Surf Forecast Glossary
Anticyclones (or high pressure systems): Atmospheric circulations that rotate anti-clockwise in the Southern Hemisphere and clockwise in the northern hemisphere. Anticyclones are areas of higher pressure and are generally associated with lighter winds and fine and settled conditions.
Atmosphere: The mixture of gases surrounding a planet.
Atmospheric Pressure: The weight per unit area of the column of air above a certain point. We use hectopascals to measure this. An average figure is 1013 hPa. This equals about 10 tonnes per square metre. Sounds like a lot right? But we're built to handle it.
Australian Eastern Standard Time (EST): Local time for Eastern Australian states including NSW, Queensland and Victoria. Time zone offset is UTC +10 hours.
AWS: Automated Weather Station
Baroclinic leaf shield: A pattern of cloud on a satellite picture that looks like a leaf. These often signal the impending formation of a surface low.
Baroclinic Zone: A region where the temperature changes abruptly in the horizontal. Fronts are baroclinic zones for example. Baroclinic zones can also exhibit large changes in wind speed and direction with height, and the temperature gradient supplies a lot of the energy associated with low-pressure systems in the mid-latitudes.
Barometer: Measures air pressure.
Bomb Low: A low pressure system that is deepening really quickly. Depending on who you talk to the central pressure has to drop 12 hPA in 12 hours or 24 hPA in a day to qualify. It's now becoming a bit of a cliché because the media love to use the term weather bomb and have over-used it something shocking.
Boundary Layer: The lowest kilometre or so of the atmosphere. This is where the friction of the earth becomes important and where a lot of the exchange of heat between the earth and atmosphere occurs. It's also the place with the most diurnal variation; temperatures and winds go up during the day and down at night, fog comes and goes etc.
Beaufort wind scale: A scale that uses observations of the effects of wind to estimate its speed.
Captured Fetch: A travelling fetch that moves at the same speed and in the same direction as the swell being generated, thereby rapidly compounding the size of the swell.
Cirriform: Thin wispy high clouds, six to thirteen kilometres above the earth.
Cirrocumulus: High clouds, six to thirteen kilometres above the earth, that look patchy or ripply.
Cirrostratus: Flat high cloud, six to thirteen kilometres above the earth, looking like transparent whitish veil.
Cirrus: That classic wispy high cloud, six to thirteen kilometres above the earth, that gets its name for the latin word for a curl of hair.
Climate: The average, over a long period, of observed weather.
Closed Low: A low pressure area with a clear centre of circulation. On a weather chart they are enclosed by one or more isobars or height lines. At the surface, a closed low can often form along a cold front. In the upper atmosphere closed lows often progress more slowly than the troughs from which they form. These upper features drive what's going on at the surface and so if they slow down so will the surface features.
A low pressure area with a clear centre of circulation. On a weather chart they are enclosed by one or more isobars or height lines. At the surface, a closed low can often form along a cold front. Once it does, it alters the flow around that front, often slowing things down and altering the distribution of rainfall. This can be good or bad for snowfall depending on the position of the low at the time. In the upper atmosphere closed lows often progress more slowly than the troughs from which they form. These upper features drive what's going on at the surface and so if they slow down so will the surface features.
Cold Front: A parcel of cold, dense air advancing north, forcing warm air aloft over its sloping surface. Cold polar air is replacing warm tropical air. A cold front is the boundary where cold air moves to replace, and undercut, warmer and less dense air. Associated cloud and weather may vary enormously according to the properties of the air masses, but tends to be concentrated near the front. As a typical cold front approaches, winds freshen from the north or northwest, and pressure falls. After the front passes, winds shift direction anticlockwise ('backing' to the west or southwest) and pressure rises. Cold fronts are much more frequent and vigorous over southern Australia than elsewhere.
Coriolis Force: A moving body on the surface of the earth experiences a tendency to turn to the left (right) in the southern (northern) hemisphere due to earth's rotation. This tendency (which is an artifice of the rotating reference frame rather than an actual force) is known as the Coriolis force (or acceleration) is only noticeable with larger scale motions such as ocean currents and winds (despite the myth of bathtub drain vortices rotating in opposite directions on either side of the equator). The Coriolis force affects the direction with which the tide propagates around an amphidrome and can also affect the propagation of the tide as it moves up a broad channel (most noticeably by tilting the water surface to the left or right of the direction of propagation).
Cold Pool: A pool of cold air. Kind of like a low centre for temperature. The coldest air aloft is often the region of greatest instability, so cold pools are where you get hail and lightning out of winter thunderstorms. The cold air also makes snow more likely.
