Weather Glossary

A large body of air with broadly uniform temperature and moisture characteristics acquired over its source region. Air masses are classified by origin (for example maritime or continental) and whether they are relatively warm or cold. Their movement and modification strongly influences cloud, precipitation, wind and temperature patterns.

The force per unit area exerted by the weight of the atmosphere above a point, most commonly expressed in hectopascals (hPa) in the UK. Air pressure varies with weather systems, altitude and temperature structure. Changes in pressure are central to diagnosing approaching or departing synoptic features.

A pressure value adjusted to mean sea level used primarily in aviation to calibrate aircraft altimeters. It differs from simple station pressure because it includes standardised assumptions about the atmosphere. Although mainly an aviation term, it relates directly to how pressure is reported and compared between locations.

An instrument used to measure wind speed, typically mounted at a standard height in an open exposure to reduce local sheltering effects. Common types include cup and ultrasonic anemometers. Accurate wind measurement is essential for gust reporting, coastal forecasts and severe wind warnings.

A region of relatively high atmospheric pressure with outward-flowing winds at the surface (clockwise in the Northern Hemisphere). Anticyclones are often associated with more settled conditions, lighter winds and reduced precipitation, though they can also bring fog, low cloud or poor air quality when stagnant. In the UK, persistent anticyclones can produce winter temperature inversions with fog and frost, or summer spells of warm, dry weather.

A cold air mass originating from high latitudes, typically characterised by low temperatures and, depending on its route, variable moisture content. When modified over the sea it can become unstable and generate showers. Over the UK, Arctic air frequently brings wintry showers to northern and eastern coasts, especially when winds are northerly or north-easterly.

Water present in the atmosphere in vapour, liquid or solid form, including humidity, cloud droplets and ice crystals. It is governed by temperature, pressure and air mass history. Atmospheric moisture is fundamental to cloud formation, fog, precipitation and visibility.

The pressure exerted by the atmosphere, measured at the surface or at various levels aloft. Surface pressure patterns help identify lows, highs, fronts and pressure gradients, while upper-level pressure fields reflect the broader flow and stability. For UK forecasting, pressure distribution is a core indicator of wind strength, frontal progression and the likelihood of unsettled weather.

The trailing boundary of a precipitation area, marking where rain, sleet or snow is ending as the system moves away. It is distinct from the “cold front” or “occlusion” itself, though it may coincide with them. Operationally, identifying the back edge is important for timing improvements in travel conditions and visibility.

A region where temperature changes sharply over a short distance, leading to strong horizontal density gradients and enhanced wind shear. Baroclinic zones are favoured regions for the development and intensification of mid-latitude depressions. Around the UK, these zones commonly lie along frontal boundaries in the Atlantic storm track.

The change in atmospheric pressure over a specified time period, often measured over 3 hours in observational practice. It helps indicate whether a location is coming under rising pressure (often stabilising) or falling pressure (often destabilising). In UK synoptic situations, a steady fall can signal an approaching depression, while a rise often follows the passage of fronts.

A descriptive scale that relates wind speed to observed effects on land and sea, originally designed for maritime use. While modern forecasting uses measured speeds, Beaufort terms (such as “gale”) still appear in marine forecasts. It provides a consistent way to communicate wind impact, especially for coastal and shipping interests.

A thin, often transparent layer of ice on surfaces, formed when water freezes on contact with roads, pavements or other structures. It is difficult to see and can form under clear skies with sub-zero surface temperatures, or when rain falls onto cold ground. In UK winter weather, black ice is a key hazard after evening cooling, following showers, or during marginal thaw–refreeze cycles.

A severe winter weather condition defined by a combination of strong winds and blowing snow that reduces visibility significantly for a sustained period. The defining hazard is the visibility and drifting, not necessarily heavy snowfall at the time. In the UK, true blizzard conditions are relatively uncommon but can occur in exposed upland areas and during powerful northerly or easterly events.

A period of twilight when the Sun is below the horizon and the sky takes on a predominantly blue tone due to scattering and the balance of direct and indirect light. It occurs both before sunrise and after sunset, with timing influenced by latitude, season and atmospheric clarity. For UK locations, the duration varies through the year and is often shortest in summer and longer in winter at higher latitudes.

