102 INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 



Hurricdjic. — A tropiral cyclone; es^pccially one of the "West Indian region. (A 

 cyclone originatini;- in tliis region and passing iiortliward into tlie Temperate 

 Zone is often called a "West India hnrricane", even after it lias assumed the 

 character of an extra-tropical cyclone, and if sufficiently severe, justifies the 

 display of "hurricane warnings" at ports of the United States. "Hurricane" 

 is also the designation of the highest wind force on the Beaufcnt scale. 



Hjidroiurtcor. — A generic term for weather phenomena such as rain, cloud, 

 fog. etc., which mostly depend upon modifications in the condition of the water 

 vapor in the atmosphere. 



Hyr/rof/niph . — A self-recording hygrometer. 



HiifiroDieter. — An instrument for measuring the humidity of the air. 



Icehcrfi. — A large mass of ice that lireaks from tlie tongue of a glacier run- 

 ning into the sea and floats away. 



IrcJiliiik. — A white, luminous appearance near the horizon caused by the 

 reflection of light from ice. 



Ice >;rrf7?c.s.— Thin crystals or shafts of ice. so light that they seem to be sus- 

 pended in the air. (See also ])art V.) 



Ire rain. — 1. A rain that causes a deposit of glaze. 2. F.-illing pellets 

 of clear ice (called .sleet by the United States Weather Bureau.) 



Ice Kfanii. — (Se(> Glaze.) 



Inclhidtion of the iriinl. — The angle which the wind direction makes VN'ith tbe 

 direction of the isobar at the place of oliservatioii. Over the ocean the angle 

 is u.sually between 2(1'' and 80°. ( Cf . Deviation of the wind.) 



Iiisohitioii. — Solar radiation, as received i)y the earth or other planets; also, 

 the rate of delivery of the same, per unit of horizontal surface. 



InstahiJifji. — A state in which the vertical distribution of temperature is 

 such that an air particle, if given either an upward or a downward impulse, 

 will tend to move away with increasing speed from its original level. (In 

 the ca.se of un.saturated air the lapse rate for instability will be greater than 

 the dry adiabatic lapse rate; in that of saturated air greater than the saturated 

 adiabatic lap.se rate.) 



Inxtrument sJtelfer. — The American name of the cage or screen in which 

 ihermometers and sometimes other instruments are exposed at meteorological 

 stations. Called thermometer screen in (jlreat Britain. 



Intertropieiil front. — The boundary Ix'tween the trade wind systems of the 

 northern and southern liemispheres. It manifests itst'lf as a fairly broad zone 

 of transition commonly known as the IJoldriiiiis. 



Inversion. — An abbreviation for "inversion of the vertical gradient of tem- 

 perature." The temperature of the air is ordinarily observed to l)ecome lower 

 with increasing height, but occasionally tlie rever.se is the case, and when the 

 temperature increases with height there is said to be an "inversion." 



Irisation. — Irregular patches or fringes of irid(>scence sometimes seen in 

 clouds (called iridescent clouds), not corresjionding in location with the ordi- 

 nary corona or the known forms of halo (such as parhelia). They are prob- 

 ably fragments of coronas of unusual size, produced by exceedingly fine chmd 

 ]iarticles. 



Isohar. — A line on a chart or diagram drawn through places or points having 

 the same barometric pressure. (Isobars are customarily drawn on weather 

 charts to show the horizontal distrilnition of atmospheric pressure reduced to 

 sea level or the pressure at some specified altitude.) 



Isofjram. — A line drawn on a chart or diagram to show the distriluition of 

 some physical condition in space or time 'or both), by connecting points cor- 

 responding to equal values of the phenomenon represented. Most of the iso- 

 grams used in meteorology are drawn on geographical charts, and show the 

 distribution of a meteorological element in space only. A special form of 

 isogram, known as the isopleth, shows the variation of an elemcMit in relation 

 to two coordinates ; one of the coordinates representing the time of the year 

 (month), and the other u.sually the time of the day (hour), but sometimes 

 space (especially altitude). The following list includes the most important 

 meteorological isograms: Animllohar, isogram of rise of barometric pressure 

 in a given time; i.sallotifn: isogram of the amount of change in barometric 

 pressure within a specified period: isanonui], or isnnonmlons line, isogram of 

 anomaly, i. e., of the departure of the local mean value of an element from 

 the mean pertaining to the latitude; ii^ohor, isogram of barometric pressure; 

 isotherm, isogram of temperature; katisaJloJiar. isogi-am of fall of barometric 

 pressure in a given time. 



