Glossary of Terms

Advection: The transfer of air mass properties horizontally by the velocity field of the atmosphere.
Air: A mixture of many discrete gases, of which nitrogen and oxygen are in which varying quantities of tiny solid and liquid particles are suspended.
Air Mass: A large body of air, usually 1600 kilometres or more across, that is characterized by homogenous physical properties at any given altitude.
Air Pressure: The force per unit area exerted by the weight of a column of air above a given point.
Anticyclone (or High): An area of high atmospheric pressure characterized by diverging and rotating winds and subsiding air aloft. Winds blow outwards and clockwise about an anticyclone in the northern hemisphere, and they blow outwards and anticlockwise about an anticyclone in the southern hemisphere.
Anticyclonic : Motion in a clockwise manner in the northern hemisphere and motion in an anticlockwise manner in the southern hemisphere.
AWS: Automatic Weather Station.
Atmosphere: The gaseous portion of a planet; the planet’s envelope of air; one of the traditional subdivisions of the earth’s physical environment.
Atmospheric pressure : Pressure (force per unit area) exerted by the atmosphere on any surfaceby virtue of its weight.
CBH: Climate Branch Head.
Celsius Scale: A temperature scale devised by Anders Celsius in 1742 and used where the metric system is used (at one time called the Centigrade
Scale. For water at sea level, 0 °C is designated the ice (freezing) point and 100 °C the steam (boiling) point.
Cirrus: One of three basic cloud forms; They are thin, delicate ice crystal clouds often appearing as veil-like patches or thin wispy fibers.
Climate: A description of weather conditions over a period of time, usually 30 years; the sum of all statistical weather information that helps describe a place or region.
Climatic Change: A study dealing with variations in climate on many different time scales, from decades to millions of years, and the possible causes of such variations.
Cloud: A form of condensation best described as a dense concentration of suspended water droplets or tiny ice crystals.
Cold Front: The boundary (discontinuity) at the forward edge of an advancing cold air mass that is displacing warmer air in its path.
Condensation: The change of state from a gas to a liquid.
Conduction: The transfer of heat through matter by molecular activity. Energy is transferred during collisions among molecules.
Convection: The transfer of heat by the movement of a mass or substance. It can only take place in fluids.
Convergence: The condition that exists when the distribution of winds within a given area results in a net horizontal inflow of air into the area. Since convergence at lower levels is associated with an upward movement of air, areas of convergent winds are regions favorable to cloud formation and precipitation.
Cumulus: One of three basic cloud forms; cumulus are billowy individual cloud masses that often have flat bases.
Cyclone (or Low): An area of low atmospheric pressure characterized by rotating and converging winds and ascending air. Winds blow in and anticlockwise about a cyclone in the northern hemisphere and they blow in and clockwise about a cyclone in the southern hemisphere.
Cyclonic : Motion in an anticlockwise manner in the northern hemisphere and motion in a clockwise manner in the southern hemisphere.
Dew: A form of condensation consisting of small water drops on grass or other objects near the ground that forms when the surface temperature falls below the dew point. Usually associated with radiation cooling on clear, calm nights.
Dew Point: The temperature to which air has to be cooled in order to reach saturation.
Discontinuity: A zone characterized by a comparatively rapid transition of meteorological elements.
Diurnal: Daily, especially pertaining to actions that are completed within 24 hours and that recur every 24 hours.
Divergence: The condition that exists when the distribution of winds within a given area results in a net horizontal outflow of air from the region. In divergence at lower levels the resulting deficit is compensated for by a downward movement of air from aloft; hence, areas of divergent winds are unfavorable to cloud formation and precipitation.
Doppler Radar: A type of radar that has the capability of detecting motion directly.
Drizzle: Precipitation from stratus clouds consisting of tiny droplets.
Elements (atmospheric): Those quantities or properties of the atmosphere that are measured regularly and that are used to express the nature of weather and climate.
Evaporation: The process by which a liquid is transformed into a gas.
