Weather Words

Orographic Clouds: Clouds formed when air forced to rise when flowing over mountains or large hills reaches the condensation level.

Outflow: The outward flow of air from a weather system. From a thunderstorm, it is the result of cold downdrafts, and its passage includes a wind shift and temperature drop. On the Canadian Pacific Coast, an outflow usually describes the movement of cold air off the continent through mountain passes and over coastal waters.

Ozone: A molecule made up of three oxygen atoms. It absorbs both ultraviolet-B and heat radiation. It is formed in the upper atmosphere through the dissociation of oxygen molecules by solar radiation. In the lower troposphere, it is a pollutant which characterizes photochemical or Los Angeles-type smog. It is formed by the photochemical reactions of nitrogen oxides and hydrocarbon gases.

Ozone Column Thickness or Total Column Ozone: The total amount of ozone present in a column of the Earth's atmosphere from the surface to the top of the atmosphere. It includes both surface level ozone and ozone found in the ozone layer. Measurement is either in Dobson Units or atmosphere-cm.

Ozone Layer: About 90% of the ozone in the Earth's atmosphere is found in a layer between 22 and 30 km above the surface. This layer is within a region of the atmosphere called the stratosphere. The stratosphere is a stable layer of the atmosphere where exchanges with the lower atmosphere or troposphere are minimal. Ozone is naturally formed here through the action of sunlight on molecular oxygen (O2). The introduction of ozone-depleting chemicals into the ozone layer destroy ozone at a faster rate than the natural rate of production.

Parhelion or Parhelia (plural) : A bright, colored portion of a solar halo which forms on points 22° either side of the sun and at the same elevation. These luminous spots are caused by the refraction of light by ice crystals. The terms mock sun and sun dog are also used to describe this optical phenomenon.

Persistence Forecast: A weather forecast that the future weather will be the same as the current conditions, e.g. tomorrow's weather will be the same as today's. In the tropics where weather doesn't vary much, a persistence forecast can be quite accurate. In areas along the polar front where storm waves rule and conditions change rapidly, it can be very unreliable. However, it is often used by meteorologists against which to judge forecasting skill.

Phase: 1: The aggregate state of a substance: solid, liquid, or vapour/gas; 2: A particular shape for the appearance of the Moon: i.e., Full, New, Crescent; 3: A point of reference on a periodic wave.

Photochemistry and Photochemical Reactions: The chemical reaction of two or more substances which is caused or hastened by the interaction of light on the reactants. In the atmosphere, the source of this light is the sun.

Pillar: A vertical shaft of light extending above or below the sun, moon, or other strong light source. Also, sun pillar.

Polar Front: The boundary separating air masses of tropical origin from those of polar origin. The front is usually only present in the far-northern polar regions in summer but dips down to the more southerly temperate latitudes during the winter. Many cyclonic storm systems form along the polar front.

Polar Stratospheric Clouds: Clouds which form in the stratosphere when the temperature falls to below -80^oC. The clouds are composed of ice which provides a surface for chemical reactions that result in the release and storage of reactive chlorine and bromine. In the spring when the sun warms the clouds, they release the chlorine and bromine which then may destroy ozone.

Probability of Precipitation or POP: The POP expressed as some percentage X means that forecasters have determined that in a 100 similar weather situations, rain or snow will have fallen X times in the forecast area. Note that POP is for any point in the forecast area, not the whole area. With a POP of 100% in the forecast, rain or snow is almost a certainty somewhere in the forecast area.

Psychrometer: An instrument used to measure water vapour content of the atmosphere. A common type consists of two liquid-in-glass thermometers, one a dry bulb and another, with a muslin sock over the tip which is saturated with water, called the wet bulb. The thermometers are ventilated to cause evaporation off the wet bulb thermometer. The atmospheric humidity is determined from the readings of these two thermometers.

Pyrocumulus clouds: Cumulus cloud forms formed by fire, usually large wildfires. Similarly, pyrocumulonimbus, cumulonimbus clouds formed by fire.

Quasi-Stationary Front: A weather front which is nearly stationary, having speeds of less than 8 km/h (5 mph).

R

Radiosonde: An instrument package connected to a weather balloon that collects, and transmits by radio, meteorological data as it ascends through the atmosphere.

Rainforest: Lush forests growing in regions of high precipitation. Most well known are the tropical rainforests or jungles such as in the Amazon basin. Rainforests also grow along the northern Pacific coast of North America. These great northern and temperate rainforests covered much of coastal regions of southern Alaska, British Columbia, and Washington State.

