Editing Area density (section)

==Usage==

===Atmospheric physics===
It is a quantity commonly retrieved by [[remote sensing]] instruments, for instance the [[Total Ozone Mapping Spectrometer]] (TOMS) which retrieves ozone columns around the globe.  Columns are also returned by the [[differential absorption spectroscopy|differential optical absorption spectroscopy (DOAS)]] method<ref name="Sinreich_etal2008">
{{cite news
 |author1=R. Sinreich |author2=U. Frieß |author3=T. Wagner |author4=S. Yilmaz |author5=U. Platt | title=Retrieval of Aerosol Distributions by Multi-Axis Differential Absorption Spectroscopy (MAX-DOAS)
 | journal=Nucleation and Atmospheric Aerosols
 | year=2008
 | pages= 1145–1149
 | doi=10.1007/978-1-4020-6475-3_227
|isbn=978-1-4020-6474-6 }}</ref> and are a common retrieval product from [[nadir]]-looking [[microwave]] [[radiometers]].<ref name="Melseimer_Heygster2008">
{{cite journal
 |author1=C. Melsheimer |author2=G. Heygster | year=2008
 | title=Improved retrieval of total water vapor over polar regions from [[Advanced Microwave Sounding Unit|AMSU]]-B microwave radiometer data
 | journal=IEEE Trans. Geosci. Remote Sens.
 | volume=46
 | number=8
 | pages=2307–2322
 | doi=10.1109/TGRS.2008.918013
|bibcode = 2008ITGRS..46.2307M |s2cid=20910677 }}</ref><ref name="Melseimer_etal2009">
{{cite news
 |author1=C. Melsheimer |author2=G. Heygster |author3=N. Mathew |author4=L. Toudal Pedersen | year=2009
 | title=Retrieval of Sea Ice Emissivity and Integrated Retrieval of Surface and Atmospheric Parameters over the Arctic from [[AMSR]]-E data| journal=Journal of the Remote Sensing Society of Japan | volume=29
 | number=1
 | pages= 236–241
}}</ref>

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:
<math display=block>P = \frac{\sigma}{\rho_0}.</math>

Another closely related concept is [[optical depth]].

===Astronomy===
{{expand section|date=August 2015}}

In astronomy, the column density is generally used to indicate the number of atoms or molecules per square cm (cm<sup>2</sup>) 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 [[Extinction (astronomy)|interstellar extinction]] can be related to the column density of H or H<sub>2</sub>.<ref>{{Cite web|url=http://astronomy.swin.edu.au/cosmos/C/Column+Density|title = Column Density &#124; COSMOS}}</ref>

The concept of area density can be useful when analysing [[accretion disk]]s. In the case of a disk seen face-on, area density for a given area of the disk is defined as column density: that is, either as the [[mass]] of [[chemical substance|substance]] per unit [[area]] [[integral|integrated]] along the vertical path that goes through the disk [[Sightline|(line-of-sight)]], from the bottom to the top of the medium:

<math display=block>\sigma = \int \rho \, \mathrm{d}z,</math>

where <math>z</math> 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):

<math display=block>N = \int n \, \mathrm{d}z.</math>

===Data storage media===
{{Main article|Areal density (computer storage)}}
Areal density is used to quantify and compare different types media used in data storage devices such as [[hard disk drives]], [[optical disc|optical disc drives]] and [[tape drives]].  The current unit of measure is typically gigabits per square inch.<ref>{{cite web |url=http://www.webopedia.com/TERM/A/areal_density.html |title=Areal Density |date=3 March 1997 |publisher=Webopedia |access-date= April 9, 2014}}</ref>

===Paper===
{{main|grammage}}
The area density is often used to describe the thickness of paper; e.g., 80&nbsp;g/m<sup>2</sup> is very common.

===Fabric===
{{main|grammage}}
Fabric "weight" is often specified as mass per unit area, grams per square meter (gsm) or [[ounce]]s 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 (electromagnetic radiation)|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<sup>−2</sup>) 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.

[[Snow water equivalent]] is a quantity of type columnar mass density.