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Isobar

An isobar is a type of contour line drawn on a weather map that connects points of equal atmospheric pressure, typically measured in millibars (mb) and reduced to sea level for consistency.^[1] These lines are essential tools in meteorology for visualizing pressure patterns across a region, with isobars commonly spaced every 4 mb on standard charts.^[2] Isobars help identify key weather features, such as high-pressure systems (anticyclones) where lines form closed contours around a central maximum, and low-pressure systems (cyclones) with contours encircling a minimum, influencing weather conditions like clear skies under highs and stormy conditions under lows.^[2] The spacing between isobars indicates the pressure gradient: closely packed lines signify steep gradients and stronger winds, while widely spaced lines denote gentler gradients and lighter winds, as wind speed is proportional to the rate of pressure change over distance.^[2] On isobaric charts, which depict conditions at constant pressure levels (e.g., 500 mb), the lines represent varying altitudes rather than fixed elevations, allowing meteorologists to analyze upper-air patterns and forecast phenomena like jet streams.^[1] Beyond meteorology, the term "isobar" also refers to nuclides in nuclear physics—atoms of different elements sharing the same mass number but differing in atomic number—though this usage stems from the Greek roots "isos" (equal) and "baros" (weight), mirroring the pressure connotation.^[3] In thermodynamics, an isobaric process describes a system undergoing change at constant pressure, such as heating a gas in an open container.^[4] These concepts underscore the term's broad application in describing equilibrium or constancy in physical systems.

Meteorology

Definition

An isobar is a type of contour line drawn on a weather map that connects points of equal atmospheric pressure, typically measured in millibars (mb) or hectopascals (hPa; 1 mb ≈ 1 hPa) and reduced to sea level for consistency across varying elevations.^[1] The term derives from the Greek words isos (equal) and baros (weight), reflecting equal pressure. The concept originated in the 19th century, with the first weather map featuring isobars published by English meteorologist Francis Galton in 1863.^[5]

Mapping and Representation

Isobars are depicted as smooth, continuous curves on meteorological charts, either closed loops encircling pressure centers or open curves extending across the map, connecting points of equal atmospheric pressure reduced to mean sea level.^[6] These lines are drawn without sharp angles or intersections to reflect the gradual variation in pressure fields, with a standard contour interval of 4 millibars for surface charts, starting from values like 1000 mb and proceeding in increments such as 1004 mb or 996 mb.^[6] The spacing between adjacent isobars visually represents the pressure gradient, where closely packed lines indicate a steep gradient and stronger winds, while widely spaced lines denote a gentler gradient and lighter winds. Labeling follows standardized conventions to ensure clarity, with each isobar marked by its pressure value using the last two digits of the millibar reading—for instance, "13" for 1013 mb or "24" for 1024 mb—typically placed along the line at map edges or in open spaces for closed contours.^[6] Solid lines are used for primary isobars at standard intervals, while dashed or thinner lines may denote auxiliary levels if needed for finer detail in complex pressure fields.^[7] Pressure centers are annotated with "H" for highs and "L" for lows, positioned at the respective minima or maxima.^[6] Isobars appear on various types of meteorological maps, including synoptic charts compiled from simultaneous observations at standard times (every 6 hours, such as 00Z, 06Z, 12Z, and 18Z UTC) to capture evolving weather systems across large regions, and surface analysis maps that provide near-real-time depictions updated every 3 hours or more frequently. Map projections influence isobar shapes; for example, polar stereographic projections, commonly used for high-latitude analyses, introduce distortion near the edges, stretching isobars and altering their apparent curvature in polar regions.^[8] Historically, isobars were plotted manually by meteorologists using pencils on paper charts, interpolating pressure data from station reports to draw contours by hand. In modern practice, digital tools such as Geographic Information Systems (GIS) software automate contouring through algorithms that interpolate data points, integrating real-time inputs from satellites, radar, and automated weather stations for dynamic isobar generation.^[9] Common isobar patterns include closed circular contours around highs (anticyclones), where pressure increases outward from the center, and similar loops around lows (cyclones), where pressure decreases toward the center, forming nearly concentric shapes that outline the pressure system's boundaries.^[10] These patterns may elongate into troughs or ridges if the pressure field is asymmetric, but they remain smooth to avoid abrupt changes.^[10]