Rain Shadow: The region on the lee side of a mountain or mountain range, where precipitation is decreased compared to the windward side. The rain shadow results when moisture-laden air is forced to ascend the encircling mountains and thus drops most of that moisture on the windward side of the range summits. As the air descends from the ridge, it is drier and warmer. Not only does the compression of descent warm the air, it also evaporates much of the liquid water, causing clouds to disappear. The downward motion of the air masse further inhibits cloud formation on the lee of these ridges.

Reflectivity: A measure of the fraction of incident radiation falling on a surface that is turned back from it by reflection. Reflectivity also refers to the degree by which precipitation is able to reflect a radar beam.

Relative Humidity: The ratio of the actual water vapour content of a given mass of air to that it would hold at saturation, usually expressed as a percentage. Also, the ratio of the water vapour pressure to the its saturation vapour pressure.

Residence Time: The length of time that some substance remains in a reservoir. For example, the typical residence time of carbon dioxide in the atmosphere is the average length of time that a molecule of the gas will remain in the air before chemically transforming or moving into the hydrosphere, biosphere, or lithosphere.

Ridge: When an extensive area of high pressure is elongated, it is called a high pressure ridge, or simply a ridge.

Rime: Ice that is opaque, milky and crystalline ice forming on vertical and horizontal surfaces by the freezing of supercooled water. Rime ice is less dense than glaze ice ( 0.2 to 0.3 grams per cubic centimetre) and clings less tenaciously. Factors favouring rime formation include: small water drop size, slow accretion, high supercooling of water in the drop, and rapid dissipation of the heat of fusion on freezing.

Runoff: The movement of water across the earth's surface or just beneath the surface coming from liquid precipitation or the melting of ice/snow. Runoff ultimately reaches stream channels, rivers, lakes, or seas unless it is evaporated or taken up by the biosphere.

Santa Ana Wind: A hot, dry desert, foehn-type wind blowing from the northeast or east in the pass and river valley around Santa Ana, California. The wind may blow with great force and carry large amounts of dust. It most frequently occurs in the winter months. However, when it blows in the autumn, it can dessicate vegetation and increase the hazards from wild fires in canyons of Southern California.

Satellite Imagery: Images formed from data collected by a weather satellite that reveal visual information on the nature of the atmosphere such as the flow of water vapour, the development and movement of weather systems, or the distribution of the temperatures of cloud tops or the earth's surface.

Saturation: The condition when the partial pressure of water vapour in the atmosphere is at its maximum level for the existing temperature and pressure. For example, at saturation an equilibrium exists between water vapour and liquid water and there is no net evaporation or condensation.

Season: A division of the year according to some regularly recurring event or phenomena, usually astronomical or climatological. The four astronomical seasons: Spring, Summer, Autumn or Fall, and Winter are generally defined by the position of the sun with respect to the Earth's equatorial plane. The four climatological seasons known to most are tied to the astronomical seasons. However, in areas where climate changes little through the year, such as the tropics, there may be only two seasons: the wet and the dry. Seasons may also be defined by other criteria such as: the persistence of prevailing wind directions (the monsoons); biological processes such as growing or dormant; human energy requirements. There may be any number of seasons per year depending on the phenomena used to define them.

Short-Wave Radiation or Solar Radiation: That portion of the electromagnetic spectrum less than 4,000 nanometres which is received from the sun.

Shower: Precipitation falling from a convective cloud, characterized by the suddenness with which it starts and stops, by rapid changes of intensity and usually rapid changes in the appearance of the sky.

Singularity: A characteristic meteorological condition that tends to occur on or near a specific calendar date more frequently than chance would indicate.

Sleet: Precipitation composed of already frozen droplets — ice pellets — that bounce on impact. Outside the United States, sleet also refers to precipitation with a mixture of rain and snow or rain and ice pellets or quickly melting snow.

Snow: A type of frozen precipitation composed white translucent ice crystals in a variety of complex hexagonal forms.

Snowbelt: In general, any area where topographical influences favour increased accumulation of snow over the surrounding region. Usually refers to the areas around large lakes such as the Great Lakes where lake-effect snows produce significantly enhanced winter accumulations.

Snowburst: An intense convectively generated snow squall with large accumulations of snow at a rate of 2.5 to 7.5 cm per hour (1 to 3 inches per hour). May be accompanied by thunder and lightning; such storms are called thundersnows.

Snow Crystal: One of several types of ice crystal found in snow. A snow crystal is an individual ice crystal whereas a snowflake is usually an aggregate of many single crystals. See Ice Crystals

Snowflake: A single ice crystal or an aggregate of ice crystals falling from a cloud.

Snow Squall: Short, intense snow showers accompanied by strong, gusty winds. Short- term snow accumulations may be significant and visibility greatly reduced. Snow squalls are common along the shores of the Great Lakes and other large lakes. See also, lake-effect snowfall.

