:for computer memory comparisons, see computer storage density. for other meanings, see density (disambiguation) the area density (also known as areal density, surface density, or superficial density) of a two-dimensional object is calculated as the mass per unit area. the si derived unit is: kilogram per square metre (kgÂ·mâ2). == formulation == it can be calculated as: : \rho_a = \frac {m} {a} or : \rho_a = \rho \cdot l where :{| |- | || = average area density |- | || = total mass of the object |- | || = total area of the object |- | || = average density |- | || = average thickness of the object |} == column density == a special type of area density is called column (mass) density (also columnar mass density), denoted Ïa or Ï. it is the mass of substance per unit area integrated along a path;  it is obtained integrating volumetric density \rho over a column: :\sigma=\int \rho \; \operatorname{d}s. in general the integration path can be slant or oblique incidence (as in, for example, line of sight propagation in atmospheric physics). a common special case is a vertical path, from the bottom to the top of the medium: :\sigma = \int \rho \; \operatorname{d}z where z denotes the vertical coordinate (e.g., height or depth). columnar density \rho_a is closely related to the vertically averaged volumetric density \bar{\rho} as :\bar{\rho} = \frac{\rho_a}{\delta z}, where \delta z = \int 1 \; \operatorname{d}z; notice that \bar{\rho}, \rho_a, and \delta z have units of, for example, grams per cubic metre, grams per square metre, and metres, respectively. === column number density === column number density refers instead to a number density type of quantity: the number or count of a substanceârather than the massâper unit area integrated along a path: :n = \int n \; \operatorname{d}s. ==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 optical absorption spectroscopy (doas) method {{cite news | author=r. sinreich, u. frieÃ, t. wagner, s. yilmaz and u. platt | title=retrieval of aerosol distributions by multi-axis differential absorption spectroscopy (max-doas) | journal=nucleation and atmospheric aerosols | year=2008 | pages= 1145â1149 | doi
Atmospheric_sciences Classical_mechanics Density