In the
natural sciences, especially in
atmospheric
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
and
Earth sciences involving
applied statistics, an ''anomaly'' is a persisting
deviation in a
physical quantity
A physical quantity is a physical property of a material or system that can be quantified by measurement. A physical quantity can be expressed as a ''value'', which is the algebraic multiplication of a ' Numerical value ' and a ' Unit '. For examp ...
from its expected value, e.g., the
systematic
Systematic may refer to:
Science
* Short for systematic error
* Systematic fault
* Systematic bias, errors that are not determined by chance but are introduced by an inaccuracy (involving either the observation or measurement process) inheren ...
difference between a measurement and a trend or a model prediction.
[Wilks, D.S. (1995) ''Statistical Methods in the Atmospheric science'', Academic Press. (page 42)]
Similarly, a standardized anomaly equals an anomaly divided by a
standard deviation
In statistics, the standard deviation is a measure of the amount of variation or dispersion of a set of values. A low standard deviation indicates that the values tend to be close to the mean (also called the expected value) of the set, while ...
.
[
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
In statistics, an outlier is a data point that differs significantly from other observations. An outlier may be due to a variability in the measurement, an indication of novel data, or it may be the result of experimental error; the latter are ...
.
There are examples in atmospheric sciences and in geophysics.
Calculation
The 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
In statistics, an outlier is a data point that differs significantly from other observations. An outlier may be due to a variability in the measurement, an indication of novel data, or it may be the result of experimental error; the latter are ...
s, are sometimes used as the basis of the transformation.[
]
Examples
Atmospheric sciences
In the atmospheric sciences, the climatological annual cycle
An annual cycle refers to a set of changes or events that uniformly, or consistently, take place at the same time of year.
In biology, the annual cycle for plants and animals details behavioral and chemical changes that take place as the seasons ...
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 NAO plays an important role for European weather by modification of the exit of the Atlantic storm track.
A climate normal can also be used to derive a climate anomaly.[{{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]
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
The Bangui magnetic anomaly is a local variation in the Earth's magnetic field centered at Bangui, the capital of Central African Republic. The magnetic anomaly is roughly elliptical, about , and covers most of the country, making it one of the "l ...
, 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
The method of least squares is a standard approach in regression analysis to approximate the solution of overdetermined systems (sets of equations in which there are more equations than unknowns) by minimizing the sum of the squares of the res ...
* Least-squares spectral analysis
References
Time series
Climate and weather statistics
Geophysics