Forecasts and Outlooks:
Glossary of Climate Terms

Acre-foot: The volume of water that would cover one acre of land (43,560 square feet) to a depth of one foot, equivalent to 325,851 gallons of water. An acre-foot is the basic measure of agricultural water use.

Aerial Sketch Mapping: An efficient and economical method of detecting and monitoring forest health over large areas by mapping surface conditions from an aerial survey.

Anomaly: Difference between a given quantity or observation and its average value. This is the same as “departure from average.” For example, if the average rainfall for June is 5 inches, but this year there is 100 inches of rainfall in June, then the anomaly is +95 inches.

Aquifer: Porous, water-saturated subsurface layers of sand, gravel, and rock that can yield an economically significant amount of water.

Brightness Temperature: The measured temperature of a surface over a specific wavelength region in the electromagnetic spectrum.

CL: see Climatological Probabilities.

Climate: The general or typical atmospheric conditions for a place and/or period of time. Conditions include rainfall, temperature, thunderstorms, lightning, freezes, etc.

Climate Division: A region within a state that is reasonably homogeneous with respect to climatic and hydrologic characteristics. Arizona is divided into 7 climate divisions and New Mexico, into 8.

Climate Prediction Center (CPC): A branch of the National Oceanographic and Atmospheric Agency (NOAA) whose mission is to assess and forecast the impacts of short-term climate variability. The CPC produces official U.S. climate forecasts.

Climatological Probabilities (CL): In CPC forecasts, CL denotes areas for which no anomaly prediction is offered. A default guess of 33.3% chance of above-average, a 33.3% chance of average, and a 33.3% chance of below-average conditions is offered based on the historical probabilities of the period 1971-2000. This notation is being replaced by Equal Chances (EC).

CMI: See Crop Moisture Index.

CNA: See Comisión Nacional de Aqua.

Comisión Nacional de Aqua (CNA): The Mexican water commission.

Cooling Degree Day: A quantitative index that reflects demand for energy to cool homes and businesses. A mean daily temperature of 65°F is the base for cooling degree day computations. Cooling degree days are summations of positive differences from the 65°F base. Thus, a day with a temperature of 72°F would count as 7 cooling degree days. Days with the following temperatures, 72, 71, 75, would result in 23 cooling degree days.

CPC: See Climate Prediction Center.

Crop Moisture Index (CMI): The CMI is derived from the Palmer Drought Severity Index and shows short-term moisture supply across major agricultural regions.

Cutoff low: Upper atmosphere low-pressure system that originates as part of a low-pressure trough but becomes displaced to the south and cut off from the main west-to-east wind flow. Cutoff lows may be associated with precipitation and flooding.

Deciles: A categorization tool that groups the occurrences of precipitation (or temperature or any other measurement) into deciles (tenths of the data distribution).

Degree Day: A quantitative index that reflects demand for energy to heat or cool homes and businesses. A mean daily temperature of 65°F is the base for both heating and cooling degree day computations. Heating degree days are summations of negative differences between the mean daily temperature and the 65°F base; cooling degree days are summations of positive differences from the same base.

Digital elevation model: A map representing the elevation of the land surface.

Discharge: The volume of water that flows in a given period of time. It commonly is measured in cubic feet per second (cfs) or in cubic meters per second (m³s^-1). One m³s^-1 equals about 35 cfs.

Drought: There is no definitive definition of drought based on measurable processes; scientists evaluate precipitation, temperature, and soil moisture data for the present and recent past to determine drought status. Very generally, it refers to a period of time when precipitation levels are low, impacting agriculture, water supply, and wildfire hazard.

Dry Season: A designation used in IRI forecasts for areas experiencing a period of time when conditions are normally dry (less than 3 cm of precipitation); the IRI refrains from giving a forecasts for these regions because of the high variability of precipitation.

EC: See Equal Chances.

El Niño: Refers to a sustained warming of sea surface temperatures (SSTs) across a broad region of the eastern and central tropical Pacific Ocean. This tends to be associated with drier winters in the Pacific Northwest and wetter winters in the Southwest United States. El Niño events are also called warm events.

El Niño Southern Oscillation (ENSO): The term currently used by scientists to describe basin-wide changes every 2 to 7 years in air-sea interaction in the equatorial Pacific Ocean. El Niño/La Niña is the oceanic component and the Southern Oscillation is the atmospheric component of the phenomenon.

ENSO: See El Niño-Southern Oscillation.