Solar Seasons: A division of the year for non-tropical latitudes into four three-month periods based on the daylength or amount of potential solar radiation. During solar winter and solar summer, the day length changes most slowly from day to day, with the least variation around the solstice. During solar spring and fall -- the transistion seasons -- the day to day change in daylength is very rapid, peaking at the equinox.

Solar Summer: The three-month period when day length is longest and solar radiation above the clouds is the greatest. In the Northern Hemisphere, the period from approximately May 1 to August 1.

Solar Winter: The three-month period when day length is shortest and solar radiation above the clouds is the least. In the Northern Hemisphere, the period from approximately November 1 to February 1.

Solstice: Either of two days when the sun's position overhead at noon is farthest north or south of the equator. These dates are June 21 for the Northern Hemisphere summer solstice, and about December 22 for the Northern Hemisphere winter solstice. The solstices are reversed for the Southern Hemisphere, with the winter solstice on June 21.

Southern Oscillation (ENSO): A periodic reversing of the surface air pressure pattern across the tropical Pacific Ocean. The Southern Oscillation is associated with El Niño events so that the two are often referred together by the acronym ENSO.

Specific Humidity: The ratio of the mass of water vapour to the total mass of air in a given volume.

Speed of Sound: The speed that a sound wave travels through a given medium. The speed of sound depends on several properties of the medium, most importantly its density. In the atmosphere, the speed changes with air temperature and pressure. At standard sea-level pressure (101.3 kPa), the speed of sound is 340 metres per second (760 mph).

Squalls: 1) A strong wind which arrives suddenly, lasts minutes and ends with a sudden decrease in speed. 2) A severe local storm with strong. gusty winds and usually precipitation, may be accompanied by thunder and lightning.

Stable Air: An air mass or portion thereof in which vertical motions are inhibited. Usually found in regions of the atmosphere where an inversion temperature profile is found.

State: The form of a substance: solid, liquid, or gas/vapour.

Stratus: The lowest clouds, generally found below 3000 metres altitude and often appearing as an overcast deck. Can also be found as scattered patches. Individual cloud elements have very ill-defined edges. Stratus is comes from the Latin word for "layer."

Stratosphere: The region of the atmosphere extending from the tropopause (8 to 15 km altitude) to about 50 km. The temperature of the stratosphere is warmer than the upper troposphere thus making it a region of high stability and low humidity.

Storm Surge: An abnormal rise local rise in sea level accompanying an intense storm system, either tropical or extratropical caused by the storm pushing a wall of water ahead of it. Storm surge is often the most damaging and deadly part of a tropical hurricane or cyclone, particularly if it arrives at high tide.

Storm Track: The path followed by the centre of a low pressure system or cyclone over a given period of time, usually its lifetime.

Sublimation: The transition of a substance from the solid phase directly to the vapour phase without being in the liquid phase. The opposite of sublimation is deposition.

Summer: 1) The warmest season of the year (except in some tropical regions) and the high sun season. 2) The period between the Summer Solstice and the Autumnal Equinox (June, July and August in the Northern Hemisphere; December, January, February in the Southern Hemisphere).

Summer Solstice: The date on which the sun reaches the greatest distance north (in the Northern Hemisphere) or south (in the Southern Hemisphere) of the celestial equator. In the Northern Hemisphere, the summer solstice occurs when the sun in overhead at the Tropic of Cancer (latitude 23° 27' N) about June 21. In the Southern Hemisphere, the summer solstice occurs when the sun in overhead at the Tropic of Capricorn (latitude 23° 27' S) about December 22.

Sun Dog: A popular term used to describe the parhelion, a bright portion of a solar halo which forms on points 22° either side of the sun and at the same elevation. Sun dogs are colored, luminous spots caused by the refraction of light by ice crystals. The term mock sun is also used to describe this optical phenomenon.

Sun Pillar: A pillar forming from sunlight. See Pillar

Supercooled: A state when the temperature of liquid water falls below 0^oC (32^oF) without freezing. Although we commonly speak of 0^oC as the freezing point of water, water, especially in the droplet form, rarely freezes at this temperature. (Pure ice, on the other hand, melts at 0^oC, thus, this temperature should be more correctly called the melting point of ice.) The temperature of freezing varies with the size of the droplet and the concentration of any impurities in the water. Very small droplets of pure water may not freeze spontaneously until the temperature has fallen to -40^oC (-40^oF).)

Surface Analysis: The weather map depicting surface weather conditions plotted from reported data or generated by computer models. Surface Analysis maps generally display isobars, fronts and centres of High and Low pressure.

The Weather Doctor's Weather Glossary ©2006, Keith C. Heidorn, PhD. All Rights Reserved.
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