Below is a list of weather terms that are described in weather reports, forecasts and readings.


A horizontal movement of any weather condition by the movement of air. For example along the West Coast summer fog occurs when warmer moist air moves over the cold upwelled waters near shore.

Advection Fog, marine layer clouds

Fog or clouds that develops when warm moist air moves over a colder surface. As that air cools to below its dew point water vapor condenses into tiny water droplets and is visible as fog.

Air mass

A large body of air with similar temperature and moisture levels. For example a marine air mass.

Aleutian low, Gulf of Alaska low

A semi-permanent area of low pressure located in the Gulf of Alaska. This low gives birth to storms and passing lows get stronger in this area. In winter and spring it sends storms towards the West Coast. During the summer this low weakens and retreats towards the North Pole. During this time the North Pacific High dominates the north pacific.

Anemometer, wind sensor

An instrument used to determine wind speed. Wind sensor

Anticyclone, North Pacific High, Pacific high, Bermuda high.

A large area descending air creating high pressure. The hich winds blow outward from this high and are turned clockwise in the Northern Hemisphere. On the west coast the North Pacific High is commonly found over the offshore waters of the eastern pacific.

Backing Wind

Wind that changes its direction in a counter clockwise fashion. For example a Northwest wind changing to a West wind. See veering wind.

Barometric Pressure (see pressure gradient)

The pressure exerted by the atmosphere over a given point. This pressure is expressed in millibars (mb) or in inches of mercury (Hg). The greater the barometric pressure between two points the stronger the wind. See isobars.

Blocking High

A high pressure area, often over the south central USA, that stays in place for a prolonged period. This acts to stop the normal progression of weather systems from west to east for long periods of time. This tends to maintain the current wind pattern

Boundary Layer

The layer of air near the earth’s surface within which the effects of friction are significant. It is roughly the lowest one or two kilometers of the atmosphere. Within this layer the speed and direction of the wind is affected by friction with the earth’s surface

Buoys/Weather buoys

Buoys placed in the near shore coastal waters to transmit information weather information. The wind at the buoys may be an indicator of the wind at the beach later in the day. Along the West Coast the buoys are 9-13 miles from shore. They are useful for looking at hourly wind strength, directions, pressure, swell Ht. and swell direction. In certain conditions, especially in the spring, they are a good indicator of the winds to be expected later in the day at the coast. In other conditions especially in the summer it can blow at the buoys and be light at the beach. Generally the buoys have stronger winds than at shore but when the upper level winds are NW and very strong the beach will have stronger winds.

Catalina Eddy

This eddy like wind pattern is common in Southern California in the summer. Wind sweeping from the NW past Pt. Conception curves over the Southern California bight and creates SW to ESE winds along the shore. This is most likely to happen the the inland valleys are warm. This eddy tends to deepen the marine layer and fog. A strong Catalina Eddy will keep the coast foggy and windless most of the day.


The bottom of the lowest cloud layer above the surface.


High clouds made of ice crystals usually around 18,000 feet. The upper level wind direction and speed can very roughly judged by these clouds.


The long term record of average daily and seasonal weather events

Closed Low

See Cutoff Low

Cloud Train

Commonly seen near the Hawaiian islands as long line of puffy cumulus clouds carried along by the NE trade winds from the North Pacific High.

Cold Front

The leading edge of an advancing cold air mass associated with a surface low-pressure area. As a cold front passes the pressure rises and the wind clocks from southwest to northwest. Rain or snow is usually along or ahead of the front.

Combined Seas

The combined height of swell (generated by strong winds hundreds or thousands away) and wind waves (generated by local daily winds)


Upward motion of air caused by surface heating. If the air is moist this process may cause rain or thunderstorms. Convection is often associated with gusty up and down winds.


Horizontal movement of air into a particular region. Convergence at lower levels is associated with upward motion.

Coriolis Force

A force produced by the earth’s rotation. This force acts to change the wind direction. The strength of the coriolis force depends on the latitude and speed of the wind. In the Northern Hemisphere the air is deflected to the right. For example the North Pacific High winds rotate outward from the high making a turn to the right. This produces NW winds along the California coast and NE trade winds in Hawaii.

