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| To find definitions of technical terms, scroll down the list or click on the initial letter to jump to the required section. Each entry has associated pages where appropriate. |  |
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| Abiotic factors | The non-living components of an ecosystem, eg light, temperature, moisture and nutrients | Climate and ecosystems | |||
| Acid rain | Rain formed by the reaction of atmospheric moisture with sulphur dioxide from industrial emissions and nitrogen oxides from car exhausts. Normal rainfall lies between pH 5.3 and 6.0. Rainfall in many urbanised countries is around pH 4 - 5, ie more acid than would naturally occur. | Regional air quality | |||
| Adiabatic cooling | A dynamic cooling process where rising air expands and the increase in volume results in a lowering of temperature. | Relief and rainfall | |||
| Adiabatic lapse rate | As a result of adiabatic cooling, rising air progressively cools. If the air is unsaturated the adiabatic lapse rate is 10°C per kilometre rise. Saturated air cools more slowly at 6°C per kilometre rise. | Heating
and cooling
Air stability |
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| Agriculture | Food production is affected by climate. Arable agriculture tends to be more climatically sensitive than pastoral agriculture. | West Africa | |||
| Air mass | A body of air possessing nearly uniform characteristics of temperature and humidity over a large area (the source area). As the air moves out from the source area, its character will progressively change. | USA
case study
Air stability Air mass introduction Air mass weather |
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| Air quality | Refers to the character and chemical composition of the air. Air carrying pollution loads will be chemically different (eg acid rain) or physically different (eg smoke and particulate matter). | Local
climate
Urban climate Regional air quality Air pollution trends |
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| Aleutian low | An area of low atmospheric pressure commonly observed near the Aleutian Islands in the northern Pacific. | USA case study - Salem | |||
| Altitude | The height of the land - an important factor in controlling local climates, especially temperature, cloud cover and precipitation | European case study | |||
| Anabatic winds | Warm, gentle upslope winds driven by pressure differences between hillsides and valley centres. | Mountain winds | |||
| Anticyclone | An area of high atmospheric pressure, maintained by subsidence of air from the higher levels of the troposphere (the "weather" layer of the atmosphere"). Associated with extremes of temperature, low rainfall and low windspeeds. Can worsen pollution loads in an area. | Synoptic
chart interpretation
Animated weather Anticyclone formation Anticyclonic weather Anticyclones v depressions |
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| Boreal forests | The climatic climax vegetation for subpolar and cooler temperate regions. Dominated by trees of the pine and fir species. | Boreal forests | |||
| California current | A cold water current flowing adjacent to the coast of California and influencing fog frequency in the area. | USA case study - Los Angeles | |||
| Carbon sink | The natural mechanisms that remove carbon dioxide from the atmosphere - include plant and plankton growth. | Global warming solutions | |||
| CFC | Chlorofluorocarbon - a compound in which some of the hydrogen atoms have been replaced by chlorine and fluorine atoms to produce an unreactive, highly stable compound. Chlorofluorocarbons can drift into the upper atmosphere and react destructively with the ozone layer. In addition, they are powerful greenhouse gases (even if only at a low concentration). | Global warming evidence and causes | |||
| Chinook wind | The native Indian name for a warm, dry wind descending unexpectedly from the Rocky Mountains. Adiabatic cooling and heating create this unusual condition. | Relief and rainfall | |||
| Climate | The average condition of weather over a long time | Defining climate | |||
| Climate change | A natural process showing immense variability but now much complicated by the influence of human activity. | Changes
in time
Global warming evidence and causes Sea level rise |
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| Climate graph | A graph showing the variation in average monthly temperatures and rainfall. | Climate graphs | |||
| Cloud | Condensed water vapour in the air. Cloud shape, height and form can give important clues about the weather processes at work in the atmosphere. | Cloud
types
Interpreting clouds Interpreting satellite images |
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| Cold front | The junction observed between two air masses when colder air encroaches on warmer air. | Cold fronts | |||
| Condensation | The process by which water vapour in the air loses energy by cooling and turns back into liquid state. | Evaporation and condensation | |||
| Continentality | The phenomenon caused by land masses heating and cooling more quickly than water bodies. Further from the sea, the temperature regime becomes more extreme (or "continental"). | Spatial
