Weather Glossary – I


ICE JAM – an accumulation of broken river ice caught in a narrow channel that frequently produces local floods during a spring break-up.

ICE STORM – liquid rain falling and freezing on contact with cold objects creating ice build-ups of 1/4th inch or more that can cause severe damage.

INFLOW– Wind speed, in knots, of the average PBL windspeed.

INDIAN SUMMER – An unseasonably warm period near the middle of autumn, usually following a substantial period of cool weather.

INVERSION – An increase in temperature with height. The reverse of the normal cooling with height in the atmosphere.

INTERPOLATION– Drawing an isopleth between known numerical values.

INVERTED TROF– This is a trough which bulges to the north. Mid-latitude troughs have a north to south amplitude but in the tropics the opposite is the case. Inverted trough look like ridges but there is lower pressure at their centers of curvature. Inverted troughs can occur across areas of the tropics and sub-tropics.

INVOF– In the Vicinity OF

IRIDESCENCE – brilliant patches of green or pink sometimes seen near the edges of high- or medium-level clouds.

ISALLOBAR– A line of equal surface pressure change.

ISENTROPIC LIFT/DECENT– Lifting or sinking of air along constant potential temperature (theta) surfaces. WAA (especially over shallow frontal boundaries) leads to lift while CAA leads to decent.

ISOBAR – A line of equal barometric pressure on a weather map.

ISODROSOTHERM– A line of constant dewpoint temperature.

ISOHYET– A line of equal rainfall.

ISOLATED – Showers covering less than 15 percent of an area.

ISOPLETH– A line of a constant meteorological value.

ISOTACH– A line of constant wind speed.

ISOTHERM – the line of equal temperature denoted on surface weather maps.

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.