Comma Cloud: A cloud pattern shaped like a comma that is seen on satellite pictures in association with some low-pressure systems.
Condensation: The change of state of a substance from vapour to liquid.
Convection: Transporting heat and moisture by moving the fluid that contains them. Meteorologists use convection to describe the motion that results when air is warmed at the surface and bubbles up through the atmosphere, often forming puffy convective clouds, and in extreme cases thunderstorms.
Convective Clouds: Puffy clouds with plenty of height, ranging from cumulus to thunderstorms (cumulonimbus)
Convergence: When more air is entering a given area on a horizontal surface than is leaving. In the immortal words of Yaz and the Plastic Population, "the only way is up", with some air forced to eave the area by rising. Convergence is prominent in areas relatively low pressure, whether that's a low centre, a cold front, or more local features such as a thunderstorm.
Crepuscular Rays: Those awesome rays of sunshine that break through clouds to light up just a little patch of earth, like a flashlight through swiss cheese.
Cumuliform: Puffy clouds.
Cumulus: Average sized puffy clouds with mounds, domes, and turrets at the top. Look at them for long enough and you'll see how much bubbling goes on.
Cumulus Congestus: A really big cumulus, top like a cauliflower, but not quite a thunderstorm yet.
Cut off low: Cut-off lows are low pressure systems which have broken away, or are cut-off, from the main belt of low pressure which lies to the south of Australia. They can be at any level in the atmosphere, and therefore may not show on the surface charts. A cut-off low may develop when a low pressure system forms on an active cold front. Alternatively, they may form in an unstable easterly flow on the northern flank of a slow-moving or blocking high. This dual system is sometimes referred to as a blocking pair.
Cyclone (see low pressure system): Atmospheric circulations that rotate clockwise in the Southern Hemisphere (anti-clockwise in the Northern Hemisphere). Cyclones are areas of lower pressure and generally associated with stronger winds, unsettled conditions, cloudiness and rainfall.
Cyclogenesis: When a low is deepening. The genesis of the cyclone.
Daylight Saving Time (DST): Also known as summer time, Daylight Saving Time occurs when the time on local clocks is advanced forward by one hour at the beginning of the defined period of DST, and returned back by one hour at the end of DST. The exact dates between which DST is to apply can be obtained from the relevant Australian State governments. Within Australia DST across the three time zones is generally denoted by: EDT - Australian Eastern Daylight Time; CDT - Australian Central Daylight Time; and WDT - Australian Western Daylight Time.
Depression: A region of low pressure that might not have a closed low in it yet, but probably has clouds and some rain.
East Coast Low (ECL): Intense low-pressure systems that form off the eastern coast of Australia, predominantly off the southern Queensland, NSW and eastern Victorian coasts. Although they can occur year-round they are more common during Autumn and Winter with a maximum frequency in June. East Coast Lows will often intensify rapidly overnight making them one of the more dangerous weather systems to affect the NSW coast. East Coast Lows may form in a variety of weather situations. In summer they can be ex-tropical cyclones. At other times of the year, they will most often develop rapidly just offshore within a pre-existing trough of low pressure due to favourable conditions in the upper atmosphere. ECL's may also develop in the wake of a cold front moving across from Victoria into the Tasman Sea. The sea surface temperature gradients associated with the warm eddies of the East Australian Current also contribute to the development of the lows. East Coast Low’s are often sources of large mid period groundswells across the Eastern Seaboard originating from the east to south quadrant, fuelled by gale force winds generated within close range of the coast.
Eastern Standard Time (EST):
Easterlies: Winds from the east.
ECMWF: The European Center for Medium-Range Weather Forecasts. These guys have a global weather model which runs out to 10 days that is the envy of many.
Ensemble Forecast: A technique of running the same weather model many times simultaneously, with tiny differences in the starting conditions of each individual model run, called and ensemble member. Gives an idea of how the atmosphere will evolve as a spread of probabilities. Ensembles are generally better than any single model from about 72h hours ahead.
Equinox: A region of low pressure that might not have a closed low in it yet, but probably has clouds and some rain.
Feet (ft): A unit of length, originally derived from the length of the human foot. It is divided into 12 inches and equal to 30.48 centimeters.
Front: A boundary between two different air masses. Often the location of significant weather.
Gale force wind: Winds averaging from 34 knots and up to 47 knots.
Geostationary Satellite: A satellite that stays over the same pleace on the equator by orbiting with the same rotational speed as the earth. The satellite pictures we see of the whole Oceania region come from the Japanese Geo-stationary satellite MT-SAT.