Cumulonimbus Cloud

A large vertically developed cloud often capped by an anvil shaped cloud producing tornadoes, hail, lightning, strong winds and heavy rain. These are an important wind factor in some parts of the USA.

Cumulus Clouds

A cloud in the shape of individual detached domes, with a flat base and a bulging upper portion resembling cauliflower.

Cutoff Low

An area of low pressure cut off from its associated jet stream. These low will wobble about in a region in an unpredictable fashion making forecasting very difficult. In the winter along the pacific coast they can cause prolonged rain and long periods of strong southerly winds. In the summer they can cause deep fog and long periods of fog and wind in the S. F. Bay Area

Cyclonic Wind

A large area of closed pressure circulation with rotating and converging winds. Due to coriolis force the circulation around the low is counterclockwise in the Northern Hemisphere.


See Low Pressure System


The temperature to which the air must be cooled for water vapor to condense.

Diablo Wind

Found in the S.F. Bay Area these winds are similar to Santa Ana winds in southern California. Powerful Diablo winds occur below canyons in the East Bay hills near Mt. Daiblo when a high pressure develops over Nevada and lower pressure along the central California coast.


A horizontal outflow of air away from a particular region. Divergence at lower levels is associated with a downward movement of air from aloft.

Doppler Radar

Specialized radar that measures the direction and speed of rain and wind.

Ebb Current

Movement of a tidal current away from shore or down a tidal. When the wind opposes this current chop and swell will become steeper.

ECMF model

European Centre for Meteorology Forecast model.

El Nino

A cyclical warming of East Pacific Ocean seawater temperatures off South America. During an El Niño event warm equatorial Pacific waters displaces the colder waters along the South America coast and slowing the normal upwelling of cold water. Strong El Niños can result in significant changes in weather patterns in the United States and elsewhere. El Niño increases the number of storms and rain over California. For windsurfers this means more winter sailing and warm waters. But in the summer it means a warmer ocean poor thermals especially at Rio and the North Bay

El Norte

A strong N to NE winds that roars down the Sea of Cortez in the winter when a high settles into the 4 corners region of the USA.

ETA model

A forecast model generated every 12 hours by NMC.


The area and distance over which ocean ground swell is generated by the unobstructed wind. The greater the fetch the longer the distance or period from crest to crest and the size of the swell.


A warm dry wind blowing down the lee side of a mountain range. Air is compressed during its drop so its temperature increases. This is a type of katabatic wind.


An area where vast numbers of visible tiny water droplets suspended in the air and limit visibility. Fog occurs when the temperature and the dew point of the air have become nearly the same. This often occurs on the west coast when warm moist air from the south is carried over colder ocean water.


The transition zone between two air masses of different densities and different temperatures. The basic frontal types are cold fronts, warm fronts and occluded fronts. Cold fronts are often associated with rain and wind.


Wind speeds from 39 to 54 mph (34 to 47 knots).

Geostationary Satellite

A satellite that rotates at the same rate as the earth so it remains over the same spot above the equator.

Geostrophic Wind

The theoretical horizontal motion of air along parallel isobars in a pressure or contour field. It is assumed that there is no friction, that the flow is straight with no curvature and there is no divergence or convergence with no vertical acceleration.

GOES Satellite

A geostationary satellite weather satellite that holds the same position over the equator. GOES stands for Geostationary Operational Environmental Satellite. These provide critical weather imagery.
GOES West Visible Imagery Loop

Ground Swell

This is swell generated by distant storms. Along the west coast ground swell comes from the S. hemisphere or the Gulf of Alaska. The time it takes for each swell crest to pass a point is over 15 seconds so we say it has a long period. The longer the period the bigger the surf when the swell encounters the bottom in the surf zone. Ground swell has cleaner and well-defined sets.

Gulf Stream

A swift, relatively narrow ocean current off the east coast of the United States.


A sudden major but temporary increase in wind speed. Technically a gust has a duration of less than 20 seconds and an increased speed greater than 10 mph. The sensor records the peak gust not the avenge of the gusts.