The lowest part of the atmosphere directly influenced by the Earth’s surface, typically extending from tens of metres to a couple of kilometres depending on conditions. Turbulence, surface heating, friction and moisture exchanges dominate within this layer. In the UK, boundary-layer processes strongly affect fog formation, low cloud, wind gustiness and the dispersion of pollutants.

A large-scale, quasi-stationary pattern that disrupts the usual west-to-east progression of weather systems, often involving persistent high pressure at mid to high latitudes. Blocking can divert the jet stream and prolong certain weather regimes. Over the UK, blocking patterns can lead to extended cold spells with easterlies in winter, or prolonged dry and warm periods in summer, depending on the block’s position.

The tendency of an air parcel to rise or sink depending on its density relative to the surrounding air, governed by temperature and moisture content. Positive buoyancy supports convective cloud growth and showers; negative buoyancy suppresses vertical development. Buoyancy is central to thunderstorm potential, especially during warm-season convection in the UK.

The standard temperature scale used in the United Kingdom for meteorological reporting and public forecasts. Zero degrees Celsius corresponds to the freezing point of pure water at standard atmospheric pressure, and 100°C to its boiling point. All routine UK surface observations, warnings and climatological statistics are expressed in Celsius.

A condition in which little or no cloud is present, typically defined operationally as negligible cloud cover. Clear skies often occur beneath anticyclones or behind cold fronts where subsiding air suppresses cloud formation. In the UK, clear overnight conditions can enhance radiative cooling, increasing the likelihood of frost or fog in winter.

The lowest altitude at which a cloud layer is observed, usually measured in metres or feet above ground level. It is a critical parameter in aviation and also influences surface visibility and light levels. Low cloud bases are common in maritime air masses affecting western and northern parts of the UK.

The proportion of the sky obscured by cloud, traditionally expressed in oktas (eighths of the sky) or as a percentage. Cloud cover affects temperature ranges, solar radiation and perceived brightness. Persistent high cloud cover in the UK can moderate daytime heating and reduce nocturnal cooling.

The leading boundary of a cooler air mass advancing into a warmer one. It is typically associated with a band of cloud and precipitation, followed by a drop in temperature and a change in wind direction. Across the UK, cold fronts often arrive from the Atlantic and may bring periods of rain, squally showers or clearer, cooler conditions in their wake.

A short-lived period of markedly colder-than-average conditions for the time of year. It may arise from the advection of Arctic or continental air masses. In the UK, cold snaps are often linked to northerly or easterly flows and can increase frost and ice risk.

An air mass originating over large land areas, typically characterised by relatively low moisture content. In winter it is often cold and dry; in summer it may be warm or hot and dry. When easterly continental air affects the UK, it can produce dry, clear conditions or, if modified over the North Sea, snow showers along eastern coasts.

Vertical motion in the atmosphere driven by buoyancy, often resulting in cumuliform cloud development and showers. Convective intensity depends on instability, moisture and lifting mechanisms. In the UK, convective activity is common in showery polar maritime air and during summer heating inland.

A quantitative measure of atmospheric instability, representing the energy available for buoyant ascent. Higher CAPE values indicate greater potential for vigorous convective development, though other factors such as wind shear are also critical. In UK forecasting, CAPE values are typically lower than in continental climates, but even modest levels can support thunderstorms.

A region of low atmospheric pressure characterised by inward-spiralling surface winds (anticlockwise in the Northern Hemisphere). Cyclones are associated with cloud, precipitation and stronger winds due to pressure gradients. Mid-latitude cyclones moving across the North Atlantic are a primary driver of unsettled UK weather.

The length of time between sunrise and sunset on a given date at a specific location. It varies seasonally due to the tilt of the Earth’s axis and latitude. In the UK, daylight duration ranges from short winter days, particularly in Scotland, to long summer days with extended twilight.

The temperature to which air must be cooled for saturation to occur at constant pressure, leading to condensation. It is a direct indicator of atmospheric moisture content. Higher dew points in the UK often accompany humid summer air masses and can contribute to overnight discomfort and mist formation.

The standard UK term for a mid-latitude cyclone, referring to a low-pressure system with associated fronts. Depressions frequently bring bands of rain and strong winds. Atlantic depressions are a defining feature of UK autumn and winter weather.

Very light precipitation composed of small droplets falling from low cloud, typically stratiform in nature. Drizzle often reduces visibility without producing significant rainfall totals. It is common in moist maritime air under overcast conditions across western Britain.

