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Climate Glossary

A glossary of terms commonly used in climate science.

30 Year Normal

The Snow Survey and Water Supply Forecasting (SSWSF) normal is a measure of central tendency for a data type (such as snow water equivalent) at a site location, over a 30-year period. The 30-year interval was chosen in agreement with World Meteorological Organization (WMO) standards. The SSWSF Program has chosen the median as the default normal for all data types. Both the median and average are available for all data types in reporting applications. Learn more here. See also Normal.


Air Temperature

See Temperature, Air.



The ratio of the amount of radiation reflected by a body to the amount of radiation incident upon it; expressed as a percentage.

Climate The synthesis of weather, or averaging of weather conditions over a given time period.

Climatic Data Elements

A climatic data element is a measured parameter which helps to specify the climate of a specific location or region, such as precipitation, temperature, wind speed and humidity. Descriptive terminology for climaticdata elements are:

Element Name - The full description of the element being referenced at the climate station (e.g., maximum temperature).

Element ID - Is a shortened identifier for the element, usually 4 characters in length (e.g., TMAX (maximum daily temperature), TMIN (minimum daily temperature), PRCP (precipitation, etc).

Element Duration - The interval between measurements of a data element. Common data element durations available for the station could include monthly, daily, or hourly.

Climate Station Metadata

Climate stations are locations at which climatic data are gathered. Biographical and index information describing the climatic station, called "metadata," are used in conservation applications and resource evaluations.

Station ID - Identification number for the climate station assigned by the agency responsible for the particular station.

Station Name - The full name of the climate station as recognized by the agency responsible for the climate station.

Station Latitude - Latitude defines a site's location based on its relative distance from the equator going toward the North or South poles. Station latitude is measured in degrees, minutes, and seconds, with 0 degrees being on the equator, and 90 degrees north or south being the North and South Poles, respectively. The latitude of a particular climate station is determined by the agency managing the station and is generally recorded to the nearest minute.

Station Longitude - Longitude defines a site's relative distance, up to 180 degrees, west or east of a North-South line running through Greenwich, England. The longitude of a particular station is determined by the agency managing the station. Measurement is generally made to the nearest minute.

Station Elevation - The elevation of a climate station is usually measured in feet above mean sea level.

Degree Days, Cooling

A value used to estimate the energy requirements for air conditioning of homes and buildings. One cooling degree day is given for each degree the daily mean temperature is above 75 degrees Fahrenheit.


Degree Days, Growing

Growing degree days (GDD) measures the day-to-day accumulation of the difference between the average daily temperature and a threshold temperature for a specific crop. GDD's give an indication of the amount of heat available for crop growth.


Degree Days, Heating

A value used to estimate the energy requirements for heating homes and buildings. One heating degree day is given for each degree the daily mean temperature is below 65 degrees Fahrenheit.



The temperature to which air is cooled for water vapor to begin condensing.



Very small, numerous, and uniformly dispersed water drops that may appear to float while following air currents. Unlike fog droplets, drizzle falls to the ground.



The period or time increment to which an observed or computed value applies.


Exceedance Probability

The probability of exceeding a given amount of precipitation for a given duration at least once in any given year at a given location. It is an indicator of the rarity of precipitation amounts and is used as the basis of hydrologic design.



The physical process by which a liquid is transformed to a gaseous state. Evaporation is influenced by solar radiation, air temperature, vapor pressure, wind, and possibly atmospheric pressure. Evaporation varies with latitude, altitude, season, time of day, and sky condition. Accurate evaporation readings requires careful maintenance of an evaporation pan which contains water. The water depth is measured daily and adjusted for any precipitation which may occur.



The combined process of evaporation and transpiration.



A visible collection of minute water droplets suspended in the atmosphere near the earth's surface. Fog reduces visibility below one kilometer (0.62 miles).



A freeze occurs at any time the surface air temperature reaches 28 degrees or less. This temperature causes damage to most vegetation except certain species which are resistant to freezing.


Freeze Free Period

Freeze free period is the number of consecutive days where the air temperature does not fall below 28 degrees Fahrenheit.


Freeze, Killing

A killing freeze occurs at or below 24 degrees Fahrenheit and causes permanent damage to almost all vegetation.


Freeze Free Period, Killing

Killing freeze free period is the number of consecutive days where the air temperature does not fall below 24 degrees Fahrenheit.



