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9.26 Airmasses and Fronts

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Cold Front

The strong advancing cold air mass pushes the warm air mass causing the warm air to recede and is pushed up over the steep cold front. If laden with moisture, numerous cumulonimbus clouds develop with little separation over a rather narrow zone; often bring rainfall initially as thunderstorms and after the front passes cumulus clouds with showers.

The change of conditions experienced as a Cold Front passes are listed below.



Prior to Front

  • Temperature: Warm
  • Pressure: Decreasing Steadily
  • Wind: West
  • Precipitation: Showers
  • Cloud: Cirrostratus Clearing to Cumulus and Cumulonimbus

During Front

  • Temperature: Cool Quickly
  • Pressure: Steady then Increasing
  • Wind: Backing
  • Precipitation: Heavy Rain or Snow, Hail Sometimes
  • Cloud: Cumulus and Cumulonimbus

After Front

  • Temperature: Steadily Cooling
  • Pressure: Steady Rising
  • Wind: Southwest
  • Precipitation: Showers to Clearing
  • Cloud: Cumulus


Warm Front

Warm front weather begins prior to the front. They develop when low pressure, warmer, moist air overtakes a colder air. The warm air glides up and over the cold air mass causing the colder to recede. Precipitation is strung out over a broader area and thick nimbostratus and other stratified cloud types are characteristic. The diagram and table pertain to the passage of a Warm Front.

The change of conditions experienced as a Warm Front passes are listed below.

Prior to Front

  • Temperature: Cool
  • Pressure: Decreasing Steadily
  • Wind: Northwest
  • Precipitation: Rain, Snow, Sleet or Drizzle
  • Cloud: Cirrus, Cirrostratus, Altostratus, Nimbostratus

During Front

  • Temperature: Warming Quickly
  • Pressure: Steady
  • Wind: Backing
  • Precipitation: Drizzle
  • Cloud: Stratus

After Front

  • Temperature: Steadily Warm
  • Pressure: Slight Rise then Decrease
  • Wind: West
  • Precipitation: None
  • Cloud: Scattered Stratus to Clearing

Wave Depressions

The development of the cyclone follows a general sequence of stages beginning with the advance of an Arctic Cold Front into cooler air (to the south in the northern hemisphere) and often ending with an occluded phase.

In the five diagrams below, read the captions for information about the processes of a wave depression in the southern hemisphere.

1: The beginning of a large-scale cyclonic development occurs as a southern cold air mass moves north (and often with an eastern component) against an air mass that is cooler, or even describable as warm; each air mass is moving. At this first stage, the boundary between air masses becomes stationary and air above it is in a pressure trough as air diverges horizontally.

2: Air from the surface replaces the upper air and this leads to a pressure drop or a low along the front. At the surface, winds move towards two lower pressure centres and begin to circulate as a clockwise inner spiral. The process is aided by imbalances in the jet stream where air is forced into uplift.

3: The two fronts - cold and warm - are connected by an extra-tropical cyclone. This strengthens aloft and the process of cyclogenesis begins to produce stormy conditions. With continued pressure drop, the cold front advances into the warm sector and the angle between air masses lessens.

4: During this mature stage, prominent wave shapes are developed in each front (wave cyclones). If the storm tracks to the north of an observation point, that area will receive much rain if temperatures are warm or snow if the near surface conditions are cold.

5: The faster moving cold front eventually overtakes the warm front, developing an occluded state, driving the warm air overhead. In time, the cyclone weakens as the storm moves more to the east. The horizontal pressure gradient diminishes, dissipating the front (frontolysis) and the dissolving stage is reached. The steering of this frontal system is controlled by the Westerlies.

Occluded Front

A third situation results when colder air overtakes cool air associated with a cyclonic low in later stages of dissipation. The cold air behind a Cold Front overtakes a Warm Front and forces the relatively warmed (cool) air upwards, causing precipitation. This produces what is known as an Occluded Front, shown in its symbol version and in a cross-section drawing. If laden with moisture, numerous cumulonimbus clouds develop with little separation over a rather narrow zone; often bring rainfall initially as thunderstorms and after the front passes cumulus clouds with showers.

Tropical Cyclones

A tropical cyclone (or tropical depression, tropical storm, typhoon, or hurricane, depending on strength and geographical context) is a type of low pressure system which generally forms in the tropics. While they can be highly destructive, tropical cyclones are an important part of the atmospheric circulation system, which moves heat from the equatorial region toward the lower latitudes in the southern hemisphere.