Groundswell: An ocean wave that carries most its energy below the sea surface. Generally denoted by a wave period greater than 10 seconds.
Gust: A rapid, temporary increase in wind speed.
Hectopascal: The unit of pressure we use. One Hectopascal equals 100 pascals, the downwards force exerted by 10 kilograms spread evenly over a square metre. Atmospheric pressure is usually around 1012 hPA, or the weight of 10,000 kilograms spread evenly over a square metre. That's how much the atmosphere weighs.
High pressure system: (See Anticyclone) Atmospheric circulations that rotate anti-clockwise in the Southern Hemisphere and clockwise in the northern hemisphere. Anticyclones are areas of higher pressure and are generally associated with lighter winds and fine and settled conditions.
Instability: When cold air lies over warm air, and air at the surface has a tendency to gain buoyancy and keep rising once it gets an initial kick upwards.
IR: Abbreviation used for infra-red satellite imagery, which displays the temperature at the top of clouds based on the peak wavelengths of light they emit.
Isobar: A line connecting points of equal pressure on a weather chart.
Isopleth: A line connecting points of equal value on a weather chart.
Isotach: A line connecting points of equal wind speed on a weather chart.
Isotherm: A line connecting points of equal temperature on a weather chart.
Jet stream: Strong winds in the upper atmosphere created by large horizontal temperature gradients.
Knots: A measure of wind speed. One knot equals one nautical mile per hour.
Latent Heat: The energy that is absorbed or released when a substance changes state between solid, liquid and gas. When you boil water, first you have to get it to 100 degrees, then it needs an extra kick to evaporate. This extra kick is the latent heat that's present in the gas form. If you then condense that water vapour back to liquid the same amount of energy is released. The same thing happens between freezing and melting. This doesn't sound super exciting but latent heat is one of the key methods of heat exchange in the atmosphere. Showers and thunderstorms get most of their energy from water vapour condensing back to liquid and releasing heat that makes the air in the storm more buoyant than the surrounding environment.
Longwave Trough: A trough in the westerly flow around the mid-latitudes that can easily stretch across a third of the globe. The shortwave troughs that generate our weather will be superimposed on these long waves.
Low pressure system: (see cyclone) Atmospheric circulations that rotate clockwise in the Southern Hemisphere (anti-clockwise in the Northern Hemisphere). Cyclones are areas of lower pressure and generally associated with stronger winds, unsettled conditions, cloudiness and rainfall.
Madden Julien Oscillation (MJO): The MJO is associated with weekly to monthly variations in wind, cloudiness, and rainfall across the tropical atmosphere. It’s best characterised as an eastward moving 'pulse' of cloud and rainfall near the equator that typically recurs every 30 to 60 days, but isn’t always discernible.
Meridional Flow: Flow in the north-south direction, along a meridian (longitude line). Compare zonal flow which is in the east-west direction, along lines of latitude.
Mid-Latitude Areas: The areas roughly between 30 and 60 degrees of latitude. This is the transtion zone between cold polar air and warm sub-tropical air, where westerlies and weather systems such as highs and lows dominate.
Monsoon: The northern Australian monsoon season generally lasts from December to March. It is associated with the inflow of moist west to northwesterly winds into the monsoon trough, producing convective cloud and heavy rainfall over northern Australia. These moisture-laden winds originate from the Indian Ocean and southern Asian waters. The north Australian wet season encompasses the monsoon months but can extend several months on either side. Parts of the north Queensland coast also receive significant rainfall throughout the cooler months. In the Top-End of the NT, the Bureau considers the wet season as being from 1 October to 30 April, while in some other parts of tropical Australia, particularly in WA, the wet months are often only from about January to March.
MSLP: Mean Seal Level Pressure Map: Depicts weather conditions at the earth’s surface.
Nautical Mile: A standard unit of measurement used in marine forecasting. The distance is equivalent to 1.852 kilometres or roughly 6,076 feet.
Numerical Weather Prediction: Using computers to forecast the weather.
Occluded front: When the cold front moves faster than the warm front, and as it overtakes the warm front, the warm sector is closed and a combine front forms. This process is called occlusion.
Peak Wave Period: Wave Period measuring the time in seconds between the highest energy swells within the swell spectrum. Longer wave periods are associated with higher wave energy within individual swells.
Polar Front: The transition zone between warm tropical air and polar air. Where the energy and gradients for our low pressure systems generally comes from.