Frost is the process of deposition of frozen atmospheric water vapor on surfaces whose surface air temperature is below 32 degrees Fahrenheit. A frost can occur at any time the surface air temperature falls to 32 degrees Fahrenheit or less. This temperature may cause damage to very young vegetation or vegetation that has no resistance to frost. Most fruit falls in this category.


Frost Free Period

Frost free period is the number of consecutive days where the surface air temperature does not fall below 32 degrees Fahrenheit.


Frost, First

First Frost is the first date following the growing season that the minimum temperature drops below an index temperature, usually 32 degrees Fahrenheit. The first frost usually occurs in the fall of the year, but it may occur during the winter months, or in some locations may not occur at all.


Frost, Last

Last Frost is the last date preceding the growing season that the minimum temperature drops below an index temperature, usually 32 degrees Fahrenheit. The last frost usually occurs in the spring of the year but may occur very early in the summer or not at all in some locations. First and Last frosts are analyzed at three temperatures (32, 28, and 24 degrees Fahrenheit) specifically relating to damage caused to vegetation by the sub-freezing temperatures.


Growing Season

Growing Season is the number of consecutive days where the temperature has not gone below an index temperature for specific vegetation. If vegetation is more resistant to cold temperatures the index temperature would be lower. The index temperatures used in growing season analysis usually include 24, 28, and 32 degrees Fahrenheit.


Growing Season Period

Growing Season Period is the period of time, beginning date and ending date, that defines the period that the temperature has not dropped below the index temperature.



Precipitation in the form of balls or irregular lumps of ice with a diameter of 5 mm or more, always produced by convective clouds, nearly always cumulonimbus.


Humidity, Relative

A measure of the amount of water in the air compared to the amount of water vapor the air has the potential to hold. (Note: the potential of air to hold water changes with air temperature. Therefore, relative humidity can change as air temperature changes without an actual change in the amount of water vapor.)


Index Temperature

A temperature which denotes the beginning of a specific event such as 28 degrees Fahrenheit. The 28 degree temperature denotes a freeze that can damage plants.


Master Station

Much of the data collected at SNOTEL sites are transmitted to one of two master stations. At the master station, the remote site data are checked for completeness. If complete, an acknowledgment message, returning over the same path, tells the remote site not to transmit again during this polling period. The entire process takes place in less than a tenth of a second. Two master stations -- at Boise, Idaho, and Dugway, Utah -- cover the 10 western states, an area of about 1 million square miles. By cable, the master stations feed the data to the Water and Climate Information System in Portland, Oregon.



The Snow Survey and Water Supply Forecasting (SSWSF) normal is a measure of central tendency (median or average) for a data type, such as snow-water equivalent, at a site location. The statistics are calculated over a 30-year period and updated every decade, in agreement with World Meteorological Organization (WMO) standards. Learn more here. See also 30-Year Normal


Period of Record (POR)

The time interval during which meteorological and climatic data have been gathered at a climatic station.



Precipitation refers to all forms of water, liquid or solid, that fall from the atmosphere and reach the ground. Precipitation includes, but is not limited to, rain, drizzle, snow, hail, graupel, sleet, and ice crystals. It is one of the most basic data elements collected by any climate station. Dew, frost and rime are excluded, since they are a result of water vapor in air condensing or freezing onto a surface.

The standard U.S. precipitation gage has an eight-inch diameter mouth and height of about 30 inches. Non-recording gages simply collect precipitation; amount of precipitation must be measured by an observer. Recording gages have instrumentation which records the time, duration, and intensity of precipitation. Most recording gages store information on a paper strip, which is generally changed weekly by an observer. Precipitation intensity and duration, useful information for many NRCS design activities, can be derived from information gathered by precipitation gages.

The biggest factor in precipitation measurement error is wind. Strong winds during precipitation events can cause considerable differences between measured and actual precipitation. Measurement errors can also result from small amounts of dew, frost, and rime accidentally included in the total measured precipitation. Even with careful placement, all gages underestimate the real precipitation, particularly with snowfall.



Probability is a statistical process that provides for the analysis of data to determine the potential of an individual value to occur at a specified time, in a given year, or in a given period of time. An example might indicate that a certain value has a 10 percent chance of occurrence in any year, or that the value has a chance of returning once in a period of ten years.



