Occlusions & frontal waves

What catches up, and which air remains at the ground?

Compare the cold air behind the cold front with the cold air ahead of the warm front.

Cold and warm occlusions

An occlusion develops when a cold front catches up with a warm front and lifts the warm-sector air away from the surface. The surface occluded front separates two relatively cold air masses; the warmest air is elevated. The occlusion point, or triple point, is where the occluded, warm and cold fronts meet. It is not the same feature as the centre of the low.

In a cold occlusion, the air arriving behind the cold front is colder and denser than the air ahead of the warm front. It undercuts that air as well as lifting the warm sector, leaving a more cold-front-like lower section. In a warm occlusion, the air ahead of the warm front is the coldest; the less-cold post-cold-frontal air rides over that deeper cold wedge, leaving a more warm-front-like lower section. Judge the two cold air masses relative to each other, not by whether either is below zero Celsius.

Sketch three labelled air masses for each type: the warm sector elevated above both; the coldest air at the ground on the appropriate side; and the less-cold air rising above it. Mark the surface occluded front and the elevated junctions. An occluded front may remain active and carry extensive cloud and precipitation. Occlusion does not mean instant disappearance of the temperature gradient or the low.

How does weather differ across the two occlusion structures?

Expect combinations of warm- and cold-frontal weather, modified by moisture and stability.

Cloud, visibility and hazards

A cold occlusion can combine widespread As and Ns ahead with more vigorous uplift, embedded CB or showers near the advancing colder wedge. Continuous rain or snow, low cloud and poor visibility can occur before passage; more showery cold-air weather may follow. Ice, turbulence, convection and low-level wind shifts depend on the actual vertical profile. The colder undercutting air does not require every cold occlusion to be thundery.

A warm occlusion commonly gives a broad cloud and precipitation shield resembling a warm front, with lower cloud and poor visibility over the coldest air ahead. The less-cold air and elevated warm sector ascend above that wedge. Embedded convection remains possible if the elevated air is unstable. A shallow cold surface layer with a warm layer aloft can support freezing precipitation. Neither type has a universally greater severity: moisture, lift, stability, terrain and the low’s evolution decide it.

In a European winter, low freezing levels, snow, wet snow, freezing rain and persistent low cloud or fog can dominate. In summer, higher moisture content and instability can increase heavy rain and embedded CB risk. Poor visibility can persist in prolonged precipitation in either season. Assess the actual temperature profile and whether the occluded system is deepening, filling or being fed by a secondary wave; the purple chart symbol alone does not decide the flight conditions.

What is stationary when wind still blows along the front?

Separate the cross-front advance of the boundary from flow within the air masses.

A nearly stationary frontal zone

A stationary front is a boundary with little appreciable movement at the surface. Winds can blow strongly approximately parallel to it on either side while the component carrying the front across the ground is small. The symbols on a surface chart alternate red warm semicircles and blue cold triangles on opposite sides. Blue triangles point into the warm air and red semicircles bulge into the cold air; they do not imply two fronts racing in opposite directions.

Persistent moist ascent can produce extensive layered cloud, prolonged rain or snow, low ceiling and fog, particularly over cold ground. Where the lifted warm air is unstable, embedded or repeating convective cells can give heavy rain, hail, lightning, icing and turbulence. Repeated cells moving along a slow boundary can maintain precipitation over the same area for hours. A dry or weakening stationary front can have little cloud, so motion alone does not fix intensity.

Terrain, a slow upper trough or blocking can hold the boundary in place. A disturbance can travel along it and develop a wave or secondary low, changing both intensity and timing. Check successive analyses and forecasts: a forecast of late clearance can be delayed by a new wave even if the large-scale air masses remain on the same sides.

Which component of a 30 kt wind actually advances the boundary?

Use a geometric component first, then apply the stated empirical speed rule.

Direction and speed estimates

For a simple translating front, the relevant wind component is normal, or perpendicular, to the local frontal line. A wind parallel to the front provides no normal component. If the wind makes angle alpha with the frontal line, the normal component magnitude is V × sin(alpha). If the angle is instead given from the normal, use V × cos(angle). The direction of the signed component determines which way the boundary is being carried; do not infer direction from the magnitude alone.

A traditional first estimate moves a cold front with approximately the geostrophic wind component normal to the front at the front. For a warm front a common broad approximation is about two-thirds of that component; the Met Office table gives roughly four-fifths to five-sixths over sea and one-half to two-thirds over land. These fractions multiply the normal geostrophic component. Use the fraction explicitly supplied in a problem. Cold fronts therefore commonly advance faster than their associated warm fronts and close the surface warm sector.

