ICE
The solid form of water. It can be found in the atmosphere in the form of ice crystals, snow, ice pellets, and hail for example.

ICE CRYSTALS
Precipitation in the form of slowly falling, singular or unbranched ice needles, columns, or plates. They make up cirriform clouds, frost, and ice fog. Also, they produce optical phenomena such as halos, coronas, and sun pillars. May be called "diamond dust." It is reported as "IC" in an observation and on the METAR.

ICE FOG
Fog that is composed of minute ice particles. It occurs in very low temperatures under clear, calm conditions in the polar latitudes and may produce a halo around the sun or moon.

ICE JAM
An accumulation of broken river ice caught in a narrow channel, frequently producing local flooding. Primarily occurs during a thaw in the late winter or early spring.

ICELANDIC LOW
A semi-permanent, subpolar area of low pressure in the North Atlantic Ocean. Because of its broad area and range of central pressure, it is an area where migratory lows tend to slow down and deepen. It is strongest during a Northern Hemisphere winter and early spring, centered over Iceland and southern Greenland, and is the dominate weather feature in the area. During the summer, it is weaker, less intense, and might divide into two parts, one west of Iceland, the other over the Davis Strait between Greenland and Baffin Island. Then the Azores or Bermuda High becomes the dominate weather feature in the North Atlantic.

ICE PELLETS
Precipitation in the form of transparent or translucent pellets of ice, which are round or irregular in shape. They have a diameter of 0.2 inches (5 mm) or less. They are classified into two types: hard grains of ice consisting of frozen rain drops or largely melted and refrozen snowflakes; pellets of snow encased in a thin layer of ice which have formed from the freezing of droplets intercepted by pellets or water resulting from the pa tial melting of pellets. It is reported as "PE" in an observation and on the METAR.

ICE STORM
A severe weather condition characterized by falling freezing precipitation. Such a storm forms a glaze on objects, creating hazardous travel conditions and utility problems. ICICLE
Ice that forms in the shape of a narrow cone hanging point down. It usually forms when liquid water from a sheltered or heated source comes in contact with below-freezing air and freezes more or less rapidly as it flows.

ICING
The forming or depositing of ice on an object.

INCHES OF MERCURY (Hg)
The name comes from the use of mercurial barometers which equate the height of a column of mercury with air pressure. One inch of mercury is equivalent to 33.86 millibars or 25.40 millimeters. First devised in 1644 by Evangelista Torricelli (1608-1647), an Italian physicist and mathematician, to explain the fundamental principles of hydromechanics.

INDIAN SUMMER
A period of abnormally warm weather in mid to late autumn with clear skies and cool nights. A first frost normally precedes this warm spell. INFRARED
The long wave, electromagnetic radiation of radiant heat emitted by all hot objects. On the electromagnetic spectrum, it can be found between microwave radiation and visible light. Water vapor, ozone, and carbon dioxide are capable of absorbing or transmitting infrared radiation. May be referred to as IR.

INSOLATION
Solar radiation or heating received at the earth's surface. The name is derived from INcoming SOLar radiATION.

INSTABILITY
The state of equilibrium in which a parcel of air when displaced has a tendency to move further away from its original position. It is the condition of the atmosphere when spontaneous convection and severe weather can occur. Air parcels, when displaced vertically, will accelerate upward, often forming cumulus clouds and possibly thunderstorms.

INSTRUMENT FLIGHT RULES (IFR)
Refers to the general weather conditions pilots can expect at the surface and applies to the weather situations at an airport during which a pilot must use instruments to assist take off and landing. IFR conditions for fixed wing aircraft means the minimum cloud ceiling is greater than 500 feet and less than 1,000 feet and/or visibility is greater than 1 mile and less than 3 miles.

INSTRUMENT SHELTER
A boxlike structure designed to protect temperature measuring instruments from exposure to direct sunshine, precipitation, and condensation, while at the same time time providing adequate ventilation.

INTERMOUNTAIN HIGH
An area of high pressure that occurs during the winter between the Rocky Mountains and the Sierra-Cascade ranges. It blocks the eastward movement of Pacific cyclones. Also called Plateau High or Great Basin High.

INTERNATIONAL DATE LINE
The line of longitude located at 180 East or West (with a few local deviations) where the date changes by a day. West of the line it is one day later than east of the line.

INTERTROPICAL CONVERGENCE ZONE (ITCZ)
An area where the Northern and Southern Hemispheric trade winds converge, usually located between 10 degrees North and South of the equator. It is a broad area of low pressure where both the Coriolis force and the low-level pressure gradient are weak, occasionally allowing tropical disturbances to form. It fluctuates in location, following the sun's rays, so that during the Northern Hemisphere summer, the ITCZ moves northward over the southern North Atlantic and southern Asia.

INVERSION
A departure from the usual increase or decrease of an atmospheric property with altitude. It usually refers to an increase in temperature with increasing altitude, which is a departure from the usual decrease of temperature with height.

IONOSPHERE
A complex atmospheric zone of ionized gases that extends between 50 and 400 miles (80 to 640 kilometers) above the earth's surface. It is located between the mesosphere and the exosphere and is included as part of the thermosphere.

ISALLOBAR
The line of equal change in atmospheric pressure during a certain time period. It marks the change in pressure tendency.

ISOBAR
The line drawn on a weather map connecting points of equal barometric pressure.

ISODROSOTHERM
The line drawn on a weather map connecting points of equal dew point.

ISOHEL
A line drawn through geographic points having equal duration of sunshine or another form of solar radiation during a specified time period.

ISOHYET
The line drawn through geographic points recording equal amounts of rainfall during a given time or for a given of storm.

ISOPLETH
A line connecting equal points of value. Also called an isoline.

ISOTACH
A line connecting equal wind speeds.

ISOTHERM
The line of equal or constant air temperature. If something is isothermal, it is of equal or constant temperature with respect to either time or space.