Precipitation in the form of liquid water drops which have diameters greater than 0.02 in (0.5 mm).


Relative Humidity

See Humidity, Relative


Reservoir Storage

The volume of usable water stored in a reservoir.



A type of precipitation consisting of transparent or translucent pellets of ice 5 mm or less in diameter. Sleet forms when raindrops fall through a layer of below-freezing air near the earth's surface.


Soil Temperature

See Temperature, Soil.


Solar Radiation

A type of precipitation consisting of transparent or translucent pellets of ice 5 mm or less in diameter. Sleet forms when raindrops fall through a layer of below-freezing air near the earth's surface.


Solar Radiation, Incoming

Incoming solar radiation is the total electromagnetic radiation emitted by the sun striking the earth.



Precipitation composed of white or translucent ice crystals, chiefly in complex branch hexagonal form and often agglimerated into snowflakes.

Snow, New

New snow is the incremental amount of snow that has fallen since the last snow depth observation. Delineating between new snow and old snow presents a challenge. A snow board (generally a sheet of plywood) can provide an artificial surface at the top of the existing snow. Snow boards are laid on top of old snow when there is any possibility of new snow falling. After each observation of new snow, the board is cleaned and placed in a new location. Board placement and measurement location are the greatest source of error in determining new snow.

Snow Depth

The actual depth of snow on the ground.

Snow Telemetry (SNOTEL) network

The NRCS operates the automated Snow Telemetry (SNOTEL) network of over 800 stations in the western U.S. Beginning October 1, these stations report accumulated seasonal precipitation, snow water equivalent, and temperature (maximum, minimum, current and average) daily. This network was established in the late 1970s to support water supply forecasting. It uses meteorburst technology to transmit data from remote sites to data gathering locations. SNOTEL augmented and partially replaced the cooperative network of manual snow courses that NRCS acquired and established the mid 1930s.

Snow Water Equivalent

Snow Water Equivalent (SWE) is a common snowpack measurement. It is the amount of water contained within the snowpack. It can be thought of as the depth of water that would theoretically result if you melted the entire snowpack instantaneously. For example, say there is a swimming pool that is filled with 36 inches of new powdery snow at 10% snow water density. If you could turn all the snow into water magically, you would be left with a pool of water 3.6 inches deep. In this case, the SWE of your snowpack would equal 36" x 0.10 = 3.6 inches. The NRCS measures SWE at many remote SNOTEL sites and uses the data for streamflow forecasting. However, many scientists and recreationists are interested in snow depth instead of SWE. The relationship between the two values is explained here. To determine snow depth from SWE you need to know the density of the snow. The density of new snow ranges from about 5% when the air temperature is 14°F, to about 20% when the temperature is 32°F. After the snow falls its density increases due to gravitational settling, wind packing, melting and recrystallization. Most snow that falls in the Cascade Mountains of Washington and Oregon tends to be higher density snow. In the Cascades, snowpack densities are around 20-30% in the winter to 30-50% in the spring. However, east of the Cascades, the snowpack density is much less. Typical values are 10-20% in the winter and 20-40% in the spring. To determine the depth of snow using snow water equivalent and density, use the following formula: [SWE] ÷ [Density] = Snow Depth (Density must be in decimal form. For example: 25% = 0.25).

Soil Temperature

Soil temperature measures the hotness or coldness of soil. Soil temperature is very important to the agricultural industry. Most seeds require a certain soil temperature in order to germinate. Soil temperatures are commonly measured at 2, 4, 8, 20, 40, 60, and 120 inches with the 4-inch reading being the most frequently observed. Readings are usually observed and recorded daily. Maximum, minimum, and current temperatures are generally recorded above 8 inches. At greater depths, where temperature changes more slowly, only the current temperature is normally recorded. Different species of plants have specific soil temperature ranges in which they will grow.

Solar Radiation

Incoming solar radiation is the total electromagnetic radiation emitted by the sun striking the earth. Much solar radiation is absorbed by air molecules, reflected back into space, or refracted as it passes through the atmosphere. A pyrheliometer measures the direct solar radiation that passes through the atmosphere unimpeded. It consists of an enclosed radiation sensing element with a small aperture through which the direct solar rays enter. A pyranometer measures the combined incoming direct solar radiation and diffuse sky radiation. It is mounted such that it views the entire sky. Both instruments can be connected to electronic recording devices to collect the measurements. Solar radiation sensors must be cleaned regularly and exposed properly to accurately measure solar radiation.