Role in Weather Analysis

Isobars are essential in weather analysis for illustrating horizontal pressure gradients that drive atmospheric motion through the pressure gradient force, which accelerates air from higher to lower pressure regions. The closeness of isobars reflects the gradient's intensity, with tighter spacing indicating stronger forces and faster winds; in geostrophic balance, this force is counteracted by the Coriolis effect, causing winds to flow parallel to the isobars rather than directly across them. In the Northern Hemisphere, the Coriolis deflection results in winds circulating clockwise around high-pressure centers and counterclockwise around lows, with low pressure typically to the left of the wind direction.^[11]^[12] Isobar patterns reveal key weather associations, as closed contours of high pressure denote anticyclones with subsiding air that inhibits cloud formation, leading to clear skies and light winds, whereas low-pressure systems feature rising air that promotes condensation, storms, and unsettled conditions. Sharp kinks or discontinuities in isobars often signify frontal boundaries, where abrupt pressure changes delineate the edges of contrasting air masses and signal potential shifts in temperature, humidity, and precipitation.^[13] For forecasting, isobar configurations enable predictions of system evolution, such as tracking storm paths via the movement of low-pressure centers or identifying intensifying cyclones through progressively tighter isobars that forecast escalating winds and precipitation. Upper-level isobar analyses on constant-pressure charts further delineate jet streams as narrow zones of closely packed lines, influencing mid-latitude storm tracks and large-scale weather patterns.^[14] Surface isobar maps, however, only capture horizontal pressure distributions at a reference level like sea level and overlook vertical variations critical for understanding full atmospheric dynamics, requiring supplementation with upper-air charts on constant-pressure surfaces to assess three-dimensional structure.^[15] In modern practice, isobaric fields from numerical weather prediction models, such as the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecasting System, are integrated with radar data to construct comprehensive 3D analyses, enhancing short-term nowcasting of precipitation and storm intensity while improving medium-range forecasts of pressure-driven phenomena.^[16]^[17]

Nuclear Physics

Definition

In nuclear physics, isobars are defined as nuclides—atomic nuclei characterized by a specific number of protons and neutrons—that possess the same mass number A, which is the total number of nucleons (protons Z plus neutrons N, so A = Z + N), but differ in their atomic number Z. Consequently, isobars belong to different chemical elements, as the varying Z determines the element while A remains constant.^[18]^[19]^[20] This terminology distinguishes isobars from related nuclear concepts: isotopes share the same Z but have different A, whereas isotones have the same N but different A. The term "isobars" (originally "isobares") was coined by British chemist Alfred Walter Stewart in 1918, derived from the Greek words isos (equal) and baros (weight), reflecting their equal mass in early atomic weight scales; it emerged alongside the discovery of isotopes by Frederick Soddy around 1913 and was formalized in nuclear nomenclature during the 1930s through advancements in mass spectrometry that enabled precise identification of nuclidic masses.^[18]^[21] Isobars are conventionally notated using the symbol ^{A}_{Z}\mathrm{X}, where \mathrm{X} is the chemical symbol of the element; for a fixed A, multiple elements can form isobars, such as those with A = 40: ^{40}_{18}\mathrm{Ar}, ^{40}_{19}\mathrm{K}, and ^{40}_{20}\mathrm{Ca}. This concept is grounded in the conservation of nucleon number A during nuclear reactions, such as beta decay, where Z changes but A remains unchanged. Stable isobars are rare, as differences in nuclear binding energies typically result in only one stable nuclide per mass number A, with the others being radioactive.^[18]^[20]^[22]

Properties and Stability

Isobars, nuclei sharing the same mass number A but differing in atomic number Z, exhibit variations in binding energy primarily due to the imbalance between neutrons and protons, which affects nuclear stability. The semi-empirical mass formula (SEMF) provides an approximation for the binding energy B(A,Z) of these nuclides, capturing the dominant contributions from nuclear structure. The formula is given by