Eye: A roughly circular area of relatively light winds and fair weather at the centre of a tropical cyclone (tropical storm or hurricane). Eye Wall : The doughnut-shaped area of intense cloud development and very strong winds that surrounds the eye of a tropical cyclone. Fahrenheit Scale : A temperature scale devised by Gabriel Daniel Fahrenheit in 1714 and used in the English system. For water at sea level, 32 °F is designated as the ice (freezing) point and 212 °F the steam (boiling) point.
Fog : A suspension of very small, usually microscopic, water droplets in the air generally reducing the horizontal visibility at the earth’s surface to less than 1 kilometre.
Front : A boundary (discontinuity) separating air masses of different densities, one warmer and often higher in moisture content than the other.
Global Circulation : The general circulation of the atmosphere; the average flow of air over the entire globe.
GMT : Greenwich mean time (now replaced by UTC).
Gust : Sudden and brief increase of the wind speed over its mean value.
Hail : Precipitation in the form of hard, round pellets or irregular lumps of ice.
High : See anticyclone.
Humidity : A general term referring to water vapor in the air.
Hurricane : A tropical cyclone having minimum winds of 119 kilometres per hour; also known as typhoon (western Pacific) and cyclone (Indian Ocean).
Hurricane Season : That portion of the year having a relatively high incidence of hurricanes (or tropical cyclones). In the Atlantic, the Caribbean and the Gulf of Mexico, it is the period from June 1 to November 30.
Hydrological Cycle : The continuous movement of water from the oceans to the atmosphere (by evaporation), from the atmosphere to the land (by condensation and precipitation), and from the land back to the sea (via stream flow). Intertropical Convergence Zone (ITC or ITCZ) : The zone of general convergence between the northern and southern hemisphere trade winds.
Isobar : A line drawn on a map connecting points of equal barometric pressure.
Isohyet : A line connecting places having equal rainfall.
Isotach : A line connecting points of equal wind speed.
Isotherm : A line connecting points of equal air temperature.
Jet Stream : Swift airstreams (currents of air) in the upper troposphere whose axis is along a line of maximum speed and which is characterized by great speeds and strong wind shears.
Land Breeze : A local wind blowing from land towards the sea during the night in coastal areas.
Lightning : A sudden flash of light generated by the flow of electrons between oppositely charged parts of a cumulominbus cloud or between the cloud and the ground.
Low : See cyclone.
Mesosphere : The layer of atmosphere above the stratosphere where temperatures drop fairly rapidly with increasing height.
METAR : Routine Aviation report.
Millibar (mb) : The standard unit of pressure measurement. One millibar (mb) equals 100 newtons per square metre.
Monsoon : The seasonal reversal of wind direction associated with large continents, especially Asia. In winter the wind blows from land to sea; in summer it blows from sea to land.
NHC : National Hurricane Centre.
NMC : National Meteorological Centre.
Occluded Front : A front formed when a cold front overtakes a warm front.
ODPEM : Office of Disaster Preparedness and Emergency Management
Precipitation : Solid or liquid water particles falling from clouds through the atmosphere. eg. hail, snow and rain.
Pressure Gradient : The amount of pressure change occurring over a given distance.
Pressure Tendency : The nature of the change in atmospheric pressure over the past several hours.
Radiation: The wavelike energy emitted by any substance that possesses heat. This energy travels through space at 300,000 kilometres per second (the speed of light).
Radiosonde : A lightweight package of weather instruments fitted with a radio transmitter and carried aloft by a balloon.
Rain : Precipitation of liquid water particles, either in the form of drops more than 0.5 millimetres (mm) in diameter, or of smaller widely scattered drops.
Rainbow : A luminous arc formed by the refraction and reflection of light in drops of water.
Relative Humidity : The ratio of the air’s water vapor content to its water vapor capacity.
Ridge : An elongated region of high atmospheric pressure.
Saturation : The maximum possible quantity of water vapor that the air can hold at any given temperature and pressure.
Sea Breeze : A local wind blowing from the sea during the afternoon in coastal areas.