influences
European case study USA case study - Minneapolis Regional climates |
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| Convectional rain | Rain caused by unstable air convecting upwards and then cooling and condensing to form deep rainclouds. | USA
case study - Galveston
USA case study - Chicago Satellite interpretation |
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| Cyclogenesis | The mechanisms forming depressions. These involve movement of the upper winds which trigger the conditions needed for low pressure to develop. | Depression formation | |||
| Cyclone | A circulating storm around a low pressure centre. The term cyclone is usually restricted to tropical storm systems (temperate ones are called depressions"). A cyclone with windspeeds in excess of 120 kph is a hurricane in the Atlantic, a typhoon in the western Pacific or a tropical cyclone in the Indian Ocean. The winds circulate anticlockwise in the northern hemisphere | Orissa Cyclone | |||
| Depression | A temperate low pressure system with winds circulating anticlockwise in the northern hemisphere. These systems are associated with wet and windy weather, and can develop into severe storms. Depressions are created along the polar front by cyclogenesis. As depressions approach an area, a distinctive sequence of weather is often experienced. | Synoptic
chart interpretation
Animated weather Anticyclones v depressions Depression origin Depression formation Depression structure Fronts and weather |
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| Dew point | The temperature at which cooling air becomes saturated and water vapour starts condensing. | Evaporation and condensation | |||
| Effective rainfall | The rainfall left after evapotransiration has taken place. Effective rainfall turns into either soil moisture replenishment or stream runoff. | Water balance graphs | |||
| El Nino | A semi regular warm current flowing in the eastern Pacific off the coast of Peru. It causes a chain of abnormal weather conditions around the globe, set off by complex "teleconnections". | ENSO | |||
| Environmental lapse rate | The observed decrease in temperate with height in still air (ie not rising or falling). Since air is heated from the ground, layers nearer the ground are warmer and layers further away are cooler. | Heating
and cooling
Air stability |
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| Evaporation | The process by which moisture gains energy from its surroundings (eg on a hot sunny day) and can change into gas state as molecules become excited. | Evaporation and condensation | |||
| Extensive agriculture | Farming with few inputs and few outputs, but on large areas of land. | Climate and people | |||
| Fog/ mist | Ground level cloud due to air in the lowest layers being cooled below dewpoint. | Water influences | |||
| Föhn wind | The Alpine name for a warm, dry wind descending unexpectedly from the mountains. Adiabatic cooling and heating create this unusual condition known as the Föhn effect. | Relief and rainfall | |||
| Friagem | An outbreak of cold polar air that occasionally reaches the Amazonian basin, influencing conditions on the southern margins of the area. | Amazonia case study | |||
| Front | The line forming the junction between two contrasting air masses. | Fronts and weather | |||
| Frontal rainfall | Rainfall caused by warm air rising over a colder air mass at a front. | Satellite interpretation | |||
| Frost hollow | A low lying area where cold air drainage creates a persistently cold microclimate. | Mountain winds | |||
| Global circulation | The large scale circulation of air driven by the temperature contrasts between equatorial heat and polar cold. This circulation determines global pressure belts, rainfall belts and wind belts. | Spatial
influences
Global circulation |
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| Global warming | The observed increase in global temperatures. Global warming has happened before without human influence. The worry is that the added influence of human activity may make the system much more unstable. Another complication is that millions of people are going to be affected by climate change - a situation which has no historical precedent. | Global
warming evidence and
causes
Sea level rise Impacts on health Impacts on ecosystems Solutions |
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| Greenhouse effect | The process by which incoming radiation enters the Earth's atmosphere more easily than outgoing radiation can escape. This results in net heating. The greenhouse effect is normal, but humans seem to be causing an enhanced greenhouse effect. | Global warming evidence and causes | |||
| Hadley cell | The part of global circulation that takes place near the equator. | Global circulation | |||
| Humidity | The amount of moisture in the air. Can be expressed as "absolute humidity" (grams per cu. cm) or "relative humidity" (% of the air's capacity to hold water). The latter is temperature dependent. | Local
climate
Evaporation and condensation |