B(A,Z) \approx a_v A - a_s A^{2/3} - a_c \frac{Z(Z-1)}{A^{1/3}} - a_a \frac{(A - 2Z)^2}{A} \pm \delta,

where a_v represents the volume term, accounting for the attractive strong force throughout the nucleus; a_s the surface term, reflecting reduced binding at the nuclear surface; a_c the Coulomb term, arising from electrostatic repulsion among protons; a_a the asymmetry term, which penalizes deviations from the ideal neutron-to-proton ratio; and \delta the pairing term, which favors even-even or odd-odd configurations for enhanced stability./01%3A_Introduction_to_Nuclear_Physics/1.02%3A_Binding_energy_and_Semi-empirical_mass_formula) The asymmetry term in the SEMF is particularly influential for isobars, as it increases the binding energy penalty for neutron-rich or proton-rich configurations relative to the optimal N/Z ratio, leading to lower overall binding energies and reduced stability for those nuclides. For a fixed A, isobars with Z far from the value that minimizes the asymmetry term (A - 2Z)^2/A are less bound and prone to decay. Stability trends among isobars favor even Z for even A, as even-even nuclei benefit from the positive pairing term \delta \approx +11 A^{-1/2} MeV, enhancing their binding compared to neighboring odd-A or odd-odd isobars. Beta decay processes link isobars within the same A chain, with neutron-rich isobars undergoing \beta^- decay to convert a neutron to a proton, shifting toward more stable, proton-richer configurations, while proton-rich isobars may undergo \beta^+ or electron capture.^[23]/01%3A_Introduction_to_Nuclear_Physics/1.02%3A_Binding_energy_and_Semi-empirical_mass_formula) In visualizations of isobaric stability, the mass excess \Delta = [M(A,Z) - Z m_H - (A-Z) m_n] c^2, where M is the atomic mass, m_H the hydrogen mass, and m_n the neutron mass, is plotted against Z for fixed A, forming a parabolic curve derived from the SEMF's dominant volume, Coulomb, and asymmetry terms. The parabola's minimum occurs at the Z yielding the most stable isobar, corresponding to an optimal N/Z ratio of approximately 1 for light nuclei (A \lesssim 20) and increasing to about 1.5 for heavy nuclei (A \gtrsim 200) due to the growing Coulomb repulsion requiring more neutrons for stability. Isobars above or below this minimum lie on the parabola's ascending arms and are unstable against beta decay toward the valley minimum.^[24]/01%3A_Introduction_to_Nuclear_Physics/1.02%3A_Binding_energy_and_Semi-empirical_mass_formula) Most isobars are unstable, with stability limited to specific cases where binding energies align closely with the parabola minimum; for low mass numbers like A=1 to $3, there are no multiple stable isobars, as only single nuclides (e.g., ^1H for A=1) achieve sufficient binding without viable alternatives. For higher A, such as A=80, two stable isobars exist: ^{80}_{34}\mathrm{Se} and ^{80}_{36}\mathrm{Kr}, though most other isobars for that mass number decay via beta processes to reach the stable line. In general, at most two stable nuclides exist for any given A. Experimental determination of isobar properties relies on precision mass spectrometry, such as Penning trap measurements, to resolve atomic masses and binding energies, complemented by decay studies that measure Q-values and half-lives to map stability along isobaric chains.^[24]^[25]^[26]^[27]

