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.
- PDSI
- See Palmer Drought Severity Index
- PHDI
- See Palmer Hydrological Drought Index
- PMDI
- See Modified Palmer Drought Severity Index
- PRISM (Parameter-elevation Relationships on Independent Slopes Model) data
- Climate observations gathered from a wide range of monitoring networks by the PRISM Climate Group at Oregon State University to reveal short- and long-term climate patterns. These datasets incorporate a variety of modeling techniques and are available at multiple spatial/temporal resolutions, covering the period from 1895 to the present.
- 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
- SPI
- 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).
- ZNDX
- See Palmer's Z-Index.