Structurally: a tropical cyclone is a large, rotating system of clouds, wind and thunderstorm activity. Its primary energy source is the release of the heat of condensation from water vapour condensing at high altitudes, the heat ultimately derived from the sun. Therefore, a tropical cyclone can be thought of as a giant vertical heat engine supported by mechanics driven by physical forces such as the orbital revolution and gravity of the Earth. Continued condensation leads to higher winds, continued evaporation, and continued condensation, feeding back into itself. This gives rise to factors that give the system enough energy to be self-sufficient and cause a positive feedback loop where it can draw more energy as long as the source of heat, warm water, remains. Factors such as a continued lack of equilibrium in air mass distribution would also give supporting energy to the cyclone. The orbital revolution of the Earth causes the system to spin, giving it a cyclone characteristic and affecting the trajectory of the storm.

The factors to form a tropical cyclone include a pre-existing weather disturbance, warm tropical oceans, moisture, and relatively light winds aloft. If the right conditions persist and allow it to create a feedback loop by maximizing the energy intake possible, for example, such as high winds to increase the rate of evaporation, they can combine to produce the violent winds, incredible waves, torrential rains, and floods associated with this phenomenon.

Condensation as a driving force is what primarily distinguishes tropical cyclones from other meteorological phenomena, and because this is strongest in a tropical climate, this defines the initial domain of the tropical cyclone. By contrast, mid-latitude cyclones, for example, draw their energy mostly from pre-existing horizontal temperature gradients in the atmosphere. In order to continue to drive its heat engine, a tropical cyclone must remain over warm water, which provides the atmospheric moisture needed. The condensation of this moisture is driven by the high winds and reduced atmospheric pressure in the storm, resulting in a sustaining cycle. As a result, when a tropical cyclone passes over land, its strength diminishes rapidly.

Condition of Formation require five factors necessary to make tropical cyclone formation possible:

  1. Sea surface temperatures above 26.5 degrees Celsius to a depth of 64 feet or greater. The moisture in the air above the warm water is the energy source for tropical cyclones.
  2. Upper-atmosphere conditions conducive to thunderstorm formation. Temperature in the atmosphere must decrease quickly with height, and the mid-troposphere must be relatively moist.
  3. A pre-existing weather disturbance. This is most frequently provided by tropical waves—non-rotating areas of thunderstorms that move through tropical oceans.
  4. Nearly all of them form between 10 and 30 degrees of the equator and 87% form within 20 degrees of it. Because the Coriolis Effect initiates and maintains tropical cyclone rotation, such cyclones almost never form or move within about 10 degrees of the equator, where the Coriolis Effect is weakest.
  5. Low vertical wind shear (change in wind speed or direction over height). High wind shear can break apart the vertical structure of a tropical cyclone.

Tropical Cyclones Season: Worldwide, tropical cyclone activity peaks in late summer when water temperatures are warmest. However, each particular basin has its own seasonal patterns. In the Southern Hemisphere, tropical cyclone activity begins in November and ends in April. Southern Hemisphere activity peaks in mid-February to early March. Worldwide, an average 80 tropical cyclones form each year.

Tropical Cyclones Development:Tropical cyclones are classified into three main groups: tropical depressions, tropical storms, and a third group whose name depends on the region.

A tropical depression is an organized system of clouds and thunderstorms with a defined surface circulation and maximum sustained winds of less than 33 knots or 62 km/h. It has no eye, and does not typically have the spiral shape of more powerful storms. It is already becoming a low-pressure system, however, hence the name "depression".

A tropical storm is an organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds between 34–63 knots or 62–117 km/h. At this point, the distinctive cyclonic shape starts to develop, though an eye is usually not present.

At tropical cyclones intensity, they tend to develop an eye, an area of relative calm (and lowest atmospheric pressure) at the centre of the circulation. The eye is often visible in satellite images as a small, circular, cloud-free spot. Surrounding the eye is the eye wall, an area about 16 to 80 kilometres wide in which the strongest thunderstorms and winds circulate around the storm's centre.