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| Hurricane | A cyclone with windspeeds in excess of 120 kph is a hurricane in the Atlantic, a typhoon in the western Pacific or a tropical cyclone in the Indian Ocean. The winds circulate anticlockwise in the northern hemisphere | Hurricane Mitch | |||
| Intensive agriculture | Agriculture with high inputs (eg fertilizers and machinery) and a high expected output in terms of yield. | Climate and people | |||
| Inter Tropical Convergence Zone (ITCZ) | More or less coincident with the "heat equator" - the belt on the Earth's surface with highest mean temperatures. This is the part of the Hadley Cell where easterly winds from either side of the equator converge into a belt of low pressure and rising air. | Seasons
Amazonia case study Savanna West Africa |
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| Katabatic winds | Cold downslope winds in mountainous areas generated by both temperature contrasts and gravitational pull on a dense fluid (cold air). | Mountain winds | |||
| Koppen classification | A classic attempt at dividing the complexity of the Earth's climate into manageable divisions. | Defining climate | |||
| Lake effect snow | Localised snowfalls generated when very cold air moves over warmer water in winter. The bottom layer of the air picks up heat and moisture from the lake and produces localised snowfalls downwind. This is the effect that gives eastern England occasional snowfalls under polar continental air in winter. | Water influences | |||
| La Nina | A "colder than normal" ocean current flowing off the coast of Peru. La Nina is associated with a range of weather extremes in teleconnected events across the world. The Atlantic hurricane season is more vigorous during La Nina. | ENSO
Hurricane Mitch |
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| Latent heat of condensation | When condensation of water vapour takes place, heat is released into the air. This process enables condensation in rising damp air to create vigorous thunderstorm activity. | Evaporation and condensation | |||
| Latitude and climate | Latitude influences the maximum angle at which the sun appears in the sky. At high latitudes, the sun is low in the sky and achieves little in the way of heating. | Spatial
influences
European case study USA case study - Washington Regional climate |
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| Maritime influences | The sea is slow to respond to heating or cooling because it has a high specific heat capacity. Land areas near the coast have a more moderate and moist climate. | USA case study - Miami | |||
| Milankovitch cycles | A series of regular climatic changes due to variations in the orbit of the Earth. | Changes in time | |||
| Occluded front | A subtle frontal change caused when cold air behind a depression catches up with the cold air in front of the depression, lifting the warm sector off the ground. | Occluded front | |||
| Ocean currents and climate | Ocean currents transfer enormous quantities of heat around the globe. The proximity of a warm or cold current can radically alter the expected climate for a given latitude. | European case study | |||
| Pacific high | A semi-permanent high pressure system which influences climatic conditions in the western USA. | USA
case study - Salem
USA case study - San Francisco |
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| Pleistocene | The name given to the period of immense climatic variability which started about 2 million years ago and ended about 12,000 years ago. | Changes in time | |||
| PM10 | Particulate matter below 10 microns in diameter; a recently discovered health hazard - possibly a serious urban health threat accounting for tens of thousands of premature deaths a year. | Regional
air quaity
Air pollution trends |
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| Polar cell | The part of the global circulation which takes place near the poles. | Global circulation | |||
| Polar front | The boundary between the polar and tropical air masses on the Earths's surface. At the polar front, strong temperature gradients create high windspeeds in the upper atmosphere which, in turn,- result in Rossby wave patterns. These both initiate and steer depressions. The location of the polar front is a very significant influence on the climate of an area. | Spatial
influences
European case study USA case study - San Francisco USA case study - Omaha USA case study - Dallas USA case study - New York Regional climates |
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| Pollution | The degradation of natural air quality by the addition of harmful emissions resulting from human activity. | Regional
air quality
Local climate Urban climate |
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| Precipitation | Moisture from the atmosphere landing on the ground - usually rainfall, but may include sleet, snow, hail. | Local climate | |||
| Prediction | The estimation of future weather or climate by studying present conditions and trends. | Issues
and uncertainties
Weather forecasting |
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| Radiation balance | The balance between the percentage of incoming radiation that is reflected and absorbed. | Urban climate | |||
| Rain shadow | The lower rainfalls commonly observed downwind of a mountain range, due in part to the Föhn effect. | Relief and rainfall | |||
