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Also called the feels-like temperature, this is an estimate of how hot or cold the air actually feels to a human body, as opposed to what the thermometer reads. It combines air temperature with factors like humidity, wind and sometimes sunshine, because your body experiences the rate at which it gains or loses heat rather than the air temperature directly. In summer it usually reflects the heat index; in winter, the wind chill.
A measure combining air temperature and humidity to express how hot it truly feels. High humidity makes hot weather feel worse because it prevents sweat from evaporating and cooling the body, so the heat index can run many degrees above the actual temperature. Heat warnings are based on this figure rather than the raw temperature because heat illness tracks the felt heat.
A measure of how cold the air feels to exposed skin once wind is taken into account. Wind strips away the thin layer of warmed air next to your skin and accelerates heat loss, so a moderate temperature in a strong wind can feel bitterly cold and even pose a frostbite risk. Cold-weather forecasts emphasise wind chill because your body loses heat at the rate it describes, not the thermometer reading.
The temperature to which air must be cooled for its water vapour to begin condensing into liquid. It is one of the most honest measures of how much moisture is genuinely in the air: a high dew point means muggy, uncomfortable conditions regardless of the relative humidity percentage. When the dew point reaches the air temperature, the air is saturated and fog or dew can form.
A situation where temperature increases with height instead of decreasing, so a layer of warm air sits on top of cooler air near the ground. Inversions are very stable and trap whatever is beneath them, including pollution, fog and, importantly for farmers, spray droplets that can then drift unpredictably. They typically form on clear, calm nights as the ground radiates its heat away.
The amount of water vapour in the air expressed as a percentage of the maximum the air could hold at that temperature. Because warm air can hold far more moisture than cold air, relative humidity can be misleading: cold air at 90 percent may contain little actual moisture, while warm air at 40 percent may contain much more. For how muggy it really feels, the dew point is a better guide.
Any form of water, liquid or solid, that falls from clouds and reaches the ground, including rain, drizzle, snow, sleet and hail. It forms when cloud droplets or ice crystals grow large enough that they can no longer be held aloft by rising air. The type that reaches the ground depends largely on the temperature profile of the air it falls through.
A long, narrow corridor of concentrated water vapour flowing through the atmosphere, capable of carrying more water than the world's largest rivers. When one meets a mountain range, the air is forced up and wrings out enormous quantities of rain or snow, making atmospheric rivers a leading cause of both beneficial water supply and destructive flooding in certain regions.
A general term for the amount of water vapour present in the air. It is the invisible moisture that governs how muggy the air feels, whether dew or fog will form, and how effectively sweat can cool the body. It is also the raw fuel for clouds, rain and storms, since all precipitation begins as water vapour that condenses.
Small, translucent ice pellets that form when snowflakes partially melt as they fall through a warm layer, then refreeze into ice on the way down through colder air near the ground. Sleet bounces on impact and is distinct from freezing rain, which falls as liquid and freezes only on contact with surfaces.
The weight of the air above a given point, pressing down on everything below. It is measured in units such as hectopascals or millibars, and changes in it drive much of our weather: falling pressure often heralds unsettled or stormy conditions, while rising pressure signals improvement. Pressure also falls steadily with altitude, which is why barometers must be adjusted for height.
A region where atmospheric pressure is higher than its surroundings, associated with descending air that suppresses cloud formation. High-pressure systems typically bring settled, calm, clear weather, warm and dry in summer but cold and frosty or foggy in winter. When one stalls in place it can lock in the same conditions for days.
A region where atmospheric pressure is lower than its surroundings, associated with rising air that cools and forms clouds and precipitation. Low-pressure systems bring the unsettled, wet and windy weather typical of storms and fronts, and are steered along by the jet stream. The deeper the low, the more vigorous the weather it tends to bring.
A ribbon of very fast wind high in the atmosphere, flowing generally from west to east and marking the boundary between warm and cold air masses. It steers weather systems and, by looping north and south, determines whether a region gets mild or cold, settled or stormy conditions. When it locks into a stalled pattern, it can produce prolonged heatwaves or wet spells.
A scale that describes wind strength by its observable effects, running from 0 for calm to 12 for hurricane force. Originally devised for sailors, it links wind speeds to what you can actually see, such as leaves rustling, umbrellas turning inside out or trees being uprooted, making it a practical way to gauge wind without instruments.
A change in wind speed or direction over a relatively short distance, either horizontally or with height. Strong wind shear tears apart the vertical structure of developing thunderstorms and hurricanes, which is why it suppresses tropical cyclone activity. Near the ground, sudden shear is a serious hazard to aircraft during takeoff and landing.
The puffy, cotton-wool clouds with flat bases and rounded tops that form when the sun heats the ground and sends up bubbles of warm air. Small, scattered cumulus signals fair, settled weather, but if they build tall through a warm afternoon they can grow into towering storm clouds. They are the classic cloud of a pleasant day.