The circulation of clouds around a cyclone's centre imparts a distinct spiral shape to the system. Bands or arms may extend over great distances as clouds are drawn toward the cyclone. The direction of the cyclonic circulation depends on the hemisphere; it is counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Maximum sustained winds in the strongest tropical cyclones have been measured at more than 165 knots and 305 km/h. intense mature tropical cyclones can sometimes exhibit an inward curving of the eye wall top that resembles a football stadium: this phenomenon is thus sometimes referred to as stadium effect.

Eye wall replacement cycles naturally occur in intense tropical cyclones. When cyclones reach peak intensity they usually - but not always - have an eye wall and radius of maximum winds that contract to a very small size, around 10 to 30 kilometres. At this point, some of the outer rainbands may organise into an outer ring of thunderstorms that slowly moves inward and robs the inner eye wall of its needed moisture and momentum. During this phase, the tropical cyclone is weakening (i.e. the maximum winds die off a bit and the central pressure goes up). Eventually the outer eye wall replaces the inner one completely and the storm can be the same intensity as it was previously or, in some cases, even stronger.

While the most obvious motion of clouds is toward the centre, tropical cyclones also develop an outward flow of clouds. These originate from air that has released its moisture and is expelled at high altitude through a chimney effect of the storm engine. This outflow produces high, thin cirrus clouds that spiral away from the centre. The high cirrus clouds may be the first signs of an approaching tropical cyclone.

Dissipation: A tropical cyclone can cease to have tropical characteristics in several ways.

  • It moves over land, thus depriving it of the warm water it needs to power itself, and quickly loses strength. Most strong storms become disorganised areas of low pressure within a day or two of landfall. There is, however, a chance they could regenerate if they manage to get back over open warm water. If a storm is over mountains for even a short time, it can rapidly lose strength. This is, however, the cause of many storm fatalities, as the dying storm unleashes torrential rainfall, and in mountainous areas, this can lead to deadly mudslides. The storm loses strength slower over flatter or marshy areas than over mountainous terrain which disrupts the surface circulation of the storm more.
  • It remains in the same area of ocean for too long, sucking up all the warm water. Without warm surface water, the storm cannot survive.
  • It experiences wind shear, causing the convection to lose direction and the heat engine to break down.
  • It can be weak enough to be consumed by another area of low pressure, disrupting it and joining to become a large area of non-cyclonic thunderstorms. (Such, however, can re-strengthen the non-tropical system as a whole.)
  • It enters colder waters. This does not necessarily mean the death of the storm, but the storm will lose its tropical characteristics. These storms are extra-tropical cyclones.
  • An outer eye wall forms (typically around 90 kilometres from the centre of the storm), choking off the convection toward the inner eye wall. Such weakening is generally temporary unless it meets other conditions above.
  • Even after a tropical cyclone is said to be extra-tropical or dissipated, it can still have tropical storm force winds and drop several inches of rainfall. When a tropical cyclone reaches higher latitudes or passes over land, it may merge with weather fronts or develop into a frontal cyclone, also called extra-tropical cyclone.

Equatorial Trough

The Intertropical convergence zone (ITCZ), also known as the Equatorial Trough, is a belt of low pressure around the equator. It is formed, as its name indicates, by the convergence of warm, moist air from the latitudes above and below the equator.

The air is drawn in to the intertropical convergence zone by the action of the Hadley cell, a macro-scale atmospheric feature which is part of the Earth's heat and moisture distribution system. It is transported aloft by the convective activity of thunderstorms; regions in the intertropical convergence zone receive precipitation more than 200 days in a year.

The location of the intertropical convergence zone varies over time, as it moves back and forth across the equator in a semi-annual pattern, following the sun's zenith point. There is also a diurnal cycle, with cumulus clouds developing around midday and building to thunderstorms in mid to late afternoon.

Variation in the location of the intertropical convergence zone drastically affects rainfall in many equatorial nations, resulting in the wet and dry seasons of the tropics rather than the cold and warm seasons of higher latitudes. Longer term changes in the intertropical convergence zone can result in severe droughts or flooding in nearby areas.

Because of the strength of the Hadley cells on either side of it, weather systems familiar to mid-latitude dwellers do not have the chance to form, and as a result, there are no prevailing winds. Advection (horizontal) motion is due entirely to air from the trade winds replacing that carried aloft by convection, a slow, languorous process at best.

Early sailors named this belt of calm the doldrums because of the low spirits they found themselves in after days of no wind.