Gust Front

The leading edge of the gusty surface winds produced by thunderstorm downdrafts.


A suspension of fine particles in the air that reduce visibility and give the air a milky appearance.

Heat Wave

A period of abnormally hots weather lasting from several days to several weeks. On the west coast a marine surge and SW winds may occur at the end of a heat wave.

High Clouds

Cirrus type clouds composed of ice crystals and generally above 20,000 feet. The main types of high clouds are cirrus, cirrocumulus, and cirrostratus.

High Pressure System

An area of relative pressure with diverging winds that are turned by coriolis force At the surface the North Pacific High plays a major role in the spring NW clearing winds and the Gorge and Bay Area summer winds.

High Wind

Sustained winds greater than or equal to 40 mph.


The amount of water vapor in the air.


A tropical cyclone or low with sustained winds of 65 knots or greater. Known as a typhoon in the western Pacific.


The heat of the sun received at the earth’s surface. Insolation comes from incoming solar radiation.


Conditions when spontaneous convection can occur and air parcels will accelerate upward. Associated with gusts.

Inter-Tropical Convergence Zone

The equatorial region where the northeasterly and southeasterly trade winds converge forming a world-circling band of clouds and thunderstorms. These massive thunderstorms pump air into the upper atmosphere. This air returns to the surface thousands of mile away in the temperate zones creates permanent highs like the North Pacific High. The location of the ITCZ changes north and south with the seasons. Likewise the highs average location moves.


A temperature increase with altitude which is opposite of the usual decrease in temperature with height. Often found capping a marine layer fog. A tight inversion is associated with good thermal winds if there is a strong pressure gradient.


Lines on a surface weather map connecting points of equal barometric pressure. Useful in locating areas of low and high pressure. The tighter the isobars are together the stronger the wind.

Jet stream

Strong upper level winds concentrated in a narrow band flowing around the world from west to east. The jet stream forms as cold polar air moving towards the equator meets the warmer equatorial air moving northward toward the poles. The jet stream often “steers” surface features such as front and low-pressure systems. In some conditions these strong winds can transfer momentum to the surface producing very strong winds.


A zone of maximum wind speed along the route of a jet stream.

Katabatic Wind

A wind created by air flowing downhill. When the air is warm, it may be called a foehn wind or Chinook, or Santa Ana. When this air is cool, it is called a drainage wind, mountain breeze or glacier wind.


A nautical unit of wind speed used by sailors and meteorologists. 1 knot = 1.151 statute miles per hour. At low wind speeds below 20 knots the difference between knots and mph is not significant to sailors. At higher speeds the difference becomes more noticeable.

La Niña

La Niña in many ways is the opposite of El Niño. Easterly trade winds increase in intensity cooling water in the central and eastern equatorial Pacific Ocean since there is more upwelling along the western shores of South America. In La there is a different change in large-scale pressure patterns. The colder water along our coast mean better thermal winds in the North Bay and Rio.

Land Breeze

A daily coastal breeze that blows from the land to the sea because the sea is warmer than the adjacent land. Most common during the night and peaks around dawn.

Lapse Rate

The change of an atmospheric variable such as humidity, wind or temperature with increasing height. A steep lapse rate implies a rapid decrease in temperature with height.


The side of an object furthest away from the wind.

Low Cloud

Low clouds with a base below 6500 feet. Types include stratus, stratocumulus, cumulus and cumulonimbus.

Low Level Jet

Strong winds that are concentrated in relatively narrow bands in the lower part of the atmosphere. These are hard to forecast but can cause strong surface winds to develop fast.

Low Pressure System

A large area of closed pressure circulation with rotating and converging winds. The circulation around the low is counterclockwise in the Northern Hemisphere.


The meteorological scale covering an area ranging from the size of a continent to the entire globe.

Mare’s Tail

Thin, wispy cirrus clouds composed of ice crystals that appear as veil patches of strands, often resembling a horse’s tail.