Polar Jet Stream : The really strong upper atmosphere winds that occur along the polar front due to the sharp temperature gradient across the front. Pressure decreases much more rapidly with height on the cold side of the front, so at higher levels a north-south pressure gradient is created which, because of the spin of the earth results in very strong westerly’s at upper levels. The progression, ebb and flow of these westerly’s then helps to drive the surface development of low-pressure features.
Polar Orbiting Satellite: A satellite which orbits the earth, passing over or close to the poles as they do so. These are usually several hundred kilometres above the earth and orbit several times per day, crossing a different point each time because the earth is spinning below them. They provide better images than geostationary satellites because they are much closer, but they generally only pass over each point on earth once a day, so they aren't great for watching weather.
Pre-Frontal Trough: An area of low pressure preceding a cold front that can have its own wind change and weather.
Pressure: The force upon a surface by another surface in contact with it.
Pressure gradient: The change of air pressure over distance. The pressure gradient force acts on air to make it move. When air moves it acts as wind. For ocean winds, the pressure gradient at mean sea level is the most important driving factor for wave generation.
Primary Swell: Height and direction of the swell with the highest energy component. This is sometimes referred to as the dominant swell.
Ridge: An elongated area of high pressure extending out from an anti-cyclone. Pressures along the ridge are higher than pressures either side. There will tend to be some sinking motion in the ridge, making it difficult for cloud to form. Cloud that is present, such as stratocumulus, will likely be flat in appearance.
Sea Level Pressure: The atmospheric pressure at sea level for a given location. At elevated locations the station pressure is corrected to a sea level equivalent using equations that calculate the change in pressure with height for air at a given temperature. Using sea level pressure helps us usefully compare pressures recorded at different altitudes to work out what the differences will mean for horizontal pressure gradients.
Shear: The change in wind speed or direction with height or distance.
Shortwave Radiation: The radiation emitted by the sun. In the absence of clouds most of it passes straight through the atmosphere and is partially absorbed and partially reflected by the eart.
Shortwave Trough: A wave in the upper atmosphere that induces upwards motion and lower surface pressure ahead of it. Cold fronts at the surface will be related to the upper trough, with their intensities closely linked.
Solstice: The longest and shortest days of the year, when the overhead midday sun is furthest from the equator. Occurs around December 22 and June 21.
Squall Line: A small line of thunderstorms, moving along in an organised fashion. Smaller than a front but potentially quite damaging.
Stable: When warm air sits over cold air and mixing is inhibited.
Stationary Front: A front between two air masses that isn't moving very much.
Stratocumulus: Low level puffy clouds that have a defined top due to the presence of warmer, more stable air above. The air rising through the clouds reaches this warmer, lighter layer and cant go any further, so it spreads out instead like an oil slick, creating the flat appearance.
Stratus: Flat and boring low cloud with little or no convection occurring. Will often be associated with drizzle. Stratus on the ground is fog.
Sea Breeze: A local onshore wind. Cooler air from over the sea flows onto the shore to replace the warm air rising over the land. On sunny days the land heats up more quickly, and to a greater extent, than the sea. The air in contact with the land warms and expands and the resulting changes in the pressure and temperature differences and distributions cause the sea breeze circulation. At night, when the land cools more quickly, and to a greater extent, than the sea, the reverse land breeze circulation is set up.
Seasons: In the southern hemisphere; Summer:, the hottest quarter of the annual weather cycle, encompassing December, January and February. Autumn: The cooling transitional quarter of the annual weather cycle encompassing March, April and May. Winter: The coldest quarter of the annual weather cycle encompassing June, July and August.
Signficant Wave Height: describes the combined height of the sea and the swell that mariners experience on open waters. The height of the Combined sea and swell refers to the average wave height of the highest one third of the waves.
Spring: The warming quarter of the annual weather cycle encompassing the months September, October and November.
Storm Force wind: Winds averaging from 48 knots and up to 63 knots in coastal waters and high seas areas.
Swell:A series of ocean waves that propagate along the interface between water and air and so they are often referred to as surface gravity waves. Swell is not generated by the immediate local wind, but by distant weather systems that sustain wind from a given direction for a duration of time over a fetch of water. Swell is distinct from locally generated windswell, which is still under the influence of the mechanisms that created it e.g. A southerly windswell along the Eastern Seaboard. More generally, swell is generally not materially affected by the local wind.
Swell direction:The direction from which a swell is arriving. Swell direction is described by points on the compass, either using degrees or by abbreviations of points on the compass eg NE (northeast) or SSE (south-southeast).
