I Ice crystals A type of precipitation composed of unbranched crystals in the form of needles, columns, or plates. Usually has a very slight downward motion and may fall from a cloudless sky. Ice point The true freezing point of water. The temperature at which a mixture of air-saturated pure water and pure ice may exist in equilibrium at a pressure of one standard atmosphere. Icing-rate meter An instrument for the measurement of the rate of ice accretion on an unheated body. IFLOWS Integrated Flood Observing and Warning System. A joint undertaking by the National Weather Service and the participating States to improve flood warning capabilities by giving local communities the ability to obtain real-time rain and stream level data. IFR Abbreviation for Instrument Flight Rules, but commonly used to refer to the weather and/or flight conditions to which these rules apply, i.e. low visibility.    Illuminance The total luminous flux received on a unit area of a given real or imaginary surface, expressed in such units as the foot-candle, lux, or phot.    Illuminometer Same as photometer.     Impactor A general term for instruments which sample atmospheric suspensoids by impaction. Same as impactometer.     Inaccuracy The difference between the input quantity applied to a measuring instrument and the output quantity indicated by the instrument. The inaccuracy of an instrument is equal to the sum of its instrument error and its uncertainty.     Inch of mercury A common unit used in measurement of atmospheric pressure. Defined as that pressure exerted by a one-inch column of mercury at standard gravity and a temperature of 0°C.     Index The indicating part of an instrument. For example, the hand of a watch or the meniscus of a mercury column.    Indicator An instrument used to reveal but not necessarily measure the presence of an electrical quantity. It is used to display the output of a sensing element after suitable amplification and modification. Sometimes called display.     Inert gas Any one of six gases, helium, neon, argon, krypton, xenon, and radon, all of whose shells of planetary electrons contain stable numbers of electrons such that the atoms are chemically inactive.      Infiltration Movement of water through the soil surface into the soil, or the quantity of water entering the soil. Infiltration is equal to the total precipitation less the losses due to interception by vegetation, retention in depressions on the land surface, evaporation, and surface runoff.     Infiltration capacity The maximum rate at which precipitation can pass through the surface into the soil, for a given soil in a given condition.       Infrared radiation Electromagnetic radiation lying in the wavelength interval between 0.8 micron and 1 millimeter. At the lower limit of this interval, the infrared radiation spectrum is bounded by visible radiation, while on its upper limit it is bounded by microwave radiation.      Input (or input signal) The quantity to be measured (or modulated, or detected, or operated upon) which is received by an instrument. For a thermometer, temperature is the input quantity.      Insolation In general, solar radiation received at the earth's surface. Contracted from incoming solar radiation.      Instrument A term used to describe a sensor (or sensors), the associated transducer(s), and the data readout or recording device.       Instrument correction The mean difference between the readings of a given instrument and those of a standard instrument.       Instrument error The correctable part of the inaccuracy of an instrument.        Instrument exposure The physical exposure of an instrument. The effect of immediate environment upon the representativeness of the measurements obtained by meteorological instruments is considerable and not always correctable. The purpose of the instrument shelter is to provide as good an exposure as possible.      Instrument flight rules (IFR) A set of regulations set down by the U.S. Civil Aeronautics Board to govern the operational control of aircraft on instrument flight. The abbreviation of this term is seldom used to denote the rules themselves, but is in popular use to describe the weather and/or flight conditions to which these rules apply.       Instrument landing system (ILS) A navigational aid used to facilitate the landing of an aircraft at an airport in instrument weather, i.e. low visibility.        Instrument shelter A box-like structure designed to protect certain meteorological instruments from exposure to direct sunlight, precipitation, and condensation, while at the same time providing adequate ventilation. Instrument shelters are painted white, have louvered sides, usually a double roof, and are mounted on a stand several feet above the ground with the door side facing poleward.      Instrument weather In aviation terminology, route or terminal weather conditions of sufficiently low visibility to require the operation of aircraft under instrument flight rules.      Interceptometer A rain gauge which is placed under trees or foliage to determine the rainfall in that location. By comparing this catch with that from a rain gauge set in the open, the amount of rainfall which has been intercepted by foliage can be determined.      Interface The point (physical and/or electrical) where two distinct data processing elements meet.       International Geophysical Year By international agreement, a period during which greatly increased observation of worldwide geophysical phenomena is undertaken through the cooperative effort of participating nations. July 1757-December 1958 was the first such year. However, precedent was set by the International Polar Years of 1882 and 1932.      International index numbers A system of designating meteorological observing stations by number, established and administered by the World Meteorological Organization. Under this scheme, specified areas of the word are divided into "blocks" each bearing a two-number designator. Stations within each block have an additional unique three-number designator, the numbers generally increasing from east to west and from south to north.      International Practical Temperature Scale of 1948 (IPTS-48) Specified by the 9th General Conference of Weights and Measures held in 1948. In the IPTS-48, the name "degree Centigrade" was replaced by "degree Celsius".      International Practical Temperature Scale of 1968 (IPTS-68) Set by the 1968 General Conference of Weights and Measures. In the IPTS-68, both thermodynamic and practical units were defined to be identical and equal to 1/273.16 of the thermodynamic temperature of the triple point of water. The unit itself was renamed "the kelvin" in place of "degree Kelvin" and designated "K" in place of "°K".      International Temperature Scale of 1927 (ITS-27) Adopted by the 7th General Conference of Weights and Measures in 1927.       International Temperature Scale of 1990 (ITS-90) An approximation to the thermodynamic temperature scale, it became the internationally recognized standard on January 1, 1990. On the ITS-90 scale, the atmospheric boiling temperature of water is approximately 373.124K (99.974°C).      International synoptic code A synoptic code approved by the World Meteorological Organization in which the observable meteorological elements are encoded and transmitted in "words" of five numerical digits length. Often abbreviated synoptic code.      Isobar A line of equal or constant pressure.      Isobaric Of equal or constant pressure, with respect to either space or time.     Iso-elastic spring A spring which is designed to achieve a fixed spring constant over a wide temperature range. Usually, this involves an alloy with high nickel content such as Ni-Span C. It is common for these springs to be stress relieved at elevated temperature after forming.     Isohel A line drawn through geographical points having the same duration of sunshine (or other function of solar radiation) during a given interval of time.      Isohume A line drawn through points of equal humidity on a given surface.      Isohyet Line drawn through geographical points recording equal amounts of precipitation during a given time period or for a particular storm.      Isonep A line drawn through all points on a map having the same amount of cloudiness.      Isophane A line drawn through geographical points where a given seasonal biological event occurs on the same date.      Isopluvial A line drawn through geographical points having the same pluvial index.      Isotherm A line of equal or constant temperature.      Isothermal layer Atmospheric layer throughout which there is no change of temperature with height, i.e. a zero lapse rate.       Isotropic radiation Diffuse solar radiation which has the same intensity in all directions.       Ivory point A small pointer extending downward from the top of the cistern of a Fortin barometer. The level of the mercury in the cistern is adjusted so that it just comes in contact with the end of the pointer, thus setting the zero of the barometric scale.   J Jevons effect The effect upon the measurement of rainfall caused by the presence of the rain gauge.      Jordan sunshine recorder A sunshine recorder of the type in which the time scale is supplied by the motion of the sun. It consists of two opaque metal semi-cylinders mounted with their curved surfaces facing each other. Each of the semi-cylinders has a short narrow slit in its flat side. Sunlight entering the slits falls on light sensitive paper which lines the curved side of the semi-cylinder. One semi-cylinder covers morning hours, the other afternoon hours.      Joule A unit of energy equal to 107 ergs or to 0.2389 calories.