_Ice Saints._--A period of cold weather popularly reputed in Europe to occur yearly about May 11-13 (or, in south-central Europe, May 12-14); also, the saints whose days in the ecclesiastical calendar fall on these dates. _Ice Storm._--(See _glaze_.) _Ignis Fatuus._--Will-o’-the-wisp. (See Chapter XXII.) _Indian Summer._--A period of mild, calm, hazy weather occurring in autumn or early winter, especially in the United States and Canada; popularly regarded as a definite event in the calendar, but weather of this type is really of irregular and intermittent occurrence. (Cf. _St. Martin’s Summer_.) _Instrument-shelter._--The American name of the cage or screen in which thermometers are exposed at meteorological stations. Called _thermometer-screen_ in Great Britain. _Inversion._--More fully _temperature inversion_; an increase of air temperature with increase of altitude, instead of the normal decrease. _Ion-counter._--An instrument for determining the number of ions present, per unit volume, in a sample of air. _Isobar._--A line of equal barometric pressure. (Isobars are generally drawn on maps to show the horizontal distribution of pressure reduced to sea level, or the pressure at some specified altitude; but in a broader sense any line on a chart or diagram drawn through places of equal pressure is an isobar.) _Isohyet._--A line of equal rainfall. _Isotherm._--A line of equal temperature. _Isothermal Layer._--(See _stratosphere_.) _January Thaw._--A period of mild weather popularly supposed to recur each January, especially in New England. _Katathermometer._--A device consisting of a dry-bulb and a wet-bulb thermometer, designed for measuring the cooling power of the atmosphere, with reference to its physiological effects. It was invented by Leonard Hill. _Kiosk._--The name given by the U. S. Weather Bureau to a small street pavilion in which are displayed meteorological instruments, maps, tables, etc. _Land and Sea Breezes. Land and Lake Breezes._--The breezes that, on certain coasts and under certain conditions, blow from the land by night and from the water by day. _Lenticular Cloud._--A cloud having approximately the form of a double-convex lens, marking the position of a standing wave in the atmosphere. (See Chapter VI.) _Lightning._--A disruptive electrical discharge in the atmosphere, or, generally, the luminous phenomena attending such a discharge. The various forms of lightning are named and described in Chapter IX. _Lightning Print._--A collection of marks, often treelike in form, sometimes found on the body of a person or animal that has been struck by lightning.