# Anomaly (natural sciences)

(Redirected from Atmospheric anomaly)

In the natural sciences, especially in atmospheric and Earth sciences involving applied statistics, an anomaly is the deviation in a quantity from its expected value, e.g., the difference between a measurement and a mean or a model prediction.[1] Similarly, a standardized anomaly equals an anomaly divided by a standard deviation.[1] A group of anomalies can be analyzed spatially, as a map, or temporally, as a time series. 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 temperatures measured every hour, the effect of typical daily cycles of temperature might be remove by subtracting a time series containing mean temperature values for each hour of the day: clearly, this can be extended by including seasonal variations of temperature.

Robust statistics, resistant to the effects of outliers, are sometimes used as the basis of the transformation.[1]

## Examples

### Atmospheric sciences

In the atmospheric sciences, the climatological annual cycle is often used as the mean 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.

### Geophysics

• Bouguer anomaly, anomaly in gravimetry
• Free-air anomaly, gravity anomaly that has been computed for latitude and corrected for elevation of the station
• Gravity anomaly, difference between the observed gravity and a value predicted from a model
• 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

## References

1. ^ a b c Wilks, D.S. (1995) Statistical Methods in the Atmospheric science, Academic Press. ISBN 0-12-751965-3 (page 42)