In the meteorological world, each word or phrase typically has a very specific meaning. Since this jargon is used so freely and are so ubiquitous, we may not know what they actually mean. Even general terms like “today” (sunrise to sunset) and “this afternoon” (noon till 6 p.m.) have parameters.
For instance, here’s our winter weather forecast for today:
— NWS Chicago (@NWSChicago) February 5, 2019
As we gear up for this “ice storm” (which indeed has a specific definition), here are the words and phrases that you may hear and what they actually mean:
A thin coating of ice on terrestrial objects, caused by rain that freezes on impact. The ice is relatively transparent, as opposed to rime ice, because of large drop size, rapid accretion of liquid water, or slow dissipation of latent heat of fusion.
A rain/drizzle that falls as a liquid but freezes into glaze or rime upon contact with the cold ground or surface structures.
A fog the droplets of which freeze upon contact with exposed objects and form a coating of rime and/or glaze.
A deposit of interlocking crystals formed by direct sublimation on objects, usually those of small diameter freely exposed to the air, such as tree branches, plants, wires, poles, etc. The deposition of hoar frost is similar to the process by which dew is formed, except that the temperature of the frosted object must be below freezing. It forms when air with a dew point below freezing is brought to saturation by cooling.
An ice storm is used to describe occasions when damaging accumulations of ice are expected during freezing rain situations. Significant accumulations of ice pull down trees and utility lines resulting in loss of power and communication. These accumulations of ice make walking and driving extremely dangerous. Significant ice accumulations are usually accumulations of ¼” or greater.
Any of the following combinations of freezing and frozen precipitation: snow and sleet, snow and freezing rain, or sleet alone. Rain may also be present.
An opaque coating of tiny, white, granular ice particles caused by the rapid freezing of supercooled water droplets on impact with an object. See also clear ice.
Pellets of ice composed of frozen or mostly frozen raindrops or refrozen partially melted snowflakes. These pellets of ice usually bounce after hitting the ground or other hard surfaces. Heavy sleet is a relatively rare event defined as an accumulation of ice pellets covering the ground to a depth of ½” or more.
Precipitation in the form of ice crystals, mainly of intricately branched, hexagonal form and often agglomerated into snowflakes, formed directly from the freezing of the water vapor in the air.
Need to know the definitions of other weather terms? Check out NOAA’s glossary!