Marine Layer

Along the west coast this is a layer of moist marine air close to the surface and capped by warmer air that creates an inversion. Fog is often but not always associated with marine layer. The depth of the marine layer can range from several hundred feet to over 3000 feet. This depth is determined by many variables including: subsidence from a high, reduction of the 925mb relative humidity above the marine layer, a trough offshore or in Southern California a Catalina Eddy. Some time strong NW wind will simply erode the marine layer. The deeper the marine layer the greater its intrusion inland. Since this air is cool and dense it acts as a local surface high. If there is a pressure gradient from this high to an inland point there will be wind close to the leading edge of the marine layer. So the deeper the marine layer the more inland the wind will penetrate.

Marine Surge

In the SF Bay Area this is a rapidly developing S to SW to WSW to flow of fog laden marine air coming up the coast. This will often weakens the Peninsula wind. Depending on its directions it can make for great winds at Sherman Island and San Luis in the AM or PM and at Larkspur or Berkeley or Pt. Isabel. Often the fog will be too deep at Crissy. A marine surge can be triggered by a strong SFO to SBA south to north gradient and/or an upper level trough offshore and/or a summer time Cut-Off low to the north.

Maritime Air Mas

Moist air mass originating over the ocean.

Mean Sea Level

The average height of the sea surface water level.


A scale ranges in size from several kilometers to around 100 kilometers. Smaller phenomena are classified as microscale while larger are classified as synoptic-scale. forecasters forecast at a microscale level but look at Mesoscale and synoptic scale variables to make those forecasts.

Medium cloud

Altostratus, altocumulus and nimbostratus.


The study of the physics, chemistry, and dynamics of the atmosphere and the direct effects of the atmosphere upon the Earth’s surface, the oceans, and life in general.


A severe but brief localized wind blasting down from a thunderstorm.


A scale of weather phenomena that range in size from a few centimeters to a few kilometers.

Middle Cloud

Clouds with bases between 6,000 and 18,000 feet.


In winter, mild refers to four to seven degrees Celsius above normal.

Millibar (MB)

The standard unit of measurement for atmospheric pressure used by the National Weather Service.

Minimum temperature

The lowest temperature during a specified time period.


Consists of microscopic water droplets suspended in the air, which produces a thin grayish veil over the landscape. It reduces visibility to a lesser extent than fog. In Australia, it is described as similar to fog, but visibility remains more than a kilometer.

Mixing out

The break up of the marine layer and any fog associated with it. This is caused by a low or a trough coming overhead as cold air advection occurs. It also happens when the marine layer becomes much thicker than 3000 feet. Mixing out will often weaken the surface wind.


Computer numerical models that subject huge amounts of data input from around the world to equations of hydrodynamics to extrapolate future weather conditions.

Moderate risk

Severe thunderstorms are expected to affect between five and 10 percent of the area.

Moderate visibility

Five to 10 kilometers’ visibility.


A persistent seasonal wind, often responsible for seasonal precipitation regime. It is most commonly used to describe meteorological changes in southern and eastern Asia.


Sunrise to noon or midnight to noon depending on context.

Mountain breeze/Katabatic flow

System of winds that blow downhill during the night.


Colloquially descriptive of warm and especially humid weather.

Multivortex tornado

A tornado in which two or more condensation funnels or debris clouds are present at the same time, often rotating about a common center or about each other. Multiple-vortex tornadoes can be especially damaging.


A thunderstorm with a well-defined anvil rollover, and thus having a visual appearance resembling a mushroom.


The long-term average value of a meteorological element for a certain area. For example, “temperatures are normal for this time of year” Usually averaged over 30 years.

Offshore breeze

A wind that blows from the land towards a body of water. Also known as a land breeze.

Offshore forecast

A marine weather forecast for the waters between 60 and 250 miles off the coast.

Onshore breeze

A wind that blows from a body of water towards the land. Also known as a sea breeze.


Air that flows outward from a thunderstorm.

Outflow winds

Winds that blow down fjords and inlets from the land to the sea.


Sky condition when greater than 9/10ths of the sky is covered by clouds. In Canada overcast means grey and dull skies, with extensive cloud cover.


A condition that exists when a relatively warm air mass moves up and over a colder and denser air mass on the surface. The result is usually low clouds, fog and steady, light precipitation.