A forecasting approach using multiple model simulations with slightly varied initial conditions to assess uncertainty and possible outcomes. The spread among ensemble members provides an indication of forecast confidence. In UK medium-range outlooks, ensemble guidance helps quantify the likelihood of temperature trends or unsettled periods.

The process by which liquid water changes into vapour, influenced by temperature, humidity, wind and solar radiation. Evaporation cools surfaces and modifies local moisture levels. After rainfall in the UK, evaporation rates are typically higher during breezy, sunny conditions.

An official notification issued when hazardous conditions are expected to pose risks to life, property or infrastructure. In the UK, warnings are categorised to reflect impact and likelihood. Such warnings may relate to wind, rain, snow, ice, heat or thunderstorms.

A temperature scale historically used in some regions, with water freezing at 32°F and boiling at 212°F under standard conditions. It is not the standard for UK meteorological reporting but may appear in international comparisons.

An assessment of the likelihood and potential impact of flooding from rainfall, river overflow, surface runoff or coastal surge. Flood risk depends on rainfall intensity, duration, soil saturation and topography. In the UK, prolonged frontal rainfall and slow-moving systems are common contributors to elevated flood risk.

A suspension of tiny water droplets near the surface reducing visibility to less than 1,000 metres. Fog forms when air cools to its dew point or when moist air moves over a cooler surface. Radiation fog under clear skies and light winds is frequent in UK autumn and winter.

An evaluation of the reliability of a forecast based on model agreement, ensemble spread and synoptic predictability. Confidence decreases with increasing forecast lead time. Communicating forecast confidence is essential for informed decision-making in sectors such as transport and energy.

Fog composed of supercooled droplets that freeze upon contact with surfaces, forming rime ice. It occurs when fog persists in sub-zero temperatures. Freezing fog in the UK can create hazardous road and rail conditions due to ice accretion.

A boundary separating air masses of different temperature and moisture characteristics. Fronts are classified as cold, warm or occluded and are often associated with cloud bands and precipitation. Frontal systems crossing the UK are central to day-to-day weather variability.

A strong wind typically defined in UK marine usage as force 8 or 9 on the Beaufort Scale, corresponding to sustained wind speeds of approximately 34–47 knots (39–54 mph). Gales are commonly associated with deep Atlantic depressions and tight pressure gradients. In the UK, gales pose risks to coastal operations, transport infrastructure and exposed upland areas.

The period shortly after sunrise and before sunset when the Sun is low above the horizon, producing softer, warmer light due to increased atmospheric scattering. Its duration varies by latitude, season and atmospheric clarity. Across the UK, golden hour is shortest in midsummer and more prolonged in winter, particularly at higher latitudes.

The rate of change in atmospheric pressure over a given horizontal distance. A steeper pressure gradient corresponds to stronger winds, as air accelerates from high to low pressure. In UK forecasting, tightly packed isobars on synoptic charts indicate the potential for strong or gale-force winds.

The formation of ice crystals on surfaces when ground-level temperatures fall to or below 0°C, even if the air temperature measured at standard height remains slightly above freezing. Ground frost is frequent in clear, calm UK winter nights under high pressure.

A brief increase in wind speed above the sustained mean, typically lasting a few seconds. Gusts are influenced by turbulence, surface roughness and convective mixing. In the UK, gusts are particularly significant during frontal passages, showers and convective storms.

Precipitation in the form of solid ice particles formed within strong convective clouds where updraughts carry droplets into sub-zero regions. Hailstones vary in size depending on storm intensity. In the UK, hail is most common in showery polar maritime air and during summer thunderstorms.

A prolonged period of unusually high temperatures relative to the seasonal average, typically defined by region-specific thresholds. Heatwaves are often associated with persistent high pressure and subsidence. In the UK, heatwaves can strain infrastructure, increase health risks and elevate wildfire potential, particularly in southern and eastern regions.

The standard unit of atmospheric pressure used in UK meteorology, equivalent to one millibar. Typical sea-level pressure ranges from approximately 950 to 1050 hPa in mid-latitudes. Pressure values in hPa are fundamental to synoptic chart interpretation.

