Area density
Area density | |
---|---|
Common symbols | |
SI unit | kg/m2 |
In SI base units | m−2⋅kg |
Derivations from other quantities | |
Dimension |
The area density (also known as areal density, surface density, superficial density, areic density, mass thickness, column density, or density thickness) of a two-dimensional object is calculated as the mass per unit area. The SI derived unit is the kilogram per square metre (kg·m−2).
A related
In the paper and fabric industries, it is called
Formulation
Area density can be calculated as:
Column density
A special type of area density is called column density (also columnar mass density or simply column density), denoted ρA or σ. It is the mass of substance per unit area integrated along a path;[1] It is obtained integrating volumetric density over a column:[2]
In general the integration path can be slant or oblique incidence (as in, for example,
Columnar density is closely related to the vertically averaged volumetric density as
Usage
Atmospheric physics
It is a quantity commonly retrieved by
A closely related concept is that of ice or liquid water path, which specifies the volume per unit area or depth instead of mass per unit area, thus the two are related:
Another closely related concept is optical depth.
Astronomy
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In astronomy, the column density is generally used to indicate the number of atoms or molecules per square cm (cm2) along the line of sight in a particular direction, as derived from observations of e.g. the 21-cm hydrogen line or from observations of a certain molecular species. Also the interstellar extinction can be related to the column density of H or H2.[6]
The concept of area density can be useful when analysing
where denotes the vertical coordinate (e.g., height or depth), or as the number or count of a substance—rather than the mass—per unit area integrated along a path (column number density):
Data storage media
Areal density is used to quantify and compare different types media used in data storage devices such as
Paper
The area density is often used to describe the thickness of paper; e.g., 80 g/m2 is very common.
Fabric
Fabric "weight" is often specified as mass per unit area, grams per square meter (gsm) or ounces per square yard. It is also sometimes specified in ounces per yard in a standard width for the particular cloth. One gram per square meter equals 0.0295 ounces per square yard; one ounce per square yard equals 33.9 grams per square meter.
Other
It is also an important quantity for the absorption of radiation.
When studying bodies falling through air, area density is important because resistance depends on area, and gravitational force is dependent on mass.
Bone density is often expressed in grams per square centimeter (g·cm−2) as measured by x-ray absorptiometry, as a proxy for the actual density.
The body mass index is expressed in units of kilograms per square meter, though the area figure is nominal, being the square of the height.
The total electron content in the ionosphere is a quantity of type columnar number density.
See also
- Areic quantity
- Linear density
- Paper density
References
- ^ Egbert Boeker; Rienk van Grondelle (2000). Environmental Physics (2nd ed.). Wiley.
- ISBN 978-3-540-67420-7.
- ^
R. Sinreich; U. Frieß; T. Wagner; S. Yilmaz; U. Platt (2008). "Retrieval of Aerosol Distributions by Multi-Axis Differential Absorption Spectroscopy (MAX-DOAS)". Nucleation and Atmospheric Aerosols. pp. 1145–1149. ISBN 978-1-4020-6474-6.
- ^
C. Melsheimer; G. Heygster (2008). "Improved retrieval of total water vapor over polar regions from S2CID 20910677.
- ^
C. Melsheimer; G. Heygster; N. Mathew; L. Toudal Pedersen (2009). "Retrieval of Sea Ice Emissivity and Integrated Retrieval of Surface and Atmospheric Parameters over the Arctic from AMSR-E data". Journal of the Remote Sensing Society of Japan. Vol. 29, no. 1. pp. 236–241.
- ^ "Column Density | COSMOS".
- ^ "Areal Density". Webopedia. 3 March 1997. Retrieved April 9, 2014.