Glossary of Climatology/Climate Data Terms

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ABSOLUTE TEMPERATURE - Temperature measured in Kelvin.

ADVECTION - Energy or mass exchange by horizontal mass movement

ADIABATIC - A change of state without transfer of energy across system boundaries.

AIR MASS - A widespread body of air approximately homogenous in its properties

ALBEDO -the reflectivity of a body integrated over the solar spectrum. May be expressed as time averaged or instantaneous values. Instantaneous values are similar to reflectance measurements in remote sensing.

ANEMOMETER - Instrument for measuring wind velocity. Most common type uses cups attached to axles which rotate horizontally.

ANNUAL FLOOD - Highest flow point on a stream during any calender or water year.

ANOMALY - Deviation from a normal or average value.

ARIDITY - the degree to which air lacks effective moisture. Opposite of humidity

ASPECT - compass azimuth to which a slope faces. Ranges from 0° to 360°.

BAR - basic unit of pressure, equal to 10⁶ dyne/cm² or 29.5 in/Hg.

BAROMETER - instrument for measuring pressure

BIOCLIMATIC - pertaining to the effect of climate on life.

BOWEN RATIO - the ratio of energy flux as sensible heat to energy flux as latent heat.

BPI PAN - A type of evaporation pan. See Pan evaporation.

BULK DENSITY - In soil, the mass of the solids divided by the volume of the solids and voids.

CLASS A PAN - Also called a standard pan. A type of evaporation pan. See Pan evaporation.

CLIMATOLOGICAL OBSERVATION - Strictly speaking, this requires the following data to be evaluated at least once daily: maximum temperature, minimum temperature, total precipitation since the previous report.

CONVECTION - Mass movements in a fluid or gas resulting in a mixture of properties within the fluid or gas. Generally assumed to be vertical motion.

DAILY MEAN - average value of a climatic element over a 24-hour period. Daily means recorded using special-purpose maximum/minimum thermometers are called TRUE DAILY MEANS.

DAY - There are a surprising large number of definitions for a day. Most relevant to climate measurement is the difference between a mean solar day and a civil day. MEAN SOLAR DAY is derived from the apparent solar day (interval of time required for the sun to cross a specific meridian), modified by the equation of time. The CIVIL DAY is a mean solar day modified to make time keeping practical.

DEGREE - unit of temperature

DEGREE DAY - A measure of departure of the mean daily temperature from a given standard or threshold. The departure can be positive with respect to the threshold, as in cooling degree days, or negative, as in heating degree days or frost degree days. Degree days are generally allowed to accumulate over a season. GROWING DEGREE DAY - As above, the threshold temperature is generally defined by the vegetation types, e.g. 4 °C for C3 grasses, for 7 °C C4 grasses, etc.

DEW - water deposited on vegetation or other surface by condensation.

DEW POINT - temperature to which air must be cooled at constant pressure and water vapor content for saturation to occur.

DROUGHT - a period of abnormally dry weather sufficiently prolonged for the lack of water to cause hydrological imbalance.

DRY BULB THERMOMETER - a thermometer which directly measures air temperature. Contrast with wet bulb thermometer.

EVAPOTRANSPIRATION - combination of EVAPORATION (loss to the atmosphere from a free water surface, rocks, bare soil, or intercepted water), and TRANSPIRATION (water loss from the surface of plants). Can also be conceptualized as an exchange of energy, see Latent Heat. POTENTIAL EVAPOTRANSPIRATION - evaporation from a short, green well-watered crop which exerts negligible resistance to the flow of water.

FETCH - distance downwind to a change in surface cover or surface property.

FIELD CAPACITY - The percentage of soil moisture held with water potential less than -1/3 bar. A measure of the greatest amount of water that a soil can store under conditions of complete wetting followed by free drainage.

FIRST ORDER STATION - a climate station where automatic or hourly readings of pressure, temperature (including min/max), humidity, sunshine, wind, and precipitation are made.

FREEZING INDEX - number of degree days above and below 0 °C between the highest and lowest points of the cumulative degree-days time curve for one freezing season.

FREEZING SEASON - time interval between the highest point and the succeeding lowest point on the time curve of cumulative degree days above and below 0 °C.

FROST DAY - an observational day on which frost occurs.

GROWING SEASON - the period of the year during which the temperature of vegetation is sufficiently high to allow plant growth, generally considered to be the time between successive occurrences of some index temperature. 10 °C is often used as an index, as is annual mean temperature for a given location.. EFFECTIVE GROWING SEASON - length of growing season which prevails in 80% of all years.

HUMIDITY - a measure of water vapor in the air.

ABSOLUTE HUMIDITY - ratio of mass of water vapor to volume of air and water vapor

RELATIVE HUMIDITY - ratio of humidity present in a volume of air to the amount it could hold if saturated. A temperature dependant quantity.

HYDROLOGIC YEAR - see WATER YEAR

HYGROMETER - instrument which measures water vapor content of the atmosphere

INSTRUMENTAL CORRECTION - difference between the readings of a given instrument and those of a standard instrument.

INSOLATION - solar radiation received at the Earth’s surface

KELVIN - a unit of the absolute temperature scale. Zero Kelvin is absolute zero.