A form of oxygen in which the molecule is made of three atoms instead of the usual two. Ozone is usually found in the stratosphere, and responsible for filtering out much of the sun’s ultraviolet radiation. It is also a primary component of smog.

Ozone hole

A thinning of the ozone layer over Antarctica, which occurs each spring.

Partly cloudy

Sky condition when between 3/10ths and 7/10ths of the sky is covered. Used more frequently at night. In the United Kingdom, it is defined as less than half cloud cover.

Partly sunny

Similar to partly cloudy. Used to emphasize daytime sunshine.


A soil layer below the surface of tundra regions that remains frozen permanently.

Polar air

A mass of very cold, very dry air that forms in polar regions.

Polar front

The semi-permanent, semi-continuous front that encircles the northern hemisphere separating air masses of tropical and polar origin.

Polar Stratospheric Clouds (PSCs)

High altitude clouds that form in the stratosphere above Antarctica during the Southern Hemisphere winter. Their presence seems to initiate the ozone loss experienced during the ensuing southern hemisphere spring.

Polar vortex

A circumpolar wind circulation which isolates the Antarctic continent during the cold Southern Hemisphere winter, heightening ozone depletion.

Poor visibility

One to five kilometers’ visibility.


Probability of Precipitation. Probability forecasts are subjective estimates of the chances of encountering measurable precipitation at some time during the forecast period.

Popcorn convection

Clouds, showers and thundershowers that form on a scattered basis with little or no apparent organization, usually during the afternoon in response to diurnal heating.


Liquid or solid water that falls from the atmosphere and reaches the ground.


The force exerted by the interaction of the atmosphere and gravity. Also known as atmospheric pressure.

Pressure change

The net difference between pressure readings at the beginning and ending of a specified interval of time.

Pressure falling rapidly

A decrease in station pressure at a rate of 0.06 inch of mercury or more per hour which totals 0.02 inches or more.

Pressure rising rapidly

An increase in station pressure at a rate of 0.06 inch of mercury or more per hour which totals 0.02 inches or more.

Pressure tendency

The character and amount of atmospheric pressure change during a specified period of time, usually the three-hour period preceding an observation.

Prevailing westerlies

Winds in the middle latitudes (approximately 30 degrees to 60 degrees) that generally blow from west to east.

Prevailing wind

The direction from which the wind blows most frequently in any location.

Pulse storm

A thunderstorm within which a brief period (pulse) of strong updraft occurs, during and immediately after which the storm produces a short episode of severe weather. These storms generally are not tornado producers, but often produce large hail and/or damaging winds. See overshooting top, cyclic storm.

Qualitative forecasts

Forecasts that provide only a categorical value for the predicted variable. Examples of this include “rain/no rain” and “cloudy/partly cloudy”.

Quantitative forecasts

Forecasts in which the “amount” of the forecast variable is specified.

Quantitative precipitation forecast (QPF)

A forecast of rainfall, snowfall or liquid equivalent of snowfall.

Quasi-biennial oscillation (QBO)

Periodic variation in the direction, either easterly or westerly, of tropical lower stratospheric winds. The direction changes every 26 months.

Quasi-nonhydrostatic (QNH)

Pilots, air traffic control (ATC) and low frequency weather beacons use this pressure setting to refer to the barometric altimeter setting that causes the altimeter to read altitude above mean sea level within a certain defined region.

Quasi-stationary front

A front which is nearly stationary or moves very little since the last synoptic position. Also known as a stationary front.


An instrument used to detect precipitation by measuring the strength of the electromagnetic signal reflected back. (RADAR= Radio Detection and Ranging).


Energy emitted in the form of electromagnetic waves. Radiation has differing characteristics depending upon the wavelength. Radiation from the Sun has a short wavelength (ultra-violet) while energy re-radiated from the Earth’s surface and the atmosphere has a long wavelength (infra-red).

Radiation fog

Fog produced over the land by the cooling of the lower atmosphere as it comes in contact with the ground. Also known as ground fog.


Liquid water droplets that fall from the atmosphere, having diameters greater than drizzle (0.5 mm). In Canada the term “rain”, used alone, means liquid precipitation of significant duration and extent.