The percentage ratio of the current amount of water vapour in the air to the maximum it could hold at that temperature. Relative humidity varies with temperature and moisture content. High relative humidity in the UK often accompanies mild maritime air and may contribute to mist, fog or discomfort in summer.

Frozen water forming on surfaces such as roads, pavements and structures when temperatures fall below freezing. Ice may develop from freezing rain, melting snow or condensation. Surface ice is a common winter hazard in the UK, particularly after evening temperature drops.

A reversal of the normal temperature decrease with height, where air temperature increases with altitude over a layer. Inversions suppress vertical mixing and can trap moisture and pollutants. In the UK, winter anticyclones frequently produce low-level inversions associated with fog and frost.

A line on a weather chart connecting points of equal atmospheric pressure. The spacing of isobars indicates the strength of the pressure gradient and therefore wind speed. Closely spaced isobars over the UK signal stronger winds, especially around deep depressions.

A narrow band of strong winds in the upper troposphere, typically found near the boundary between warm and cold air masses. The jet stream influences the development and track of mid-latitude weather systems. The position and strength of the North Atlantic jet stream play a central role in determining whether the UK experiences settled or unsettled conditions.

A metric unit of speed commonly used internationally to express wind speed. While miles per hour are widely used in UK public forecasts, km/h may appear in technical or comparative contexts.

A sudden electrostatic discharge occurring within a cloud, between clouds, or between cloud and ground during thunderstorms. Lightning is associated with strong convective activity and rapid heating of surrounding air. In the UK, lightning is most frequent in summer convective storms and occasionally within vigorous winter frontal systems.

A forecast covering extended periods, typically beyond ten days and sometimes seasonal in scope. Long-range forecasts rely heavily on ensemble modelling and probabilistic methods. For the UK, long-range outlooks provide broad indications of temperature and precipitation trends rather than precise daily details.

A region of lower atmospheric pressure relative to surrounding areas, associated with rising air, cloud formation and often precipitation. Low-pressure systems moving in from the Atlantic are a primary source of rainfall and wind across the UK.

An air mass originating over the ocean, characterised by relatively high moisture content. Its temperature depends on source latitude, ranging from mild and moist (tropical maritime) to cool and unstable (polar maritime). Maritime air masses frequently influence the UK due to its proximity to the Atlantic, often bringing cloud, showers and variable winds.

A forecast covering approximately three to ten days ahead. It balances deterministic model output with ensemble guidance to assess developing patterns. In the UK, medium-range forecasts typically indicate trends in temperature, rainfall and prevailing wind direction rather than precise local timing.

The standard unit used in the UK to measure precipitation depth. One millimetre of rainfall corresponds to one litre of water per square metre. Rainfall totals in millimetres provide a consistent basis for assessing flood risk, soil saturation and drought conditions.

A commonly used unit for reporting wind speed in UK public forecasts and warnings. It provides an accessible reference for assessing wind impact.

A suspension of small water droplets reducing visibility to between 1,000 and 5,000 metres. Mist is less dense than fog and often forms under similar conditions of cooling and high humidity. Mist is common in the UK during calm, damp evenings and mornings.

A single execution of a numerical weather prediction model using a defined set of initial conditions. Major global models are typically run multiple times daily. Successive model runs allow forecasters in the UK to monitor shifts in projected storm tracks, temperature trends and precipitation timing.

The apparent shape of the Moon’s illuminated portion as seen from Earth, determined by its position relative to the Sun. Phases progress from new moon to full moon over a lunar cycle of approximately 29.5 days. While moon phase does not directly affect day-to-day weather, it is relevant to tidal forecasting and night-time illumination.

Short-term forecasting covering the next few hours, based on real-time observations such as radar, satellite imagery and surface data. Nowcasting is particularly valuable for rapidly evolving weather. In the UK, nowcasting is used to monitor convective showers, thunderstorms and sudden winter hazards such as snow bursts.

A front formed when a cold front overtakes a warm front, lifting the warm air aloft. Occlusions are commonly associated with mature mid-latitude depressions and widespread cloud and precipitation. Occluded fronts frequently pass over the UK within Atlantic low-pressure systems.

A condition in which the sky is completely covered by cloud, typically defined as 8 oktas of cloud cover. Overcast conditions often occur beneath extensive frontal or stratiform cloud layers. Persistent overcast skies in the UK can limit daytime temperature rise and suppress nocturnal cooling.