Applications and Examples

In nuclear reactions, isobaric analog states (IAS) serve as powerful probes for elucidating nuclear structure, particularly isospin symmetries and mixing effects, by allowing comparisons across isobars with the same mass number but differing proton numbers.^[28] Charge-exchange reactions, such as the (p,n) process, are commonly employed to excite these states, enabling the study of isobaric resonances that reveal details about nuclear forces and excitation energies.^[29] For instance, (p,n) reactions on targets like ^{48}Ca have been analyzed using folding models to extract isovector potentials and analog state properties, providing insights into charge-dependent nuclear interactions.^[28] In astrophysics, weak interactions play a critical role in stellar nucleosynthesis by facilitating beta decays that connect isobars, thereby influencing the final abundances of elements produced in explosive environments like supernovae.^[30] These processes, governed by stellar weak interaction rates, determine pathways between isobars during rapid neutron capture (r-process) events in core-collapse supernovae, where delayed neutron emissions and beta decays alter isotopic yields for neutron-rich nuclei.^[31] Accurate computation of these rates, as in intermediate-mass nuclei relevant to type Ia supernovae, is essential for modeling element synthesis and matching observed galactic abundances.^[30] A prominent application in medical and geochronological contexts involves the isobars ^{14}\mathrm{C} (Z=6) and ^{14}\mathrm{N} (Z=7), where the beta decay of ^{14}\mathrm{C} to ^{14}\mathrm{N} underpins radiocarbon dating for organic materials up to about 50,000 years old.^[32] This decay chain, with a half-life of 5,730 years, allows precise age determination by measuring the residual ^{14}\mathrm{C} ratio relative to stable carbon isotopes, as the production of ^{14}\mathrm{C} from cosmic-ray interactions with atmospheric nitrogen maintains equilibrium in living organisms./Nuclear_Chemistry/Applications_of_Nuclear_Chemistry/Radiocarbon_Dating) Concrete examples illustrate isobaric relationships in decay processes. The A=40 isobaric triplet consists of unstable ^{40}\mathrm{K} (Z=19), which decays primarily via beta minus emission (89.3%) to stable ^{40}\mathrm{Ca} (Z=20) and electron capture (10.7%) to stable ^{40}\mathrm{Ar} (Z=18), forming the basis for K-Ar and K-Ca geochronology in dating ancient rocks.^[33] Similarly, for A=238 in the actinide series, ^{238}\mathrm{U} (Z=92) and short-lived ^{238}\mathrm{Np} (Z=93, half-life ~2.4 days) represent isobars encountered in nuclear fuel cycles and transuranic production, where ^{238}\mathrm{Np} arises from neutron interactions and beta decays within the chain.^[34] In geochemical research, isobar suppression techniques in mass spectrometry are vital for achieving precise isotope ratio measurements, mitigating interferences from atomic isobars that overlap in mass-to-charge ratios.^[35] Methods such as laser photodetachment in accelerator mass spectrometry (AMS) selectively neutralize interfering isobars like ^{36}\mathrm{Cl} relative to ^{36}\mathrm{S}, enabling high-accuracy determinations of ratios such as ^{238}\mathrm{U}/^{235}\mathrm{U} in environmental samples for tracing geochemical processes.^[35] These approaches enhance sensitivity in inductively coupled plasma mass spectrometry (ICP-MS) for actinide isotope analyses, crucial for nuclear forensics and paleoclimate studies.^[36]