Rain gauge

An instrument used to measure rainfall amounts.

Rain showers

Rain showers stop and start suddenly and vary widely in intensity, and are gone in less than an hour.


Optical phenomena when light is refracted and reflected by moisture in the air into concentric arcs of color. Raindrops act like prisms, breaking the light into the colors of a rainbow, with red on the outer, and blue on the inner edge.

Relative humidity

The amount of water vapor in the air, compared to the amount the air could hold if it was totally saturated. It is expressed as a percentage.

Return flow

South winds on the back (west) side of an eastward-moving surface high pressure system. Return flow over the central and eastern United States typically results in a return of moist air from the Gulf of Mexico (or the Atlantic Ocean).


An elongated area of high pressure in the atmosphere. Opposite of a trough.

River flood warning

Issued when main stem rivers are expected to reach a level above flood stage.

Roll cloud

A relatively rare, low-level horizontal, tube-shaped accessory cloud completely detached from the cumulonimbus base. When present, it is located along the gust front and most frequently observed on the leading edge of a line of thunderstorms. The roll cloud will appear to be slowly “rolling” about its horizontal axis. Roll clouds are not and do not produce tornadoes.


Particles of sand carried aloft by a strong wind. The sand particles are mostly confined to the lowest ten feet, and rarely rise more than fifty feet above the ground.


A condition of the atmosphere in which a certain volume of air holds the maximum water vapor it can hold at a specific temperature.


A cloud layer that covers between 3/8ths and 1/2 of the sky.

Scud clouds

Small, ragged, low cloud fragments that are unattached to a larger cloud base and often seen with and behind cold fronts and thunderstorm gust fronts. Such clouds generally are associated with cool moist air, such as thunderstorm outflow.

Sea breeze

A wind that blows from a sea or ocean towards a land mass. Also known as an onshore breeze. It occurs when the land is warmer than the water.

Sea-level pressure

The pressure value obtained by the theoretical reduction or increase of barometric pressure to sea-level.

Secondary cold front

A front that follows a primary cold front and ushers in even colder air.

Severe thunderstorm

A strong thunderstorm with wind gusts in excess of 58 mph (50 knots) and/or hail with a diameter of 3/4 inch or more.

Shallow fog

Fog in which the visibility at 6 feet above ground level is 5/8ths of a mile or more.


A low-level horizontal accessory cloud that appears to be wedge-shaped as it approaches. It is usually attached to the thunderstorm base and forms along the gust front.

Short-fuse warning

A warning issued by the National Weather Service for a local weather hazard of relatively short duration. Short-fuse warnings include tornado warnings, severe thunderstorm warnings, and flash flood warnings. Tornado and severe thunderstorm warnings typically are issued for periods of an hour or less, flash flood warnings typically for three hours or less.


Precipitation that is intermittent, both in time, space or intensity.

Sky condition

The state of the sky in terms of such parameters as sky cover, layers and associated heights, ceiling, and cloud types.


Rain drops that freeze into ice pellets before reaching the ground. Sleet usually bounces when hitting a surface and does not stick to objects. Forms when snow enters a warm layer of air above the surface and melts and then enters a deep layer of sub freezing air near the surface and refreezes. In Australia, sleet refers to a mixture of rain and snow or falling snow that is melting into rain.

Slight chance

In probability of precipitation statements, usually equivalent to a 20 percent chance.

Slight risk (of severe thunderstorms)

Severe thunderstorms are expected to affect between two and five percent of the area. A slight risk generally implies that severe weather events are expected to be isolated.


Pollution formed by the interaction of pollutants and sunlight (photochemical smog), usually restricting visibility, and occasionally hazardous to health.


A suspension in the air of small particles produced by combustion. A transition to haze may occur when smoke particles have traveled great distances (25 to 100 statute miles or more) and when the larger particles have settled out and the remaining particles have become widely scattered through the atmosphere.


Frozen precipitation composed of ice particles in complex hexagonal patterns. Snow forms in cold clouds by the direct transfer of water vapor to ice.

Snow burst

Very intense shower of snow, often of short duration, that greatly restricts visibility and produces periods of rapid snow accumulation.