SIGMET : Significant
Sleet : Frozen or semifrozen rain formed when raindrops pass through a subfreezing layer or air.
Smog : A word currently used as a synonym for general air pollution. It was originally created by combining the words “smoke” and “fog”.
Snow : Precipitation in the form of white or translucent ice crystals, chiefly in complex branched hexagonal form and often clustered into snowflakes.
Squall : Atmospheric phenomenon characterized by an abrupt and large increase of wind speed with a duration of the order of minutes which diminishes rather suddenly. It is often accompanied by showers or thunderstorms.
Stationary Front : A front which is stationary or nearly so (conventionally, moving with a speed of less than five knots).
Storm Surge : The difference between the actual water level under influence of a meteorological disturbance, for example a storm or hurricane, (storm tide) and the level that would have been attained in the absence of the disturbance (astronomical tide).
Stratosphere : The zone of the atmosphere above the troposphere that is characterized at first by isothermal conditions and then a gradual temperature increase. The earth’s ozone is concentrated here.
Stratus : One of the three basic cloud forms. They are sheets or layers that cover much or all of the sky.
Subsidence : An extensive sinking motion of the air, most frequently occurring in anticyclones. Associated with warm, dry stable conditions.
Temperature : A measure of the degree of hotness or coldness of a substance.
Thermosphere : The zone of the atmosphere beyond the mesosphere in which there is a rapid rise in temperature with height.
Thunder : The sound emitted by rapidly expanding gases along the channel of a lightening discharge.
Thunderstorm : A storm produced by a cumulonimbus cloud and always accompanied by lightening and thunder. It is of relatively short duration and is usually accompanied by strong wind gusts, heavy rain and sometimes hail.
Tornado : A violently rotating column of air attended by a funnel shaped or tubular cloud extending downward from a cumulonimbus cloud.
Trade Winds : Two belts of winds that blow almost constantly from the subtropical highs toward the equator. The predominant directions are northeast in the northern hemisphere and southeast in the southern hemisphere.
Tropical Cyclone : A non-frontal cyclone originating over tropical or subtropical waters with organized convection and definate cyclonic wind circulation.
Tropical Disturbance : An area of organized convection with light surface winds and indications of cyclonic circulation.
Tropical Depression : A tropical cyclone with wind speed up to 33 knots.
Tropical Storm: A tropical cyclone with maximum wind speed of 34 – 47 knots.
Severe Tropical Storm : A tropical cyclone with maximum wind speed of 48 – 63 knots.
Hurricane : A tropical cyclone with maximum wind speed of 64 knots or more.
Tropical Wave : Wave-like deformation or perturbation of the wind flow in the Tropics, sometimes resulting in a zone of cloudy and/or showery conditions moving from the East to the West.
Troposphere : The lowermost layer of the atmosphere marked by considerable turbulence and, in general, a decrease in temperature with increasing height.
Trough : An elongated region of low atmospheric pressure.
UTC : Universal Coordinated Time
Visibility : The greatest distance that prominent objects can be seen and identified by unaided, normal eyes.
Vorticity : The tendency of air to rotate in either a cyclonic or anticyclonic manner.
Warm Front : The discontinuity at the forward edge of an advancing warm air mass that is displacing cooler air in its path.
Weather : The state of the atmosphere at any given time as defined by the various meteorological elements.
Weather Forecasting : Predicting the future state of the atmosphere.
Wind : Air flowing horizontally with respect to the earth;’s surface.
Wind Chill : A measure of apparent temperature that uses the effects of wind and temperature on the cooling rate of the human body. Wind Direction : The direction from which the wind blows.
Wind Shear : The local variation of the wind in a specific direction.
World Meteorological Organization (WMO) : Established by the United Nations, the WMO consists of more than 130 nations. The organization is responsible for coordinating, standardizing and improving meteorological activities throughout the world and for encouraging and facilitating the efficient exchange of information between countries, in the interest of various human activities.
Weather Analysis : The stage prior to weather forecasting which involves collecting compiling and transmitting observational data.