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Low-pressure area

Low-pressure area is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Low-pressure area rather than just read about it. In short: In meteorology, a low-pressure area (LPA), low area, or low is a region where the atmospheric pressure is lower than that of surrounding locations. It is the opposite of a high-pressure area.

Low-pressure area — main illustration
Low-pressure area — illustration

Key takeaways

  • Low-pressure area belongs to science; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Low-pressure area to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Low-pressure area from memory before moving on to harder problems.

Reference excerpt

In meteorology, a low-pressure area (LPA), low area, or low is a region where the atmospheric pressure is lower than that of surrounding locations. It is the opposite of a high-pressure area. Low-pressure areas are commonly associated with inclement weather (such as cloudy, windy, with possible rain or storms), while high-pressure areas are associated with lighter winds and clear skies. Winds circle anti-clockwise around lows in the northern hemisphere, and clockwise in the southern hemisphere, by dint of opposing Coriolis forces. Large low-pressure, rotating air masses whose winds spiral inward are termed cyclones. Low-pressure systems form under areas of wind divergence that occur in the upper levels of the atmosphere (aloft). The formation process of a low-pressure area is known as cyclogenesis. In meteorology, atmospheric divergence aloft occurs in two kinds of places:

The first is in the area on the east side of upper troughs, which form half of a Rossby wave within the Westerlies (a trough with large wavelength that extends through the troposphere). A second is an area where wind divergence aloft occurs ahead of embedded shortwave troughs, which are of smaller wavelength. Diverging winds aloft, ahead of these troughs, cause atmospheric lift within the troposphere below as air flows upwards away from the surface, which lowers surface pressures as this upward motion partially counteracts the force of gravity packing the air close to the ground. Thermal lows form through localized heating caused by greater solar incidence over deserts and other land masses. Since localized areas of warm air are less dense than their surroundings, this warmer air rises, which lowers atmospheric pressure near that portion of the Earth's surface. Large-scale thermal lows over continents help drive monsoon circulations. Low-pressure areas can also be caused by organized thunderstorm activity over warm water. When this occurs over the tropics in concert with the Intertropical Convergence Zone, it is known as a monsoon trough. Monsoon troughs reach their northerly extent in August and their southerly extent in February. When a convective low acquires a well-hot circulation in the tropics it is termed a tropical cyclone. Tropical cyclones can form during any month of the year globally but can occur in either the northern or southern hemisphere during December. Atmospheric lift will also generally produce cloud cover through adiabatic cooling once the air temperature drops below the dew point as it rises, the cloudy skies typical of low-pressure areas act to dampen diurnal temperature extremes. Since clouds reflect sunlight, incoming shortwave solar radiation decreases, causing lower temperatures during the day. At night the absorptive effect of clouds on outgoing longwave radiation, such as heat energy from the surface, allows for warmer nighttime minimums in all seasons. The stronger the area of low pressure, the stronger the winds experienced in its vicinity. Globally, low-pressure systems are most frequently located over the Tibetan Plateau and in the lee of the Rocky Mountains. In Europe (particularly in the British Isles and Netherlands), recurring low-pressure weather systems are typically known as "low levels." On English-language weather maps, low-pressure centers are identified by the letter L.

Formation

Cyclogenesis is the development and strengthening of cyclonic circulations, or low-pressure areas, within the atmosphere. Cyclogenesis is the opposite of cyclolysis, and has an anticyclonic (high-pressure system) equivalent which deals with the formation of high-pressure areas—anticyclogenesis. Cyclogenesis is an umbrella term for several different processes, all of which result in the development of some sort of cyclone. Meteorologists use the term "cyclone" where circular pressure systems flow in the direction of the Earth's rotation, which normally coincides with areas of low pressure. The largest low-pressure systems are cold-core polar cyclones and extratropical cyclones which lie on the synoptic scale. Warm-core cyclones such as tropical cyclones, mesocyclones, and polar lows lie within the smaller mesoscale. Subtropical cyclones are of intermediate size. Cyclogenesis can occur at various scales, from the microscale to the synoptic scale. Larger-scale troughs, also called Rossby waves, are synoptic in scale. Shortwave troughs embedded within the flow around larger scale troughs are smaller in scale, or mesoscale in nature. Both Rossby waves and shortwaves embedded within the flow around Rossby waves migrate equatorward of the polar cyclones located in both the Northern and Southern hemispheres. All share one important aspect, that of upward vertical motion within the troposphere. Such upward motions decrease the mass of local atmospheric columns of air, which lowers surface pressure. Extratropical cyclones form as waves along weather fronts when a shortwave aloft or upper-level jet streak passes by before occluding later in their life cycle as cold-core cyclones. Polar lows are small-scale, short-lived atmospheric low-pressure systems that occur over the ocean areas poleward of the main polar front in both the Northern and Southern Hemispheres. They are part of the larger class of mesoscale weather-systems. Polar lows can be difficult to detect using conventional weather reports and are a hazard to high-latitude operations, such as shipping and offshore platforms. They are vigorous systems that have near-surface winds of at least 17 metres per second (38 mph).

… excerpt ends here. Continue reading the full article.

Illustrations

Low-pressure area: A low-pressure system over Iceland
A low-pressure system over Iceland
Low-pressure area: This depiction of the Hadley cell shows the process which sustains low-pressure areas. Diverging winds aloft allow for lower pressure and convergence at the Earth's surface, which leads to upward motion.
This depiction of the Hadley cell shows the process which sustains low-pressure areas. Diverging winds aloft allow for lower pressure and convergence at the Earth's surface, which leads to upward motion.
Low-pressure area: QuikSCAT image of typical extratropical cyclones over the ocean. Note the maximum winds on the poleward side of the occluded front.
QuikSCAT image of typical extratropical cyclones over the ocean. Note the maximum winds on the poleward side of the occluded front.
Low-pressure area: February position of the ITCZ and monsoon trough in the Pacific Ocean, depicted by area of convergent streamlines offshore Australia and in the equatorial eastern Pacific
February position of the ITCZ and monsoon trough in the Pacific Ocean, depicted by area of convergent streamlines offshore Australia and in the equatorial eastern Pacific
Low-pressure area: A visible image of Hurricane Dennis intensifying in the Jamaica Channel
A visible image of Hurricane Dennis intensifying in the Jamaica Channel

Worked examples

Example 1 — a first encounter with Low-pressure area

Start with the simplest possible case. Write down what Low-pressure area claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Low-pressure area before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Low-pressure area ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Low-pressure area

In research
Low-pressure area appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Low-pressure area in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Low-pressure area is common in secondary-school and first-year university syllabi. It links to neighbouring topics Atmospheric dynamics, Atmospheric pressure, Meteorological phenomena, so understanding it makes those chapters shorter.
In everyday life
Look for Low-pressure area outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study Low-pressure area in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Low-pressure area means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Low-pressure area out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Low-pressure area in simple terms?

In meteorology, a low-pressure area (LPA), low area, or low is a region where the atmospheric pressure is lower than that of surrounding locations. It is the opposite of a high-pressure area.

Why does Low-pressure area matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Low-pressure area?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Low-pressure area.

Tags

  • Atmospheric dynamics
  • Atmospheric pressure
  • Meteorological phenomena
  • Types of cyclone
  • Vortices

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