View source for Anomaly (natural sciences)

{{Short description|A persisting deviation}}
In the [[natural sciences]], especially in [[atmospheric sciences|atmospheric]] and [[Earth sciences]] involving [[applied statistics]], an '''anomaly''' is a persisting [[deviation (statistics)|deviation]] in a [[physical quantity]] from its expected value, e.g., the [[systematic effect|systematic]] difference between a measurement and a trend or a model prediction.<ref name=Wilks>Wilks, D.S. (1995) ''Statistical Methods in the Atmospheric science'', Academic Press. {{ISBN|0-12-751965-3}} (page 42)</ref>
Similarly, a '''standardized anomaly''' equals an anomaly divided by a [[standard deviation]].<ref name =Wilks/>
A group of anomalies can be analyzed spatially, as a map, or temporally, as a [[time series]].
It should not be confused for an isolated [[outlier]].
There are examples in atmospheric sciences and in geophysics.
==Calculation==
The [[Central tendency|location and scale measures]] used in forming an anomaly time-series may either be constant or may themselves be a time series or a map. For example, if the original time series consisted of daily mean temperatures, the effect of seasonal cycles might be removed using a [[deseasonalization]] filter.
[[Robust statistics]], resistant to the effects of [[outlier]]s, are sometimes used as the basis of the [[Data transformation (statistics)|transformation]].<ref name =Wilks/>
==Examples==
===Atmospheric sciences===
In the atmospheric sciences, the climatological [[annual cycle]] is often used as the expected value.  Famous atmospheric anomalies are for instance the [[Southern Oscillation]] index (SOI) and the [[North Atlantic oscillation]] index.  SOI is the atmospheric component of [[El Niño]], while [[North Atlantic oscillation|NAO]] plays an important role for [[Europe]]an [[weather]] by modification of the exit of the Atlantic [[storm track]].
A [[climate normal]] can also be used to derive a climate anomaly.<ref>{{Cite book|url=https://books.google.com/books?id=ypcMBwAAQBAJ&dq=climate+anomaly&pg=PA53|title=Dictionary of Global Climate Change|date=August 15, 2008|publisher=Springer Science & Business Media|isbn=9780585295732|via=Google Books}}</ref>
===Geophysics===
* [[Gravity anomaly]], difference between the observed gravity and a value predicted from a model
** [[Bouguer anomaly]], anomaly in gravimetry
** [[Free-air anomaly]], gravity anomaly that has been computed for latitude and corrected for elevation of the station
* [[Iridium anomaly]], an unusual abundance of what is normally a very rare element in the Earth's crust
* [[Magnetic anomaly]], local variation in the Earth's magnetic field
** [[Bangui magnetic anomaly]], in central Africa
** [[Kursk Magnetic Anomaly]], territory rich in iron ores located within Kursk Oblast, Belgorod Oblast, and Oryol Oblast
** [[Temagami Magnetic Anomaly]], large buried geologic structure in the Temagami region of Ontario, Canada
==See also==
*[[Bias (statistics)]]
*[[Climate oscillation]]
*[[Frequency spectrum]]
*[[Innovation (signal processing)]]
*[[Least squares]]
*[[Least-squares spectral analysis]]
*[[Temperature anomaly]]
==References==
<references/>
[[Category:Time series]]
[[Category:Climate and weather statistics]]
[[Category:Geophysics]]