Cold front in the context of Advection

A weather front is a boundary separating air masses for which several characteristics differ, such as air density, wind, temperature, and humidity. Disturbed and unstable weather due to these differences often arises along the boundary. For instance, cold fronts can bring bands of thunderstorms and cumulonimbus precipitation or be preceded by squall lines, while warm fronts are usually preceded by stratiform precipitation and fog. In summer, subtler humidity gradients known as dry lines can trigger severe weather. Some fronts produce no precipitation and little cloudiness, although there is invariably a wind shift.

Cold fronts generally move from west to east, whereas warm fronts move poleward, although any direction is possible. Occluded fronts are a hybrid merge of the two, and stationary fronts are stalled in their motion. Cold fronts and cold occlusions move faster than warm fronts and warm occlusions because the dense air behind them can lift as well as push the warmer air. Mountains and bodies of water can affect the movement and properties of fronts, other than atmospheric conditions. When the density contrast has diminished between the air masses, for instance after flowing out over a uniformly warm ocean, the front can degenerate into a mere line which separates regions of differing wind velocity known as a shear line. This is most common over the open ocean.

A rainband is a cloud and precipitation structure associated with an area of rainfall which is significantly elongated. Rainbands in tropical cyclones can be either stratiform or convective and are curved in shape. They consist of showers and thunderstorms, and along with the eyewall and the eye, they make up a tropical cyclone. The extent of rainbands around a tropical cyclone can help determine the cyclone's intensity.

Rainbands spawned near and ahead of cold fronts can be squall lines which are able to produce tornadoes. Rainbands associated with cold fronts can be warped by mountain barriers perpendicular to the front's orientation due to the formation of a low-level barrier jet. Bands of thunderstorms can form with sea breeze and land breeze boundaries, if enough moisture is present. If sea breeze rainbands become active enough just ahead of a cold front, they can mask the location of the cold front itself. Banding within the comma head precipitation pattern of an extratropical cyclone can yield significant amounts of rain or snow. Behind extratropical cyclones, rainbands can form downwind of relative warm bodies of water such as the Great Lakes. If the atmosphere is cold enough, these rainbands can yield heavy snow.

Heat lightning (not to be confused with dry thunderstorms, which are also often called dry lightning) is a misnomer used for the faint flashes of lightning on the horizon or other clouds from distant thunderstorms that do not appear to have accompanying sounds of thunder.

The actual phenomenon that is sometimes called heat lightning is simply cloud-to-ground lightning that occurs very far away, with thunder that dissipates before it reaches the observer. At night, it is possible to see the flashes of lightning from very far distances, up to 100 miles (160 km), but the sound does not carry that far. In the United States, lightning is especially common in Florida, which is considered the deadliest state for lightning strikes in the country. This is due to high moisture content in the lower atmosphere and high surface temperature, which produces strong sea breezes along the Florida coast. As a result, heat lightning is often seen over the water at night, the remnants of storms that formed during the day along a sea breeze front coming in from the opposite coast.

An outflow boundary, also known as a gust front, is a storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature and a related pressure jump. Outflow boundaries can persist for 24 hours or more after the thunderstorms that generated them dissipate, and can travel hundreds of kilometers from their area of origin. New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary (cold front, dry line, another outflow boundary, etc.). Outflow boundaries can be seen either as fine lines on weather radar imagery or else as arcs of low clouds on weather satellite imagery. From the ground, outflow boundaries can be co-located with the appearance of roll clouds and shelf clouds.

Outflow boundaries create low-level wind shear which can be hazardous during aircraft takeoffs and landings. If a thunderstorm runs into an outflow boundary, the low-level wind shear from the boundary can cause thunderstorms to exhibit rotation at the base of the storm, at times causing tornadic activity. Strong versions of these features known as downbursts can be generated in environments of vertical wind shear and mid-level dry air. Microbursts have a diameter of influence less than 4 kilometres (2.5 mi), while macrobursts occur over a diameter greater than 4 kilometres (2.5 mi). Wet microbursts occur in atmospheres where the low levels are saturated, while dry microbursts occur in drier atmospheres from high-based thunderstorms. When an outflow boundary moves into a more stable low level environment, such as into a region of cooler air or over regions of cooler water temperatures out at sea, it can lead to the development of an undular bore.

A squall line, or quasi-linear convective system (QLCS), is a line of thunderstorms, often forming along or ahead of a cold front. In the early 20th century, the term was used as a synonym for cold front (which often are accompanied by abrupt and gusty wind shifts). Linear thunderstorm structures often contain heavy precipitation, hail, frequent lightning, strong straight-line winds, and occasionally tornadoes or waterspouts. Particularly strong straight-line winds can occur where the linear structure forms into the shape of a bow echo. Tornadoes can occur along waves within a line echo wave pattern (LEWP), where mesoscale low-pressure areas are present. Some bow echoes can grow to become derechos as they move swiftly across a large area. On the back edge of the rainband associated with mature squall lines, a wake low can be present, on very rare occasions associated with a heat burst.