References

[1]
Isobar - NOAA's National Weather Service - Glossary
Isobar: A line connecting points of equal pressure. Isobaric Chart: A weather map representing conditions on a surface of equal atmospheric pressure.
[2]
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/prs/isb.rxml
[3]
ISOBAR Definition & Meaning - Merriam-Webster
1. an imaginary line or a line on a map or chart connecting or marking places of equal barometric pressure 2. one of two or more atoms or elements having the ...
[4]
Isobars - Nuclear | Definition & Characteristics | nuclear-power.com
The term “isobars” (originally “isobares”) for nuclides was suggested by British chemist Alfred Walter Stewart in 1918. It is derived from the Greek word ...
[5]
https://www.workybooks.com/resources/climate-science/isobar
[6]
Some Basic Facts of Nuclear Physics
Different values of A or N for a given element lead to different isotopes, while nuclei with the same A and different Z are referred to as isobars.
[7]
[PDF] Structure of Atomic Nuclei
Isobars are nuclides that have the same mass number A, but different Z (and ... Its value depends on how the nuclear radius R is defined. One such ...
[8]
Stable Nuclides - an overview | ScienceDirect Topics
For odd A, usually only one stable isobar exists. For even A, usually only ... A species of atom characterized by its mass number, atomic number, and nuclear ...
[9]
Learning Lesson: Drawing Conclusions - Surface Air Pressure Map
Dec 11, 2023 · Isobars can be used to identify Highs and Lows. The pressure in a High is greater than the surrounding air. The pressure in a Low is lower than ...Missing: definition | Show results with:definition
[10]
CHART COMPARISON
Understanding pressure contour lines (isobars) is the key to interpreting the chart. Without understanding isobars, temperature advection, wind speed/direction ...Missing: WMO | Show results with:WMO
[11]
[PDF] FORECASTING MANUAL - IMD Pune
the maximum distortion is less than 1% for the area lying between the Equator and 50°N. In the case of Polar Stereographic Projection the maximum distortion.
[12]
A GIS application for weather analysis and forecasting
Sep 1, 2009 · The forecaster draws contours or isobars of pressure and marks the fronts, lows highs etc. on the chart. The contouring can be done by Arc View ...Missing: historical modern
[13]
Cyclones and Anticyclones - The Physical Environment
Cyclones usually exhibit nearly circular isobars. If isobars are oblong or elongate with the lowest pressure near the center we call them troughs. As air enters ...
[14]
Origin of Wind | National Oceanic and Atmospheric Administration
Apr 3, 2023 · The closer the isobars are drawn together the quicker the air pressure changes. This change in air pressure is called the "pressure gradient".Missing: effect | Show results with:effect
[15]
winds balanced by the Coriolis and Pressure Gradient forces
As the wind gains speed, the deflection increases until the Coriolis force equals the pressure gradient force. At this point, the wind will be blowing parallel ...
[16]
How to read Surface Weather Maps - NOAA
Sep 23, 2025 · Weather maps use fronts (cold, warm, stationary, occluded) to show air mass boundaries. Cold fronts are blue with triangles, warm fronts are ...Missing: conventions | Show results with:conventions
[17]
Mariner's Guide to the 500 – Millibar Chart
Oct 31, 2023 · Jet Streams exist due to horizontal temperature contrasts. ... These waves are responsible for the overall weather pattern or surface storm track.
[18]
Constant Pressure vs. Constant Elevation - NOAA
Apr 14, 2023 · Therefore, an isobar is a line representing the location where the pressure is equal (the same) along that line. Typical levels of constant ...
[19]
[PDF] Atmospheric Thermodynamics - ECMWF
The isobars appear as quasi-horizontal lines in Figure 2. Furthermore, knowing T and p one can compute the saturation specific humidity qvs or mixing ratio ...
[20]
Integrated Forecasting System | ECMWF
It includes a sophisticated data assimilation system and global numerical model of the Earth system, as well as supporting infrastructure to make forecast ...
[21]
[PDF] The semi-empirical mass formula (SEMF) Isobar mass chains
The semi-empirical mass formula (SEMF). The binding energy of a nucleus can be modelled as. B(Z, A) = av A−as A2/3. −aC Z(Z−1)A−1/3−asym (A−2Z)2/A+.
[22]
[PDF] Symmetry Energy in the Semi-empirical Mass Formula
Nov 9, 2021 · Equation (4) yields that the most stable isobar has Z≈ 56 for. A = 135 and it is in pretty agreement with observation. Apart from the importance ...
[23]
Direct mass measurements of A=80 isobars
Direct mass measurements of A=80 isobars were performed using the second cyclotron of SARA (Système Accélérateur Rhône-Alpes) of the IS.
[24]
[PDF] Precise mass measurements of A = 133 isobars with the Canadian ...
Sep 23, 2024 · We report precision mass measurements of 133Sb, 133g,mTe, and 133g,mI, produced at CARIBU at Argonne National.
[25]
Folding model study of the isobaric analog excitation: Isovector ...
Jul 9, 2007 · ( p , n ) A ˜ IAS reaction to isobaric analog states. However, more interesting structure information can be obtained when U 0 ( 1 ) are ...
[26]
Analysis of nuclear structure in the nuclear chart and improvement to ...
Mar 1, 2019 · The form is considered through experimental ( p , n ) reactions. FIG ... isobaric analog state. Figure 2 shows schematic diagrams of ...
[27]
[PDF] Nucleosynthesis in thermonuclear supernovae - arXiv
Apr 3, 2017 · Nucleosynthesis in thermonuclear supernovae. 19. Fuller GM, Fowler WA, Newman MJ (1982) Stellar weak interaction rates for intermediate-mass.
[28]
[PDF] arXiv:2505.03488v1 [nucl-th] 6 May 2025
May 6, 2025 · As per previous simulation results, weak interaction rates on nickel isotopes have significant influence on the stellar core vis-`a-vis ...
[29]
Radiocarbon Dating - American Chemical Society
Oct 10, 2016 · In 1946, Willard Libby proposed an innovative method for dating organic materials by measuring their content of carbon-14, a newly discovered radioactive ...
[30]
Production of 40 Ar by an overlooked mode of 40 K decay ... - GChron
Nov 26, 2020 · The decay of 40K to the stable isotopes 40Ca and 40Ar is used as a measure of time for both the K-Ca and K-Ar geochronometers, the latter of ...
[31]
[PDF] Transmission Resonance Spectroscopy of the Doubly Odd 238Np in ...
238Np is a very interesting isotope regarding hyperdeformation: it is an isobar of 238U, where a hyperdeformed third minimum has already been settled [6]. 2.
[32]
Isobar suppression in AMS using laser photodetachment
The basic idea is to neutralize the isobars by selective photodetachment with laser light whereas the radioisotope ions remain unaffected. To adopt this method ...
[33]
Recent progress on mass spectrometric analysis of artificial ...
In order to improve analyte sensitivity and minimize isobaric interferences for the determination of uranium isotopic ratios, Ohno et al. [39] applied a ...