Snow flurries

Light snow showers, usually of an intermittent nature and short duration with no measurable accumulation.

Snow grains

Light snow showers, usually of an intermittent nature and short duration with no measurable accumulation. In Canada, Snow grains are described as minute, white and opaque grains of ice. When they hit hard ground, they do not bounce or shatter. They usually fall in very small quantities, and never in the form of a shower.

Snow pellets

Precipitation of white, opaque grains of ice. The grains are round or sometimes conical. Diameters range from about 0.08 to 0.2 inch (2 to 5 mm). In Canada, snow pellets are brittle and easily crushed; when they fall on hard ground, they bounce and often break up. They always occur in showers and are often accompanied by snowflakes or rain drops, when the surface temperature is around zero degrees Celsius.

Snow shower

Snow falling at varying intensities for brief periods of time. Some accumulation is possible.

Snow squalls

Intense, but of limited duration, periods of moderate to heavy snowfall, accompanied by strong, gusty surface winds and possible lightning. In Canada, it is described that a snow squall brings strong winds, flurries and poor visibility.


The depth of new snow that has accumulated since the previous day or since the previous observation.


White ice crystals that have combined in a complex branched hexagonal form.

Special marine warning

Issued for brief or sudden occurrence of sustained wind or frequent gusts of 34 knots or more. This is usually associated with severe thunderstorms or waterspouts.


An ensemble of water droplets torn by the wind from an extensive body of water, generally from the crests of waves, and carried up into the air in such quantities that it reduces the horizontal visibility.


A strong wind characterized by a sudden onset in which the wind speed increases at least 16 knots and is sustained at 22 knots or more for at least one minute. In Canada, a squall is defined as a strong, sudden wind which generally lasts a few minutes then quickly decreases in speed. In Australia, it is described as a sudden increase of the mean wind speed which lasts for several minutes at least before the mean wind returns to near its previous value.


An indication of how easily a parcel of air is lifted. If the air is very stable it is difficult to make the parcel rise. If the air is very unstable the parcel may rise on its own once started.

Stable air

Air with little or no tendency to rise, usually accompanied by clear dry weather.

Steam fog

Fog that is formed when water vapor is added to air which is much colder than the vapor’s source. This is most common when very cold air drifts across relatively warm water.

Steering winds (steering currents)

A prevailing synoptic scale flow which governs the movement of smaller features embedded within it.


In marine usage, winds 48 knots (55 miles per hour) or greater.

Storm surge

A rise of the sea level alone the shore that builds up as a storm (usually a hurricane) moves over water. It is a result of the winds of the storm and low atmospheric pressures.

Storm warning

A marine wind warning for sustained winds greater of 48 knots (55 miles per hour) or more from a non-tropical system.

Straight line winds

Thunderstorm winds most often found with the gust front.

Subtropical storm

A low pressure system that develops in subtropical waters (north of 20 north degrees latitude) and initially has non-tropical features (see table below for a list of tropical features) but does have some element of a tropical cyclone’s cloud structure (located close to the center rather than away from the center of circulation).


Sunny or a few clouds means that less than half the sky has clouds.


A severe thunderstorm whose updrafts and downdrafts are in near balance allowing the storm to maintain itself for several hours. Supercells often produce large hail and tornadoes.


The condition which occurs in the atmosphere when the relative humidity is greater than 100 percent.


A measure of the warmth or coldness of an object or substance with reference to a standard value.


The sound caused by a lightning stroke as it heats the air and causes it to rapidly expand.


A storm with lightning and thunder produced by a Cumulonimbus cloud, usually producing gusty winds, heavy rain and sometimes hail. In Australia, they are described to be usually short-lived and hit on only a small area.

Tilted storm or tilted updraft

A thunderstorm or cloud tower which is not purely vertical but instead exhibits a slanted or tilted character. It is a sign of vertical wind shear, a favorable condition for severe storm development.


A violent rotating column of air, in contact with the ground, pendant from a cumulonimbus cloud. A tornado does not require the visible presence of a funnel cloud. It has a typical width of tens to hundreds of meters and a lifespan of minutes to hours.