| Relative humidity | The amount of water in the air expressed as a percentage of the amount of water the air is theoretically capable of holding at the same temperature. | Relief and rainfall | |||
| Relief | The height of the land. This has a significant affect on temperatures, cloud cover, precipitation and humidity. | Regional
climates
Local climates Relief and rainfall |
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| Relief rain | Rain caused by air rising over a highland area cooling and condensing to form clouds. If the clouds are sufficiently thick, rain will form. | Satellite interpretation | |||
| Rocky Mountains | A major north-south mountain range in the west of the USA. The Rockies are big enough to disturb the global westerly wind flow and encourage the formation of meanders in the upper westerly winds. This creates breeding grounds for depressions downwind. | USA case study - St Louis | |||
| Rossby waves | Large scale meanders in the upper westerly air flow. These both initiate and steer depression systems. | Depression
origins
Depression formation |
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| Satellite image | A view of Earth imaged with special scanners that can view the atmosphere in visible light and a range of infrared lights. This enables temperature and moisture information to be calibrated from the resulting images. | Interpreting satellite images | |||
| Savanna | A tropical climate with alternating wet and dry seasons. The length of the wet and dry seasons depends on the location with respect to the ITCZ. | Savanna | |||
| Sea breeze | A local wind effect caused by the temperature and pressure contrasts which result from the differential heating of the land and sea during the day. | Water influences | |||
| Seeder-feeder effect | The effect by which rainfall generation is enhanced by large raindrops splitting into smaller drops which then grow rapidly. | Relief and rainfall | |||
| Sensible heat | Heat that can be physically felt or "sensed" - warm air escaping from windows, hot air from exhausts, etc. | Urban climate | |||
| Soil moisture | Water stored in the soil. Soil moisture stores build up after wet periods, but gradually get used by plants or leak into groundwater during a dry spell. | Water balance graphs | |||
| Synoptic chart | A chart plotting current weather observations for a wide area. By observing the present pressure patterns and weather activity, it is possible to forecast future changes to some degree of accuracy. | Synoptic
charts
Synoptic chart interpretation |
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| Temperature | The energy level of a substance as measured by its heat output. | Local climate | |||
| Tropical forests | Evergreen, broadleaved woodlands which form the climax stage of equatorial climates. | Tropical forests | |||
| Urban climate | A local variation in temperatures, rainfall, wind, etc as a result of an urban area altering the physical and chemical nature of the local atmosphere. | Local climate | |||
| Urban heat island | A commonly observed phenomenon of higher temperatures in towns and cities. | Urban climate | |||
| Varzea | A fertile strip of land along the edge of the Amazon floodplain. Most Amazon agriculture takes place in the varzea due to its high fertility. | Amazonia case study | |||
| Volcanism | Volcanic activity on the surface of the earth - large eruptions can alter the transparency of the atmosphere, resulting in global cooling. | Changes in time | |||
| Warm front | The junction observed between two air masses when warmer air is encroaching on colder air. | Warm fronts | |||
| Water balance | The balance between precipitation, evapotranspiration and water storage. A water balance graph is often used to show seasonal changes in the water balance. | Urban
climate
Water balance graphs |
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| Water bodies | The climate of areas near lakes and coastlines is more humid with moderate temperatures due to the high specific heat capacity of water. | Local climate | |||
| Weather | Weather is the "condition of the atmosphere", usually taken on a short time scale and a small to medium spatial scale. | What is weather | |||
| Weather salience | This refers to the "weather awareness" of a society and their sensitivity to weather conditions. | Weather and people | |||
| Weather symbols | Symbols used on a synoptic chart to record the weather observations at a weather station. | Synoptic charts | |||
| Wind direction and climate | Wind direction is often a good predictor of coming weather conditions. The weather conditions brought by the wind will depend on the air mass it flows from and the changes that occur en route. | European
case study
USA case study - Boston USA case study - New York Air mass introduction Air mass weather |
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| Wind and local climate | In addition to the large scale climate influences brought about by wind direction, small scale contrasts in temperature and pressure create localised winds. These are most significant when regional wind flows are weak. | Urban
climate
Mountain winds Sea breezes |