A sky condition in which cloud covers a portion, but not the majority, of the sky. The term is descriptive and reflects a mix of cloud and clearer intervals. In the UK, partly cloudy conditions often occur in showery regimes with variable cloud amounts.

Any form of water, liquid or solid, falling from the atmosphere and reaching the ground, including rain, snow, sleet and hail. Precipitation results from condensation and growth of droplets or ice crystals within clouds. Patterns of precipitation are central to UK weather variability and hydrological impact.

The direction and rate of change of atmospheric pressure over time. Rising pressure generally indicates stabilising conditions, while falling pressure suggests an approaching disturbance. Monitoring pressure trends assists in identifying developing weather systems over and around the UK.

The likelihood, expressed as a percentage, that measurable precipitation will occur at a specific location during a defined period. It reflects both forecast confidence and expected spatial coverage. In UK forecasts, this value helps quantify uncertainty in showery or marginal situations.

A cool, moist air mass originating over high-latitude oceans. It is typically unstable over relatively warmer waters and land surfaces. When polar maritime air flows across the UK, it often produces frequent showers, particularly in western and northern regions.

A forecast of the expected amount of precipitation over a given period, expressed in millimetres. QPF values are derived from numerical models and observational adjustments. In the UK, QPF informs flood risk assessments and water resource management.

Fog formed by radiative cooling of the Earth’s surface under clear skies and light winds, causing near-surface air to reach saturation. Radiation fog is frequent in UK autumn and winter anticyclonic conditions.

Liquid precipitation measured as accumulated depth over a specified period. Rainfall intensity and duration determine hydrological impact. In the UK, frontal systems and slow-moving lows are common sources of significant rainfall totals.

The ratio, expressed as a percentage, of the current amount of water vapour in the air to the maximum amount it could hold at the same temperature. Relative humidity varies inversely with temperature when moisture content remains constant. In UK conditions, high relative humidity is common in maritime air masses and frequently accompanies mist, fog or low cloud.

An elongated area of relatively high atmospheric pressure extending from a high-pressure centre. Ridges are typically associated with subsiding air, lighter winds and more settled conditions. Across the UK, a transient ridge between Atlantic systems can bring brief dry and calmer intervals.

A white, opaque ice deposit formed when supercooled fog droplets freeze on contact with surfaces. It often develops in freezing fog or on exposed upland terrain. Rime ice is occasionally observed on higher ground in the UK during cold, moist winter conditions.

The state in which air contains the maximum amount of water vapour possible at a given temperature and pressure. At saturation, condensation can occur, leading to cloud, fog or dew formation. Saturation near the surface in the UK commonly occurs during nocturnal cooling under clear or moist conditions.

A local wind circulation that develops when land heats more rapidly than the adjacent sea, causing air to rise over land and cooler marine air to move inland. In coastal parts of the UK, sea breezes are most frequent in spring and summer under light large-scale winds.

Weather conditions that pose significant risk to life, property or infrastructure due to intensity or impact. Criteria vary by hazard type and regional context. In the UK, severe weather may include strong winds, heavy rainfall, disruptive snowfall, heatwaves or thunderstorms.

A forecast covering the immediate period ahead, typically up to 48 hours. It is based on high-resolution model output and recent observations. Short-range forecasts in the UK provide detailed timing of fronts, showers and temperature changes.

Precipitation consisting of a mixture of rain and snow, or partially melted snowflakes. It occurs when snow falls through a layer of above-freezing air before reaching the surface. Sleet is common in the UK during marginal winter conditions when temperatures hover near 0°C.

Solid precipitation in the form of ice crystals that combine to form snowflakes. Snowfall intensity and accumulation depend on temperature profiles and moisture availability. In the UK, snowfall is most frequent in northern and upland regions but can affect lowland areas during cold continental or Arctic outbreaks.

Energy emitted by the Sun and received at the Earth’s surface, measured in watts per square metre. Solar radiation drives atmospheric heating and influences temperature, evaporation and convection. Seasonal variation in solar radiation is pronounced across the UK, contributing to significant differences between winter and summer temperatures.

A general term for a disturbed atmospheric state involving strong winds and often precipitation. In UK usage, the term is typically reserved for named windstorms with significant impact. Storm systems affecting the UK frequently originate over the North Atlantic.