Secondary swell:A second swell running simultaneously with the primary swell. A secondary swell is subordinate to the primary swell, being smaller in height and usually exhibits a shorter peak wave period.
Synoptic Scale: A horizontal length scale of the order of 1000 kilometres (about 620 miles) or more. This corresponds to a horizontal scale typical of mid-latitude depressions (e.g. extratropical cyclones). Most high and low-pressure areas seen on weather maps such as surface weather analyses are synoptic-scale systems, driven by the location of Rossby waves in their respective hemisphere. Low-pressure areas and their related frontal zones occur on the leading edge of a trough within the Rossby wave pattern, while surface highs form on the back edge of the trough. Most precipitation areas occur near frontal zones. The word synoptic is derived from the Greek word s???pt???? (synoptikos), meaning seen together.
Trade winds: The trade winds are the east to southeasterly winds (in the Southern Hemisphere) which affect tropical and subtropical regions, including the northern areas of Australia. During the monsoon season in northern Australia, the easterly trade winds are replaced by moist northwesterly (monsoonal) winds from the Indian Ocean and southern Asian ocean waters. In the Northern Hemisphere the trade winds are east to northeasterly in direction. It means that in both hemispheres, they tend to blow from the east to the west and towards the equator. Sometimes the trade winds will just be called "easterly" to avoid having to specify the hemisphere.
Tropics: Areas of the earth within 23 degrees latitude of the equator.
Tropopause: The top of the troposphere, where temperature change with height stalls and eventually reverses into the very stable and boring stratosphere.
Troposphere: Where all the action happens. The bottom 9 to 15 km of the atmosphere with plenty of instability, vertical motion and water vapour to make things interesting.
Trough: A trough of low pressure is an elongated area where atmospheric pressure is low relative to its immediate surroundings. A trough of low pressure is sometimes indicated on the synoptic chart by a centre line or trough line denoted by a dashed line e.g. - - - - -. The trough line often extends outward from a low pressure centre, or an enclosed area of relatively low pressure.
Tropical Cyclone: Intense low pressure systems that develop over warm waters of 26C or greater, mostly north of the 26S parallel. Tropical cyclones are associated with strong winds, torrential rain and storm surges (in coastal areas). Tropical cyclones can cause extensive damage as a result of the strong wind, and flooding (caused by either heavy rainfall or ocean storm surges).
Turbulence: Lumps and bumps in the atmospheric flow.
Upper Level: No fixed rule, but generally things more than 1500m high in the atmosphere, away from the boundary layer.
UTC: Coordinated Universal Time. The base time standard by which the world regulates and keeps track of time.
Virtual Buoy: A computer modeled forecast predicting wave height, wave period and direction over time (usually seven days) at a particular offshore location. A virtual buoy takes wave data from a single grid point in a wave model and makes it easy for forecast analysis. The highest energy swell calculated in the wave spectrum is displayed as the primary wave conditions you can expect for that location. Virtual buoy locations are defined by specific a latitude/longitude position over time.
Veering: When winds shift in a clockwise direction - eg from south-west to north-west as one cold front leaves and another approaches.
Vertical Wind Shear: Change in wind speed and direction with height. Important for thunderstorms as stronger winds aloft can carry the thunderstorms downdraft away from its updraft, helping it go for longer. Too much and it will blow the whole thing apart though.
Warm Front: A boundary between warm air and cooler air, when the warm air is advancing and rising over the cooler air. This is usually a more gradual process than a cold front and the rain by comparison is lighter but more continuous over a larger area. Warm fronts are quite rare over south-east Australia because of all the land to the north, but very common for New Zealand, where they generally precede a cold front by a day or so.
Wave height: The height difference between the wave crest and the preceding trough.
Wave length: The mean horizontal distance between successive crests (or troughs) of a wave pattern.
Weather Map (or Mean Sea Level Pressure Chart):
Wave period: The time interval in seconds measuring the passage of successive wave crests (or troughs).
Wind:The movement of air on a large scale.
Windswell: Waves generated by a localised or close range weather systems, characterised by shorter peak wave periods ranging from 5 to 10 seconds. Windswells exhibit shorter wavelengths and are relatively disorganised, producing peaky, less predictable surf . Most of the swell energy sits near the surface and hence is more vulnerable to the erosive effects of headwinds and currents.
Zonal Flow: Component of atmospheric circulation along a line of latitude, towards the east or west. Atmospheric circulation along, or approximately along, parallels of latitude.
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Welcome to our first installment on meteorology basics: Swell is generated by wind and wind is the result of air pressure.
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