In meteorology, an occluded front is a type of weather front formed during cyclogenesis. The classical and usual view of an occluded front is that it starts when a cold front overtakes a warm front near a cyclone, such that the warm air is separated (occluded) from the cyclone center at the surface. The point where the warm front becomes the occluded front is the triple point; a new area of low-pressure that develops at this point is called a triple-point low. A more modern view of the formation process suggests that occluded fronts form directly without the influence of other fronts during the wrap-up of the baroclinic zone during cyclogenesis, and then lengthen due to flow deformation and rotation around the cyclone as the cyclone forms.

The Antarctic oscillation (AAO, to distinguish it from the Arctic oscillation or AO), also known as the Southern Annular Mode (SAM), is a low-frequency mode of atmospheric variability of the Southern Hemisphere that is defined as a belt of strong westerly winds or low pressure surrounding Antarctica which moves north or south as its mode of variability.

It is a climate driver for Australia, influencing the country's weather conditions – It is associated with storms and cold fronts that move from west to east that bring precipitation to southern Australia.

A warm front is a density discontinuity located at the leading edge of a homogeneous warm air mass, and is typically located on the equator-facing edge of an isotherm gradient. Warm fronts lie within broader troughs of low pressure than cold fronts, and move more slowly than the cold fronts which usually follow because cold air is denser and less easy to remove from the Earth's surface. This also forces temperature differences across warm fronts to be broader in scale. Clouds ahead of the warm front are mostly stratiform, and rainfall generally increases as the front approaches. Fog can also occur preceding a warm frontal passage. Clearing and warming is usually rapid after frontal passage. If the warm air mass is unstable, thunderstorms may be embedded among the stratiform clouds ahead of the front, and after frontal passage thundershowers may continue. On weather maps, the surface location of a warm front is marked with a red line of semicircles pointing in the direction of travel.

The 2005 Azores subtropical storm was the 19th nameable storm and only subtropical storm of the extremely active 2005 Atlantic hurricane season. It was not named by the National Hurricane Center as it was operationally classified as an extratropical low. It developed in the eastern Atlantic Ocean, an unusual region for late-season tropical cyclogenesis. Nonetheless, the system was able to generate a well-defined centre convecting around a warm core on 4 October. The system was short-lived, crossing over the Azores later on 4 October before becoming extratropical again on 5 October. No damages or fatalities were reported during that time. Its remnants were soon absorbed into a cold front. That system went on to become Hurricane Vince, which affected the Iberian Peninsula.

The subtropical nature of this unnamed system was determined several months after the fact, while the National Hurricane Center was performing its annual review of the season. Upon reclassification, the storm was entered into HURDAT, the official hurricane database.

Hurricane Ophelia, known as Storm Ophelia while extratropical, was regarded as the worst storm to affect Ireland in 50 years, and was also the easternmost Atlantic major hurricane on record. The tenth and final consecutive hurricane and the sixth major hurricane of the very active 2017 Atlantic hurricane season, Ophelia had non-tropical origins from a decaying cold front on 6 October. Located within a favourable environment, the storm steadily strengthened over the next two days, drifting north and then southeastwards before becoming a hurricane on 11 October. After becoming a Category 2 hurricane and fluctuating in intensity for a day, Ophelia intensified into a major hurricane on 14 October south of the Azores, brushing the archipelago with high winds and heavy rainfall. Shortly after achieving peak intensity, Ophelia began weakening as it accelerated over progressively colder waters to its northeast towards Ireland and Great Britain. Completing an extratropical transition early on 16 October, Ophelia became the second storm of the 2017–18 European windstorm season. Early on 17 October, the cyclone crossed the North Sea and struck western Norway, with wind gusts up to 70 kilometres per hour (43 mph) in Rogaland county, before weakening during the evening of 17 October. The system then moved across Scandinavia, before dissipating over Norway on the next day.

Three deaths can be directly attributed to Ophelia, all of which occurred in Ireland. Total losses from the storm were less than initially feared, with a minimum estimate of total insured losses across Ireland and the United Kingdom of US$87.7 million.

Inflow is the flow of a fluid into a large collection of that fluid. Within meteorology, inflow normally refers to the influx of warmth and moisture from air within the Earth's atmosphere into storm systems. Extratropical cyclones are fed by inflow focused along their cold front and warm fronts. Tropical cyclones require a large inflow of warmth and moisture from warm oceans in order to develop significantly, mainly within the lowest 1 kilometre (0.62 mi) of the atmosphere. Once the flow of warm and moist air is cut off from thunderstorms and their associated tornadoes, normally by the thunderstorm's own rain-cooled outflow boundary, the storms begin to dissipate. Rear inflow jets behind squall lines act to erode the broad rain shield behind the squall line, and accelerate its forward motion.

An arcus cloud is a low, horizontal cloud formation, usually appearing as an accessory cloud to a cumulonimbus. Roll clouds and shelf clouds are the two main types of arcus clouds. They most frequently form along the leading edge or gust fronts of thunderstorms; some of the most dramatic arcus formations mark the gust fronts of derecho-producing convective systems. Roll clouds may also arise in the absence of thunderstorms, forming along the shallow cold air currents of some sea breeze boundaries and cold fronts.

In meteorology, an undular bore is a wave disturbance in the Earth's atmosphere and can be seen through unique cloud formations. They normally occur within an area of the atmosphere which is stable in the low levels after an outflow boundary or a cold front moves through.

In hydraulics, an undular bore is a gentle bore with an undular hydraulic jump pattern at the downstream (subcritical) side.