Equal Chances (EC): In CPC forecasts, EC denotes areas for which no anomaly prediction is offered. A default guess of 33.3% chance of above-average, a 33.3% chance of average, and a 33.3% chance of below-average conditions is offered based on the historical probabilities of the period 1971-2000. This notation replaces the Climatological Probabilities (CL) notation.

Flood: Any relatively high streamflow event that overflows the natural or artificial banks of a river or stream.

Forecast: A prediction of future conditions by analysis of data. For example, precipitation forecasts are based on meteorological data.

Global Climate Models: Sophisticated computer models of the atmosphere and oceans that attempt to include all the processes known to affect climate.

Groundwater: The water stored in aquifers.

Groundwater Mining: When discharge from an aquifer, usually due to groundwater pumping for municipal and business use, exceeds recharge.

Heating Degree Day: A quantitative index that reflects demand for energy to heat homes and businesses. A mean daily temperature of 65°F is the base for heating degree day computations. Heating degree days are summations of negative differences between the mean daily temperature and the 65°F base. Thus, a day with a temperature of 62°F would count as 3 heating degree days. Days with the following temperatures, 62, 61, 55, would result in 17 heating degree days.

Historical Flood: Flood events documented by human observation but recorded prior to the development of systematic streamflow measurements.

Hydrograph: Graph of variation of stream flow over time.

Hydrologic Data and Information System: Collection of databases, research, and model information used to collect, analyze, and disseminate information to stakeholders.

Hypsometric relationship: A linear relationship between any dependent variable and elevation.

Infiltration Rate: The amount of water that is absorbed by soils in an amount of time (e.g., millimeters of water absorbed per hour or mm/hr).

Interpolate: To estimate values between measured values, usually using a mathematical function. Spatial interpolation involves estimating values on a map.

Inverse distance weighting: A method used to interpolate point data into a spatial map that relies on statistical weights that vary as a function of distance from observations.

IRI: The International Research Institute for Climate Prediction; housed at Columbia University’s Earth Institute. Its mission is to accelerate the ability of societies worldwide to cope with climate, especially those events that cause devastating impacts to humans and the environment.

Jack-knifing: The process of removing data in a modeling procedure and simulating locations where known observations exist in order to estimate model skill.

La Niña: Refers to a sustained cooling of sea surface temperatures (SSTs) across a broad region of the eastern and central tropical Pacific Ocean. This tends to be associated with wetter winters in the Pacific Northwest and drier winters in the Southwest United States. La Niña events are also called cold events.

Masking: The act of removing areas from an analysis in spatial data.

Monsoon: A wind system that reverses its direction seasonally. In the North American Monsoon system, summer winds from the south bring moisture and rainfall to the Southwest United States.

NAO: See North Atlantic Oscillation.

North Atlantic Oscillation (NAO): The NAO is the dominant mode of winter climate variability in the North Atlantic region ranging from central North America to Europe and into much of Northern Asia. The NAO is a large scale seesaw in atmospheric mass between the subtropical high and the polar low.

Oxygen Isotope Records: Records of the effect of salinity and temperature on oxygen chemistry of, for example, growth rings in corals provide a proxy record of past climate conditions.

Pacific Decadal Oscillation (PDO): A long-term El Niño-like pattern of North Pacific climate variability, with phases that persist from 20-30 years. The positive (warm) phase of the PDO is characterized by cooler than average SSTs and air pressure near the Aleutian Islands and warmer than average SSTs near the California coast; these conditions tend to enhance El Niño teleconnections. The negative (cool) phase tends to enhance La Niña teleconnections (i.e., winter wetness in the Pacific Northwest and winter dryness in the Southwest United States).

Pacific/North American Teleconnection Pattern (PNA): Variability in atmospheric pressure over the Northern Pacific and North America is associated with variability in rainfall in the southwestern United States. Wetter summers are associated with PNA phases with strong North to South pressure gradients. Drier summers have tended to follow PNA phases with weak North to South pressure gradients.

Paleoflood: A past or ancient flood event that occurred prior to the time of human observation or direct measurement by modern hydrological procedures.

Palmer Drought Severity Index (PDSI): An indicator, based on temperature, precipitation, and soil type, of long-term deficits or surpluses of soil moisture.

Palmer Hydrological Drought Index (PHDI): An indicator, based on impacts such as groundwater and reservoir levels, of long-term, hydrological drought.

PDO: See Pacific Decadal Oscillation.

PDSI: See Palmer Drought Severity Index.

