Glossary of Weather Terms – Beginning with “O”

Curious about what an occluded front is, how orographic lift causes heavy mountain rain, what an outflow boundary does to severe weather, or what an omega block weather pattern means for weeks of extreme heat or cold?

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Weather Terms Beginning with “O”

Occluded Front

An occluded front forms when a cold front overtakes a warm front, lifting the warm air mass off the surface. It is represented on weather maps as a purple line with alternating triangles and semicircles. Occlusions develop in the later stages of a mid-latitude cyclone’s life cycle. There are two types: a cold-type occlusion (when the advancing cold air is colder than the cold air ahead of the warm front, producing a cold front at the surface) and a warm-type occlusion (when the advancing cold air is warmer than the existing cold air ahead, producing a warm front at the surface). Occluded fronts are associated with persistent clouds, widespread precipitation, and gradual clearing as the parent storm system begins to weaken.

Omega Block

An omega block is a persistent upper-level atmospheric pattern in which the jet stream takes the shape of the Greek letter omega (Omega), with high pressure ridges on both sides and a low pressure trough in the middle. This pattern blocks the normal west-to-east progression of weather systems and can lock weather patterns in place for one to three weeks. Regions under the ridges of an omega block experience persistent warmth and dryness that can lead to drought; the region under the trough experiences extended cold and precipitation. Omega blocks are responsible for some of the most extreme prolonged heat waves and droughts in US history. The pattern eventually breaks down when an upstream disturbance finally disrupts the blocking configuration.

Orographic Lift (Orographic Effect)

Orographic lift is the forced rising of air as it encounters a mountain barrier. The windward side of the mountain receives enhanced precipitation as the lifted air cools, saturates, and condenses. The leeward (downwind) side falls under the rain shadow effect as the now-dry air descends and warms by compression. The contrast can be dramatic, the windward side of the Washington Cascades receives 100 or more inches of precipitation annually, while the lee side (eastern Washington) receives under 12 inches. Orographic lift also triggers much of the catastrophic flooding associated with atmospheric river events in California and the Pacific Northwest, where moist Pacific air is forced rapidly upward by the Sierra Nevada, Coast Ranges, and Cascades.

Outflow Boundary

An outflow boundary is the boundary between a thunderstorm’s cool, dense outflow air and the surrounding warmer environmental air, essentially a mini cold front produced by an individual thunderstorm. Outflow boundaries can persist for hours after the parent storm has dissipated and are easily detectable on Doppler radar as a thin arc of enhanced reflectivity. They act as a focus for new thunderstorm development when they collide with the surrounding warm, moist air. In the plains states, collisions of multiple outflow boundaries during severe weather outbreaks can trigger tornadoes and explosive new storm development, making outflow boundary tracking an important tool for NWS forecasters during active severe weather events.

Overcast

Overcast is the sky condition in which cloud cover is 8/8 of the sky, complete cloud cover with no direct sunlight, coded as “OVC” in METAR aviation weather observations. An overcast sky produces diffuse, even lighting but blocks all direct solar radiation, reducing daytime maximum temperatures and preventing the overnight temperature drop that occurs under clear skies. Overcast skies associated with thick, low clouds often indicate incoming precipitation. Overcast is distinct from “mostly cloudy” (5/8 to 7/8 cloud cover, coded BKN for Broken) and “partly cloudy” (3/8 to 4/8 coverage, coded SCT for Scattered).

Cloud Cover Terms Explained (METAR Codes and Forecast Language)

Offshore Wind

An offshore wind is wind blowing from land toward the sea. Offshore winds are typically cooler and drier than onshore flow and suppress marine fog along coastlines. They can enhance fire danger in coastal areas when combined with dry conditions and low humidity. In surfing, offshore winds hold up breaking waves, creating cleaner, more organized surf. In meteorology, offshore flow is associated with high pressure over land. The Santa Ana winds of Southern California are a type of powerful, hot, dry offshore wind that funnels through mountain gaps and significantly increases wildfire risk during fall and winter.

Rain Shadow

The rain shadow is the dry area on the downwind (leeward) side of a mountain range where precipitation is dramatically reduced. As moist air rises over the windward side of a mountain, it cools, condenses, and drops most of its moisture as rain or snow. As the air descends on the leeward side, it warms by compression (adiabatic heating) and arrives hot and dry. The contrast can be extreme, the Olympic Peninsula in Washington receives over 140 inches of rain per year, while the Cascade foothills just 60 miles to the east receive less than 15 inches. Other notable rain shadows include eastern Washington (leeward of the Cascades), Nevada (leeward of the Sierra Nevada), and the Deccan Plateau in India (leeward of the Western Ghats).