LANGLEY - unit of energy per area equal to one calorie per square centimeter. Superseded by the SI unit Wm^-2 but still commonly encountered.

LAPSE RATE - Rate of temperature change with height.

LATENT HEAT - strictly speaking, energy consumed in a phase change. In climatological usage, the energy transported to the atmosphere from the surface by the evaporation of water.

MEAN TEMPERATURE - average temperature of the air given a specified time period, usually a day, month, or year. Also called TRUE MEAN TEMPERATURE

MIXING RATIO - the mass of water vapor to the mass of dry air, a measure of humidity

NORMAL - in climatology, mean values over a specified period, usually thirty years.

OBSERVATIONAL ERROR - difference between an observed value and its true value. Can be subdivided into SYSTEMATIC ERROR (a series of measurements with similar error), RANDOM ERROR, and MISTAKES (widely discrepant readings, usually the result of error by the observer).

PAN EVAPORATION - water loss to the atmosphere as measured from a standard pan.

PAN COEFFICIENT - quantity which relates pan evaporation to actual evapotranspiration. Specific to various vegetation or crop types.

PRECIPITATION GUAGE - a device for measuring the amount of precipitation. A rain gauge should have uniform cross sectional area and be sited away from obstructions. Precipitation gauges often have shields to prevent wind from biasing the data.

PSYCHROMETER - a type of hygrometer utilizing either a wet bulb/dry bulb thermometer combination or a piezoelectric resistance element.

PYRANOMETER - a device for measuring incoming solar radiation. Measures both direct and diffuse insolation, although it is possible to modify the instrument to measure only diffuse irradiance by addition of a shadow band.

PYRGEOMETER - measures longwave radiation

PYRHELIOMETER - similar to a pyranometer but measures only direct insolation

RAIN DAY - in U.S. usage, a day with some measurable precipitation

RANGE - difference between the maximum and minimum value of a measurement

RADIOSONDE/RAWINSONDE - an instrument package attached to a balloon for making upper atmosphere measurements. The rawinsonde includes wind measurement instruments.

SENSIBLE HEAT - an exchange of energy causing a rise in temperature

SHELTER - housing for instruments.

SOIL HEAT FLUX - Flow of heat into or out of a soil, the product of the soil temperature gradient times the thermal conductivity of the soil. Expressed in Wm^-2.

SOIL TEXTURE - The distribution of particle sizes within a soil. Texture classes are SAND (particle size range 0.05 - 2.0 mm), SILT (particle size range 0.002 - 0.05 mm), CLAY (particle size < 0.002 mm), or LOAM (a soil of mixed texture).

SOIL WATER CONTENT - The amount of liquid water or water vapor contained in the soil matrix. Can be expressed as VOLUMETRIC units (volume of water per unit volume of soil) or GRAVIMETRIC units (mass of soil moisture per unit mass of soil).

SOIL WATER POTENTIAL - The mechanical work necessary to transfer a unit of soil solution from a standard or reference state (where potential = 0) to the situation of interest. This term is also frequently used to describe the concentration gradient of soil moisture between two locations separated either vertically or horizontally, or both.

SPECIFIC - observation made per unit of mass. For example, specific heat is the heat required to raise one unit of mass one degree of temperature. See specific humidity.

STANDARD - an index or hypothetical value or distribution of some quantity. E.g. standard pressure is 1000 mb.

STATION - a meteorological or climatological measurement site.

SYNOPTIC - in meteorology, an observation or forecast occurring simultaneously over a wide area.

TEMPERATURE EFFICIENCY INDEX - long range efficiency of temperature in promoting plant growth. The sum of all 12 monthly temperature efficiency ratios.

TEMPERATURE EFFICIENCY RATIO - departure of normal monthly temperature above 0 °C divided by 4, i.e. (T-0)/4

THAWING INDEX - number of degree days above and below freezing between the lowest and highest points on the cumulative degree day time curve

THAWING SEASON - the period of time between the lowest and succeeding highest points on the time curve of cumulative degree days above and below 0 °C.

THERMISTOR - a temperature measurement device composed of solid semiconductors with a negative linear resistance response to temperature change

THERMOCOUPLE - a temperature sensor which converts thermal energy directly into electrical current.

VAPOR PRESSURE - the partial pressure exerted by the water molecules in mixed gas. A measure of humidity.

WATER YEAR - any twelve month period, usually selected to begin or end during a dry period, used as a basis for analyzing hydrological data. In the U.S., the water year is commonly 1 Oct - 30 Sept.

WATT - a unit of power equal to 1 joule per second. A watt per square meter is the standard unit of radiant flux density in the SI system.

WET BULB - either a thermometer with a saturated wick covering its bulb, used along with a dry bulb thermometer to measure relative humidity, or the temperature an air parcel would obtain if cooled adiabatically to its saturation point.

WILTING POINT - The soil moisture potential at which a plant begins to lose turgor and exhibit adaptive mechanisms in response to water stress, e.g. leaf rolling, stomatal closure.

WIND VANE - arrow like instrument used to indicate wind direction

WIND VECTOR - vector describing the combined direction and velocity of the wind. WIND ROSE - a graph of wind vectors over time VECTOR MEAN WIND - the vector average of wind over time.

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