Peaks-Above-Base: All of the flow events of a size greater than the base flood flow for a particular gauging station.

Percent of Normal (Average): A comparison of conditions, such as precipitation or temperature, at any one place or time with the historical average of that condition.

PHDI: See Palmer Hydrological Drought Index.

Phenology: A branch of science dealing with the relations between climate and periodic biological phenomena, such as bird migration or plant flowering.

PNA: See Pacific/North American teleconnection.

Precipitation: Rainfall, snow, sleet, hail, etc.

Precipitation Intensity: The maximum amount of precipitation in a period of time (e.g., I,₃₀ is the maximum precipitation over thirty minutes). Precipitation intensity can be related to discharge.

Precipitation-Runoff Modeling System (PRMS): A modular-design, deterministic, distributed-parameter modeling system developed to evaluate the impacts of various combinations of precipitation, climate, and land use on streamflow, sediment yields, and general basin hydrology.

PRMS: See Precipitation-Runoff Modeling System.

RDI: See Reclamation Drought Index.

Recharge: Net accumulation of water into an aquifer from sources such as precipitation, seepage, and injection.

Reclamation Drought Index (RDI): Similar to the Surface Water Supply Index, the RDI incorporates temperature as well as precipitation, snowpack, stream flow, and reservoir levels in order to define drought on a river basin level.

Ridge: An elongated area of high pressure.

Sea Surface Temperature Anomalies (SSTAs): Difference between the measured sea surface temperature at any given time and place and the mean (average) sea surface temperature.

Servicio Meteorológico Nacional (SMN): The Mexican meteorological agency.

SMN: See Servicio Meteorológico Nacional.

SNOTEL (SNOpack TELemetry): A near real-time hydrometeorological data collection network in the West that collects SWE, precipitation, and temperature data from nearly 600 remote high-elevation stations.

Snowpack: A horizontally layered accumulation of snow from snowfall events, which may be modified by meteorological conditions over time.

Snow water content (SWC): How much liquid water is contained in a volume of solid snow (in other words, how much water would be measured if a known amount of snow was melted). Snow water content and snow water equivalent are different terms for the same parameter.

Snow water equivalent (SWE): How much liquid water is contained in a volume of solid snow (in other words, the amount of water measured from melting a known amount of snow). Snow water content and snow water equivalent are different terms for the same parameter.

Spectral band: A specific wavelength region in the electromagnetic spectrum. For example: the visible spectrum is within a wavelength region (or spectral band) ranging from 0.4 to 0.7 µm.

Spectral properties: How a specific surface on the ground (e.g. snow, vegetation, etc.) reflects or absorbs different wavelengths within the spectral band.

SPI: See Standard Precipitation Index.

SSTs: Sea surface temperatures.

Standard Precipitation Index (SPI): An index of soil moisture that considers both the long-term average and recent precipitation (up to the last 72 months).

Surface Water Supply Index (SWSI): The SWSI is similar to the Palmer Drought Severity Index but also includes hydrological elements such as mountain snowpack, stream flow, precipitation, and reservoir storage on a basin-to-basin basis, making it very useful in “mountain water dependent” areas.

SWC: See Snow water content.

SWE: See Snow water equivalent.

SWSI: See Surface Water Supply Index.

Teleconnections: Atmospheric interactions between widely separated regions that have been identified through statistical correlations (in space and time). For example, the El Niño teleconnection with the Southwest United States involves large-scale changes in climatic conditions that are linked to increased winter rainfall.

Trough: An elongated area of low pressure.

Vegetation and Temperature Condition Index (VT): A numerical index of vegetation health that ranges from 0 (extremely poor) to 100 (excellent). It reflects, indirectly, a combination of chlorophyll (photosynthetic plant material) and moisture content in vegetation, as well as thermal conditions at the surface.

VT: See Vegetation and Temperature Condition Index.

Water Supply Outlook: A summary of snowpack, reservoir, stream flow, and precipitation for watersheds and basins, which is available bi-monthly from January through April from the U.S. Department of Agriculture’s National Resources Conservation Service.

Water Year: The water year begins on October 1 and ends on September 30 of the following year. For example water year 1994 began October 1, 1993 and ended September 30, 1994.

Weather: Describes the daily conditions (individual storms) or conditions over several days (week of record-breaking temperatures) to those lasting less than two weeks.

Wildland Urban Interface: A term that refers to houses that are built close to or within forested areas.

Wind stress: The force per unit area that wind exerts on the surface of the ocean.