How an occlusion forms
An occlusion is the mature stage of a frontal depression. The cold front normally moves faster, catches the warm front, and lifts the warm sector clear of the surface. The weather of both fronts is then concentrated into one complex weather zone.
Begin with the three air masses
An occlusion is not just two fronts drawn together. Compare the two cold air masses: the air ahead of the warm front and the air behind the cold front. The colder one stays nearest the ground. The relatively warmer air is forced over it, together with the original warm sector. This decides whether the occlusion is warm or cold.
The junction of warm front, cold front and occluded front is the triple point, also called the point of occlusion. A chart symbol locates it, but cannot by itself prove the type. The temperature structure does that. This is why two apparently similar occlusions can put the main precipitation on opposite sides of the surface position.
Why the fronts meet
Joshi gives the useful planning rule that a cold front normally moves at about twice the speed of a warm front. A warm front has a shallow slope, about 1 in 150 to 1 in 180, and commonly moves at only 10 to 15 kt. It is therefore gradually overtaken. Fronts themselves belong to the extra tropical cyclone family, not to tropical revolving storms or monsoon depressions.
Before occlusion, the student should still recognise the ingredients being merged. A stable warm front produces mainly layered cloud, while unstable warm air can put Cu and Cb inside the stratiform cloud. A cold front has steeper lift and can bring roll cloud, Sc or Ac with embedded Cb, squalls and showery precipitation. The occlusion inherits both sides of that history.
Warm and cold occlusions
Warm occlusion
A warm occlusion forms when the air ahead of the warm front is colder than the air behind the cold front. The air behind is cold relative to the warm sector, but it is not cold enough to undercut the leading air. It rises over it. In plan view the occluded front follows the old warm front, while the cold front becomes discontinuous at the triple point. Most precipitation is normally ahead of the surface occlusion.
Oxford associates this pattern with European winter, when very cold continental air can lie ahead of the depression. The gradual ascent gives extensive stratiform cloud and persistent precipitation. Cumuliform cloud from the cold front can still be mixed into that layer cloud.
Joshi describes the weather as initially warm front like: broad ascent, thickening layered cloud, rain or snow and low cloud. The important change is what follows passage. In a cold occlusion the showery, more unstable cold front character can assert itself after the initially warm front type weather. In a warm occlusion the weather remains more like a warm front because the advancing air is forced to override the colder surface air.
Cold occlusion
A cold occlusion forms when the air behind the cold front is colder than the air ahead of the warm front. The advancing air undercuts the leading air and lifts the warm sector. In plan view the occlusion follows the old cold front, the warm front becomes discontinuous, and the principal rain area lies mainly behind the surface front. It is more likely in summer when air ahead has been warmed.
| Question to ask | Warm occlusion | Cold occlusion |
|---|---|---|
| Coldest air | Ahead of warm front | Behind cold front |
| Surface action | Air behind rises over leading cold air | Air behind undercuts leading air |
| Main precipitation | Ahead | Behind |
| Common tendency | Winter | Summer |
In either case, the original warm sector is no longer at the surface. Do not look for a warm sector at ground level. Look for deep ascent, thick frontal cloud and the chance that an embedded Cb is hidden inside apparently ordinary nimbostratus.
Weather, hazards and western disturbances
An occlusion compresses the weather of a frontal depression into a smaller horizontal band. A flight can therefore encounter extensive low cloud and continuous rain while also carrying hidden convective hazards.
Cloud, rain and embedded convection
Broad ascent produces the frontal cloud progression taught by Joshi: Ci, Cs, As, Ns, St ahead of a warm front. An occlusion can combine this layered structure with cumulonimbus generated along the former cold front. The Cb may be embedded in nimbostratus, with no visual gap to make it obvious.
Deep cloud and persistent precipitation bring poor visibility and icing. Close to the freezing level, supercooled liquid water and freezing rain create the specific danger of rain ice. Embedded convection can add moderate or severe turbulence, hail, strong vertical currents and squalls. A mature low moves slowly, so these conditions may endure much longer than a simple cold front passage.
| Hazard | Why it occurs | Flight implication |
|---|---|---|
| Embedded Cb | Cold front convection concealed in Ns | Layer cloud is not automatically smooth or safe |
| Icing and rain ice | Deep cloud near freezing level | Know the freezing level and preserve an escape plan |
| Poor visibility | Low cloud and persistent precipitation | Approach minima and alternate fuel matter |
| Turbulence and hail | Embedded convective lift | Allow genuine distance from cells |
Western disturbances in India
Joshi describes western disturbances as extra tropical lows or troughs moving west to east north of 20°N. A system with two or more closed isobars at 2 hPa intervals is termed a western depression; upper level troughs above 500 hPa are troughs in the westerlies. They are seen as cyclonic circulation or a trough in the middle and lower troposphere, or as a surface low.
They originate around the Mediterranean, Caspian and Black Sea region, move east across north India, and are in an occluded state over India. Orography and moisture from the Arabian Sea intensify them over northwest India. A western disturbance can induce a low or cyclonic circulation to its south. Their average frequency is about 5 to 7 per winter month, falling to nil in monsoon months and returning from October.
From Pakistan and Rajasthan they often track northeast towards the western Himalaya, sometimes along the foothills eastward. Some continue through Uttar Pradesh, Madhya Pradesh and Bihar to northeast India. Typical movement is about 250 to 300 km per day. Maximum cloud and precipitation lie along the foothills. After passage, widespread fog and cold waves are common. Fog may lift by forenoon but can persist for several days.
Back bent occlusions, movement and decay
The mature depression is still moving
A first forecast projects the warm and cold fronts forward at their expected speeds until they meet. The depression itself tends to curve anticlockwise in the Northern Hemisphere. Its speed is related to the pressure pattern: closely packed isobars normally mean stronger winds and faster movement. Growth may take about four days, while filling and decay can last much longer.
Secondary depressions can develop at the occlusion point or on the trailing cold front, bringing renewed cloud and rain after an apparent improvement. On the other hand, the temperature contrast can weaken and an old occlusion becomes nonactive or frontolytic, leaving mainly cloud.
Back bent occlusion
Near the centre of the low, an occluded front can curve south and west around it as a back bent occlusion. Oxford gives a typical loop length of 100 to 200 NM. It produces a rain band in cold air behind the cold front and can be thundery. This is why mature depression weather cannot be understood only by looking ahead of the familiar fronts.