Outflow

Outflow is the cold, dense air that flows outward from the downdraft regions of a thunderstorm at the surface, spreading in all directions from the storm’s base. The leading edge of the outflow is the gust front. Outflows are associated with sudden wind shifts, temperature drops of 10-20 degrees F, and a sharp rise in surface pressure. The balance between a supercell thunderstorm’s updraft inflow and its outflow determines storm organization, too much outflow “undercuts” the updraft and weakens the storm. In the tropics, outflow aloft from hurricanes spreads anticyclonically away from the storm’s center, transporting heat energy and helping maintain the storm’s structure.

Occluded Low

An occluded low is a mature low pressure system that has completed the occlusion process. As the low ages, the cold front catches up to the warm front, the warm sector at the surface shrinks and is eventually lifted completely off the ground. The low becomes “cold-core” throughout its depth rather than retaining the warm sector that fueled its development. An occluded low begins to weaken after occlusion because it no longer has the contrast between warm and cold air masses at the surface that drives intensification. The “bent-back” occlusion visible on radar and satellite imagery, shaped like a backwards “comma”, is a classic signature of a mature, occluding extratropical cyclone.

Frequently Asked Questions, “O” Weather Terms

What is an occluded front?

An occluded front forms when the cold front of a storm system catches up to and overtakes the warm front, lifting the warm air mass completely off the surface. It is represented on weather maps as a purple line with alternating triangles (cold front symbols) and semicircles (warm front symbols). Occluded fronts are associated with the mature stage of a storm system, persistent clouds, steady or intermittent precipitation, and the gradual end of the storm’s intensification phase. After occlusion, the storm loses its warm air energy source and begins to weaken.

What is an omega block and how does it affect weather?

An omega block is a large, persistent upper-level weather pattern shaped like the Greek letter omega, the jet stream bends upward into two ridges with a trough between them. It acts like a roadblock in the atmosphere, preventing the normal west-to-east flow of weather systems and locking weather patterns in place for one to three weeks. Areas under the ridges experience heat waves and drought; the area under the trough experiences prolonged cool, rainy weather. Some of the most destructive US droughts and heat waves, including the 2012 central US drought, were associated with omega blocking patterns.

What is orographic lift and why does it cause so much rain?

Orographic lift is the forced rising of air when it encounters a mountain barrier. As the air rises, it cools at the dry adiabatic lapse rate, eventually reaching the dew point and forming clouds. The continued rise forces more and more moisture out as rain or snow on the windward side. When an atmospheric river, a concentrated corridor of tropical moisture, hits a mountain range like the Sierra Nevada or Cascades, orographic lift can produce 10-20 inches of rain in 24-48 hours on the windward slopes, causing catastrophic flooding.

What is an outflow boundary?

An outflow boundary is the leading edge of cool, dense air that flows out of a thunderstorm’s downdraft at the surface, essentially a small-scale cold front created by an individual storm. Outflow boundaries can persist for hours after the parent storm dissipates and can be detected on Doppler radar as a thin arc of enhanced reflectivity. They are important because they can trigger new thunderstorm development when they collide with warm, humid air. In active severe weather environments, interactions between multiple outflow boundaries and large-scale fronts often produce the most violent tornadoes.

What is the difference between overcast and mostly cloudy?

In standard METAR aviation weather observations, sky cover is divided into eighths. Overcast (OVC) means 8/8 of the sky is covered, complete cloud cover with no direct sunlight. Mostly Cloudy or Broken (BKN) means 5/8 to 7/8 cloud cover, limited but possible periods of direct sunlight. Partly Cloudy or Scattered (SCT) means 3/8 to 4/8 cloud cover, alternating periods of sun and clouds. In public NWS forecasts, “mostly cloudy” corresponds roughly to BKN and “partly cloudy” to SCT. An overcast sky is the standard requirement for a Dense Fog Advisory when combined with low visibility.

What is the rain shadow effect?

The rain shadow is the dry zone on the leeward (downwind) side of a mountain range, caused by the orographic process. As moist air is forced upward over the windward slopes, it loses most of its moisture as precipitation. When the now-dry air descends on the leeward side, it warms through compression and arrives hot and arid. The contrast can be dramatic, the Olympic Peninsula in Washington receives over 140 inches of rain per year, while the Cascade foothills just 60 miles to the east receive under 15 inches. The Great Basin, Mojave Desert, and Sonoran Desert all exist in the rain shadow of western mountain ranges.

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