These are forecasting heuristics, not an exact physical law or a universal ratio between the speeds of two different fronts. Upper forcing, deformation, a developing wave, terrain and diabatic processes can accelerate, slow or redevelop the boundary. Compare successive analysed positions and forecast charts. A moving convective rain line or gust front is also not necessarily the same feature as the air-mass front.

What makes the wave develop instead of remaining a small kink?

Combine a surface thermal boundary with the mass balance and steering aloft.

The classical frontal-wave life cycle

A disturbance on a baroclinic boundary displaces warm air poleward and cold air equatorward. A wave develops with a warm front ahead, a cold front behind and a surface low near their meeting region. In the Northern Hemisphere the low-level circulation is anticlockwise and inward across the isobars; it is clockwise in the Southern Hemisphere. Supportive upper-level divergence can remove mass from the column faster than surface convergence replaces it, allowing pressure to fall and the low to deepen.

As the wave amplifies, a warm-sector wedge opens between the fronts. The cold front commonly catches up near the low first, starting an occluded segment. As occlusion proceeds, the triple point extends away from the low along the frontal system, the surface warm sector narrows, and warm air is lifted into a wrapping cloud shield. A plan sketch should show the initially short occlusion lengthening and wrapping around the low while the triple point moves along the remaining warm and cold fronts. It need not follow a straight path or coincide with the low centre.

When the low becomes vertically stacked with the upper low, the supply of favourable thermal contrast and upper support often weakens; the system fills and cloud and precipitation gradually decay. This classical sequence is a conceptual model. Some lows deepen strongly during occlusion, stall, merge or spawn secondary lows; occlusion itself does not prove immediate filling. A new wave on the trailing front can prolong poor weather even as the original centre weakens.

Which motion estimate applies to the centre of the low?

Distinguish surface frontal advection from movement of the whole synoptic disturbance.

Upper steering and empirical estimates

A mobile frontal depression commonly travels broadly along the middle-tropospheric steering flow, often represented by the 500 hPa height contours or winds. A traditional rough speed estimate is about half the 500 hPa wind speed; NOAA gives a range of about 30–50 percent for lows or fronts embedded in winter zonal flow. Use the factor stated in a question. This rule is a first estimate under specified conditions, not a universal relationship for every pressure system.

A historical surface-chart rule for a young, unoccluded depression with straight warm-sector isobars moves the low approximately along those isobars at the speed of the warm-air flow. The U.S. Navy training reference distinguishes that rule from upper-air steering and favours upper-air guidance if the estimates conflict. A stated surface wind, geostrophic wind and upper wind are different inputs: apply the reference wind and fraction actually specified in the question. The warm-sector rule is less dependable as the system occludes or where the upper flow differs strongly from the surface pattern. Do not apply the two-thirds warm-front component rule to the depression centre: the centre, its cold front and its warm front need not have identical motion.

A mature occluded or cut-off low often slows, loops or follows an upper closed circulation rather than a simple west-to-east track. Blocking diverts the flow; rapid deepening and redevelopment can shift the analysed centre. Observed centre displacement, pressure-tendency fields, upper charts and successive forecasts establish the best estimate. For timing a crossing, locate the relevant front or weather belt, not just the low centre.

How do the plan, section and station traces agree?

Use a stated Northern Hemisphere track through both fronts, with the low passing to the north.

A connected sequence of surface and upper changes

On a Northern Hemisphere plan view, draw a low with a warm front extending generally east or north-east and a cold front extending south or south-west. Mark the warm sector between them. A station south of a mobile low can experience the warm front, warm sector and then cold front. A station farther north may instead remain in cold air and encounter the occlusion; the order is conditional on track, not a universal itinerary for every station.

For the stated warm-sector crossing, pressure falls ahead of the warm front, changes less rapidly within the sector unless the low is deepening, and rises after the cold-front trough. Temperature and dew point generally rise at the warm front and fall at the cold front. Surface wind commonly backs before the warm front, veers into a south-westerly warm sector, then veers again towards west or north-west at the cold front. Low ceilings and prolonged precipitation ahead, drizzle or low cloud within the sector, a cold-frontal rain band and post-frontal showers provide a compatible weather sequence.

In the vertical section, warm advection ahead of the low is commonly associated with wind veering with height in the free atmosphere in the Northern Hemisphere; cold advection behind is associated with backing with height. Reverse those thermal-wind associations in the Southern Hemisphere. Friction also turns surface wind relative to the free wind, so a single surface-to-upper comparison is not enough to infer advection. Place the sloping warm-frontal cloud shield ahead, the warm-sector layer between, and the colder wedge and possible convective band at the cold front. Mark possible icing in saturated sub-zero cloud and embedded CB wherever instability exists.

Can you apply the ideas?

Four original questions. Decide from the facts, then review the explanation.