Trade winds

Persistent tropical winds that blow from the subtropical high pressure centers towards the equatorial low. They blow northeasterly in the northern hemisphere.

Tropical storm

An organized low pressure system in the tropics with wind speeds between 38 and 74 miles per hour. In Australia, it is a term used in the northern hemisphere for a tropical cyclone.

Tropical storm warning

A warning issued when sustained winds of 39 to 73 miles per hour (34 to 63 knots) are expected within 24 hours.


A warning issued when sustained winds of 39 to 73 mph (34 to 63 knots) are expected within 24 hours. In Canada, it is described as an elongated area of relatively low pressure extending from the centre of a region of low pressure.


A warning issued when sustained winds of 39 to 73 mph (34 to 63 knots) are expected within 24 hours. In Canada, it is described as an elongated area of relatively low pressure extending from the centre of a region of low pressure.


A warning issued when sustained winds of 39 to 73 miles per hour (34 to 63 knots) are expected within 24 hours.


A colloquial term for a tornado.


A hurricane that forms in the Western Pacific Ocean. It is a term used in Australia for a tropical cyclone in the northwestern Pacific with maximum winds above 117 kilometers per hour (63 knots).

Unstable air

Air that rises easily and can form clouds and rain.

Very cool

Very cool, in the summer, refers to more than seven degrees Celsius below normal.

Very mild

In the winter, very mild means more than seven degrees Celsius above normal.


In summer, warm refers to four to seven degrees Celsius above normal.

Warm front

A narrow transitions zone separating advancing warmer air from retreating cooler air. The air behind a warm front is warmer and typically more humid than the air it is replacing. In Canada, it is defined as the trailing edge of a retreating cold air mass and moves in such a way that the warmer air replaces the colder air.


In meteorology any pattern identifiable on a weather map that has a cyclic pattern or a small cyclonic circulation in the early stages of development that moves along a cold front. In Canada, A wave, in meteorology, is the intersection of warm and cold fronts.


State of the atmosphere with respect to heat or cold, wetness or dryness, calm or storm, clearness or cloudiness. Also, weather is the meteorological day-to-day variations of the atmosphere and their effects on life and human activity. It includes temperature, pressure, humidity, clouds, wind, precipitation and fog.


Wind is the horizontal movement of air relative to the earth’s surface and is caused by variations in temperature and pressure (for instance, air rises as it warms and a cool breeze moves in to take the place of the rising air.) Wind is also known as moving air.

Wind chill

The additional cooling effect resulting from wind blowing on bare skin. The wind chill is based on the rate of heat loss from exposed skin caused by the combined effects of wind and cold. The (equivalent) wind chill temperature is the temperature the body “feels” for a certain combination of wind and air temperature.

Winter storm

A heavy snow event. A snow accumulation of more than six inches in 12 hours or more than 12 inches in 24 hours.

Winter storm warning

Issued when seven or more inches of snow or sleet is expected in the next 24 hours, or 1/2 inch or more of accretion of freezing rain is expected. A warning is used for winter weather conditions posing a threat to life and property.


An electromagnetic wave of very short wavelength, able to pass through many materials opaque to light.


An inert gaseous chemical element, present in trace amounts in the air and used in some kinds of electric light.

Yellow wind

A strong, cold, dry west wind of eastern Asia that blows across the plains during winter and carries a yellow dust from the desert.


A hot wind during unsettled summer weather in the Mediterranean.

Zonal flow

A small amplitude pattern where winds blow mostly west-to-east.

Zonal wind

Wind blowing east-to-west or west-to-east.

Zulu time

The mean solar time for the meridian at Greenwich, England, used as a basis for calculating time in communications, military, aviation, maritime and other activities that cross time zones. Zulu time, which is also known as Coordinated Universal Time (UTC), is used internally among people working in the field of weather and meteorology, as well as among weather enthusiasts, for weather reports and forecasts.

This data was collected from: the National Oceanic and Atmospheric Administration’s (NOAA) National Weather Service; the Met Office in the United Kingdom; Canada’s Weather Office; and the Australian Government Bureau of Meteorology.