Streamflow: Adjusted Volume

Volume of streamflow that would occur without the influences of major upstream reservoirs or diversions, otherwise known as unregulated or naturalized flow. Reservoirs and diversions are used as an adjustment based on size of reservoir/diversion relative to total streamflow volume and availability of historical records. Adjusted Volume datasets include streamflow points that are: 1) adjusted due to presence of major upstream reservoirs/diversions with historical records; 2) not adjusted due to lack of major upstream reservoirs/diversions; 3) not adjusted or partially adjusted due to lack of historical records for all major upstream reservoirs/diversions. Volumetric NRCS water supply forecasts are for Adjusted Volume.


Streamflow: Diversion Discharge

Flow rate as measured in or along a diversion structure.


Streamflow: Forecast Point

Point within a watershed for which volume, threshold, and/or stage forecasts are published by the NRCS Snow Survey and Water Supply Forecasting Program.


Streamflow: Observed Volume

Volume of streamflow as observed at a gaging station. In a watershed with major reservoir storage and/or diversions, the observed volume can be significantly different from the adjusted volume.



A measure of the internal energy of molecular motion in a substance.


Temperature, Air

A measure of the hotness or coldness of air. It is measured on some definitive temperature scale. Two scales are commonly used. The Fahrenheit and Centigrade temperature scales establish the freezing of water at 32/0 degrees respectively and boiling point at 212/100 degrees respectively. The Fahrenheit scale is used most frequently in the U.S. and Centigrade throughout the rest of the world. Air temperature is usually measured with either a liquid-in-glass maximum and minimum thermometer mounted in a vented, wooden box or with an electronic sensor.


Temperature, Soil

Measurement of the hotness or coldness of soil. At SCAN and SNOTEL sites soil temperatures are commonly measured at 2, 4, 8, 20, and 40 inches, with the 4-inch reading being the most frequently observed.


Temperature Anomaly

A departure from a reference value or long-term average. A positive anomaly indicates that the observed temperature was warmer than the reference value, while a negative anomaly indicates that the observed temperature was cooler than the reference value.


Temperature Threshold

A temperature that denotes the boundary condition for a specific event. For example, a crop specific temperature below which the growth of that crop is minimal.



The process by which water in plants is transferred to the atmosphere as water vapor.


Water and Climate Information System

The Water and Climate Information System (WCIS) is composed of the database, applications, and tools which support the extensive data collection network of the Snow Survey and Water Supply Forecasting (SSWSF) Program. Data collected at automated and manual data collection sites are transmitted either to intermediate Master Stations or directly back to centralized database servers, which are located in a secure, fail-safe environment. Snowpack, precipitation, streamflow, and reservoir data are also collected from the U.S. Army Corps of Engineers (USACE), the U.S. Bureau of Reclamation (BOR), the Applied Climate Information System (ACIS), the U.S. Geological Survey (USGS), and other entities and entered into the WCIS database. Depending on the type of information, data are processed on both an hourly and a daily basis and made available to WCIS applications, such as Report Generator, Update Report, and displayed on the Interactive Map. Learn more about Snow and Water Data Collection and Stewardship.


Water Supply Forecast

A water supply forecast is a prediction of streamflow volume that will flow past a point on a stream during a specified season, typically in the spring and summer.



The instantaneous or short-term state of the atmosphere.



Wind is the motion of air relative to the surface of the earth. Wind speed and direction, the two primary elements, are usually measured with an anemometer and wind vane, respectively. Wind speed is generally measured in miles per hour; direction is measured in degrees to the nearest ten(s) (10 to 360) with 360 degrees being north, 90 degrees being east, 180 degrees representing south, and 270 degrees being west. Wind measurement accuracy is primarily influenced by sensor height and nearby objects.


Wind Rose

A type of analysis that describes wind measurements graphically and tabularly as a combination of the cardinal direction that the wind was coming from and the average speed from that direction for a particular time interval. A wind rose depicts the frequency of occurrence of winds in each of the specified wind direction sectors and wind speed classes for a given location and period of time.

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