Glossary of Climate Terms

Climate Divisions
Each state is divided into climatic regions. In the Carolinas, these are organized by county borders except for a small mountainous region in extreme northwest South Carolina. Collectively, they represent coastal, piedmont, and mountain regions.
Climate Normals
Climate Normals are thirty-year averages of climatological variables, including temperature and precipitation. Produced once every ten years by NOAA's National Centers for Environmental Information, the 1981–2010 U.S. Climate Normals dataset is the most recent one issued .
Climatological Seasons
Based on the calendar and annual temperature cycles rather than astronomical movement, climatological (or meteorological) spring includes March, April, and May; summer includes June, July, and August; fall covers September, October, and November; and winter includes December, January, and February.
Cooperative Weather Station
Departure from Normal
Modified Palmer Drought Severity Index
The Modified Palmer Drought Severity Index (PMDI) is obtained from the sum of the wet and dry terms weighted by probability values. The PMDI has the same value as the PDSI during established dry or wet spells but can be different during transition periods.
Palmer Drought Severity Index
The Palmer Drought Severity Index (PDSI) uses monthly temperature and precipitation data to calculate a simple soil water balance. The index is a relative measure that typically ranges from -4 (extremely dry) to +4 (extremely wet) and represents how soil moisture availability differs from that expected for a given place and time of year. The PDSI includes a "memory" component that considers past conditions and persistence of soil moisture surplus or deficit. More information on the PDSI and other drought indexes can be found at: http://drought.unl.edu/Planning/Monitoring/HandbookofDroughtIndices.aspx
Palmer Hydrological Drought Index
The Palmer Hydrological Drought Index (PHDI) is a specific version of the PDSI that accounts for longer-term drought that reduces surface and groundwater supply.
Palmer's Z-Index
Palmer's Z-Index (ZNDX) represents the departure from normal monthly moisture conditions. It is the first step in the calculation of the Palmer Drought Indices, wherein precipitation and temperature are used to estimate a simple water balance for one month. Limited to just one month, it is a short-term drought measure without memory from previous months.
See Palmer Drought Severity Index
See Palmer Hydrological Drought Index
See Modified Palmer Drought Severity Index
Probability Density Function
Shows the range and probability of a given event occurring over a specified period.
Recurrence Intervals
Probability of either not reaching or exceeding specified totals during a given period.
Return Period
See Standardized Precipitation Index
Standardized Precipitation Index
The Standardized Precipitation Index (SPI) compares observed total precipitation for a given period, typically ranging from 1 to 24 months, against long-term averages for the same period. The SPI is constructed from observed precipitation data that are fit to a gamma or other representative probability distribution. The cumulative probability associated with given precipitation totals is transformed to a normal (Gaussian) probability distribution. This provides a measure of relative wetness and dryness that typically ranges between -3 and +3. The SPI is standardized such that a value of -1 (-2) approximates one (two) standard deviation(s) drier than the long term average. Arbitrary adjectives are often associated with ranges of SPI values: dry (-1 to -1.5), moderately dry (-1.5 to -2), extremely dry (<-2), wet (1 to 1.5), moderately wet (1.5 to 2), and extremely wet (>2). SPI names reflect the number of months considered (e.g. SPI 3 represents a 3-month precipitation total).
See Palmer's Z-Index.