Subj : Wx Terms (I)(3) To : All From : Daryl Stout Date : Fri Nov 10 2017 12:05 am Iridescence Brilliant spots or borders of colors in clouds, usually red and green, caused by diffraction of light by small cloud particles. The phenomenon is usually observed in thin cirrus clouds within about 30ø of the sun and is characterized by bands of color in the cloud that contour the cloud edges. Iridescent Clouds Clouds that exhibit brilliant bright spots, bands, or borders of colors, usually red and green, observed up to about 30 degrees from the sun. The coloration is due to the diffraction with small cloud particles producing the effect. It is usually seen in thin cirrostratus, cirrocumulus, and altocumulus clouds. Irrigation In hydrologic terms, the controlled application of water to arable lands to supply water requirements not satisfied by rainfall. Irrigation Requirement In hydrologic terms, the quantity of water, exclusive of precipitation, that is required for crop production. It includes surface evaporation and other economically unavoidable wastes. Isallobar A line of equal change in atmospheric pressure during a specified time period. Isentropic Analysis A way in the forecaster can look at the atmosphere in 3-dimensions instead of looking at constant pressure surfaces (such as the 850 mb, 700 mb, 500 mb, etc.) which are in 2-dimensions. In this analysis method, the forecaster looks at constant potential temperature (the temperature that it would take if we compressed or expanded it adiabatically to the pressure of 1000 mb) surfaces. Air parcels move up and down these surfaces; therefore, the forecaster can see where the moisture is located and how much moisture is available. Isentropic Lift Lifting of air that is traveling along an upward-sloping isentropic surface. Isentropic lift often is referred to erroneously as overrunning, but more accurately describes the physical process by which the lifting occurs. Situations involving isentropic lift often are characterized by widespread stratiform clouds and precipitation, but may include elevated convection in the form of embedded thunderstorms. Isentropic Surface A two-dimensional surface containing points of equal potential temperature. Isobar A line connecting points of equal pressure. Isobaric Chart A weather map representing conditions on a surface of equal atmospheric pressure. For example, a 500 mb chart will display conditions at the level of the atmosphere at which the atmospheric pressure is 500 mb. The height above sea level at which the pressure is that particular value may vary from one location to another at any given time, and also varies with time at any one location, so it does not represent a surface of constant altitude/height (i.e., the 500 mb level may be at a different height above sea level over Dallas than over New York). Isobaric Process Any thermodynamic change of state of a system that takes a place at constant pressure. Isobath In hydrologic terms, an imaginary line on the earth's surface or a line on a map connecting all points which are the same vertical distance above the upper or lower surface of a water-bearing formation or aquifer. Isochrone A line on a chart connecting equal times of occurrence of an event. In a weather analysis, a sequence plotted on a map of the frontal positions at several different observation times would constitute a set of isochrones. Isodop A contour of constant Doppler velocity values. Isodrosotherm A line connecting points of equal dew point temperature. Isoheight Same as a contour depicting vertical height of some surface above a datum plane. Isohel A line on a weather map connecting points receiving equal sunlight. Isohyet A line connected points of equal precipitation amounts. ISOL Isolate(d) Isolated A National Weather Service convective precipitation descriptor for a 10 percent chance of measurable precipitation (0.01 inch). Isolated is used interchangeably with few. ISOLD Isolated Isopleth A broad term for any line on a weather map connecting points with equal values of a particular atmospheric variable (temperature, dew point, etc.). Isotherms, isotachs, etc. are all examples of isopleths. Isotach A line connecting points of equal wind speed. Isotherm A line connecting points of equal temperature. Isotropic Having the same characteristics in all directions, as with isotropic antennas. Directional or focused antennas are not isotropic. Issuance Time The time the product is transmitted. ITCZ Inter-tropical Convergence Zone. The region where the northeasterly and southeasterly tradewinds converge, forming an often continuous band of clouds or thunderstorms near the equator. ITWAS Integrated Terminal Weather System --- þ Synchronet þ The Thunderbolt BBS - wx1der.dyndns.org .