Atmospheric pressure measured at the Earth’s surface and adjusted to mean sea level for comparison across locations. It forms the basis of synoptic chart analysis. Surface pressure patterns determine wind flow and the development of weather systems over the UK.

A weather map displaying pressure patterns, fronts and other large-scale features over a broad area at a specific time. Synoptic charts are fundamental tools for analysing mid-latitude weather systems. UK forecasters use synoptic charts to interpret Atlantic depressions, ridges and frontal structures.

A measure of the thermal state of the air, expressed in degrees Celsius in UK meteorology. Temperature influences air density, humidity capacity and atmospheric stability. Daily temperature ranges in the UK are moderated by maritime influence but can vary significantly under clear or continental conditions.

A layer in the atmosphere where temperature increases with height, contrary to the usual decrease. Inversions suppress vertical mixing and can trap moisture and pollutants. Winter anticyclones over the UK frequently produce low-level inversions associated with fog and poor air quality.

The rate of temperature change over a given horizontal or vertical distance. Strong thermal gradients often occur near fronts and contribute to baroclinic development. In UK mid-latitude systems, thermal gradients are central to the formation and intensification of depressions.

A convective storm characterised by lightning and thunder, often accompanied by heavy rain, gusty winds and occasionally hail. Thunderstorms form in unstable air with sufficient moisture and lift. In the UK, thunderstorms are most common in summer but may also occur in vigorous frontal systems.

An elongated region of relatively low atmospheric pressure, often extending from a depression. Troughs are typically associated with rising motion, cloud development and showery or unsettled conditions. Across the UK, troughs frequently follow cold fronts and can sustain bands of showers, particularly in polar maritime air.

The period of diffused natural light occurring before sunrise and after sunset, when the Sun is below the horizon but its light is still scattered by the atmosphere. Twilight is divided into civil, nautical and astronomical phases based on solar elevation. In the UK, twilight duration varies considerably by season and latitude, being prolonged in northern regions during summer.

The pattern of winds in the mid and upper troposphere, typically analysed on pressure surfaces such as 500 hPa. Upper-level flow governs the movement and development of surface weather systems. The configuration of upper-level flow over the North Atlantic strongly influences whether the UK experiences zonal (west–east) progression or blocked patterns.

A standardised measure of the strength of ultraviolet radiation reaching the surface, scaled to indicate potential skin exposure risk. The index depends on solar elevation, cloud cover, ozone concentration and surface reflection. In the UK, UV levels are generally highest in late spring and summer, particularly under clear skies.

The maximum horizontal distance at which an object can be clearly distinguished against the background. Visibility is reduced by fog, mist, precipitation, haze or blowing snow. In the UK, visibility is frequently affected by frontal rainbands, sea fog and winter radiation fog.

The leading boundary of a warmer air mass advancing over cooler air. Warm fronts are typically associated with widespread layered cloud and steady precipitation ahead of the surface boundary. In the UK, warm fronts often bring prolonged rainfall and gradual temperature rises, particularly in autumn and winter.

An official communication indicating the expected occurrence of hazardous weather conditions, graded according to likelihood and potential impact. UK warnings are designed to support preparedness and informed response across transport, utilities and public services.

The compass direction from which the wind is blowing, measured in degrees or cardinal points. Wind direction reflects the orientation of pressure systems and air mass origins. In the UK, prevailing south-westerlies are common, though northerly and easterly flows can significantly alter temperature and precipitation patterns.

A brief, rapid increase in wind speed above the sustained mean, usually lasting a few seconds. Gust magnitude depends on turbulence, convective mixing and local exposure. Gust reporting in the UK is critical for assessing potential impacts during storms and squally showers.

A change in wind speed or direction over a short vertical or horizontal distance. Wind shear can influence storm development and aviation safety. In UK convective environments, wind shear plays a role in organising thunderstorms and enhancing gust potential.

The rate at which air moves horizontally past a fixed point, typically measured at 10 metres above ground level. Wind speed is reported as a sustained average over a standard time interval. Wind speed in the UK is closely tied to pressure gradients around depressions and anticyclones.

An atmospheric circulation pattern in which winds predominantly follow a west–east orientation, with relatively limited north–south meandering. Zonal flow supports the steady progression of weather systems. When zonal flow dominates the North Atlantic, the UK often experiences frequent frontal systems and changeable conditions.