The towering thunderstorm cloud, capable of reaching the top of the lower atmosphere and spreading into a flat anvil shape. It brings heavy rain, lightning, gusty winds and sometimes hail, and is the most dramatic and dangerous of the common cloud types. It grows from cumulus when the atmosphere is unstable and moist.
Thin, wispy clouds of ice crystals high in the atmosphere, often looking like delicate streaks or mares' tails. On their own they often accompany fine weather, but thickening cirrus spreading across the sky is frequently the first sign of an approaching warm front and coming rain, sometimes a day or more ahead.
A low, grey, featureless cloud layer that spreads across the sky like a blanket, often bringing dull, overcast conditions and sometimes drizzle. When it reaches the ground it becomes fog. Stratus signals stable, moist air rather than dramatic change, and typifies a grey, uneventful day.
Essentially a cloud at ground level, formed when air cools to its dew point and its moisture condenses into tiny suspended droplets, reducing visibility. It commonly forms on clear, calm nights as the ground cools, or when warm moist air moves over a cooler surface such as a cold sea. It often burns off as the sun warms the ground.
A powerful tropical cyclone with sustained winds of at least 119 kilometres per hour, known as a typhoon in the western Pacific. It is a vast heat engine that draws energy from warm ocean water, with a calm central eye surrounded by a ring of violent weather. Its greatest dangers are often storm surge and flooding rather than wind alone.
A violently rotating column of air extending from a thunderstorm to the ground, capable of producing the most intense winds on Earth over a small area. Tornadoes form most readily where warm, moist air meets cold, dry air with strong wind shear. They are rated by the damage they cause on the Enhanced Fujita scale.
The one-to-five scale used to categorise hurricanes by their maximum sustained wind speed and potential wind damage. Because damage rises roughly with the cube of wind speed, each step up represents a large jump in destructive power. It measures wind only, and does not capture the storm surge and rainfall that are often a hurricane's deadliest hazards.
An abnormal rise of seawater above the normal tide, pushed ashore by the winds and low pressure of a storm such as a hurricane. It is frequently the deadliest hazard of a tropical cyclone, capable of inundating low-lying coastal areas with fast-moving, destructive water. Its severity depends on the storm's strength, size, speed and the shape of the coastline.
A climate pattern in which the central and eastern tropical Pacific Ocean becomes unusually warm, part of a larger cycle called the El Nino Southern Oscillation. It shifts weather patterns worldwide, tending to suppress Atlantic hurricane activity while bringing drought to some regions and flooding to others. Its opposite phase, with unusually cool Pacific water, is called La Nina.
A seasonal reversal of prevailing winds that brings a dramatic shift between wet and dry seasons, most famously in South Asia. The summer monsoon draws warm, moisture-laden air off the ocean onto the heated land, delivering the heavy rains on which billions of people depend for agriculture and water supply.
A forecasting technique that runs a weather model many times with slightly different starting conditions to produce a range of possible outcomes. If the runs agree, confidence is high; if they scatter, the forecast is uncertain. The percentages in a probability forecast, such as a 70 percent chance of rain, often come from how many ensemble members produced that outcome.
An instrument that measures atmospheric pressure. By watching not the absolute reading but its trend, a barometer becomes a genuine forecasting tool: a falling barometer warns of approaching unsettled or stormy weather, while a rising one heralds improving conditions. It must be calibrated for local altitude to read correctly.
An instrument that measures wind speed, most commonly using spinning cups whose rotation rate is proportional to the wind. For accurate readings it must be mounted high and clear of obstructions, since nearby buildings and trees create turbulence that corrupts the measurement. It is often paired with a wind vane that shows direction.
A method of reconstructing past weather by feeding historical observations into a modern weather model to produce a complete, consistent record of conditions across the globe. Reanalysis datasets such as ERA5 allow the climate of any location to be studied in detail, and underpin the historical averages and comparisons used across this site.
A line on a weather map connecting points of equal atmospheric pressure, much as contour lines connect points of equal height. The spacing of isobars reveals the strength of the wind: closely packed isobars mean a steep pressure gradient and strong winds, while widely spaced ones indicate light winds.
A scale describing the strength of the sun's ultraviolet radiation at the surface, running from low values to extreme. It guides sun protection: the higher the index, the faster unprotected skin can burn and the more important shade, clothing and sunscreen become. It peaks around midday and is stronger at high altitude and nearer the equator.
These definitions are just the start. Our weather guides and articles explain many of these concepts in much greater depth, from how forecasts are actually made to the science of feels-like temperature and the weather behind great wine. Explore the Learn hub to find them organised by topic.
An expanded reference covering everything from everyday words to specialist meteorology. Looking to learn actively rather than just look things up? Try the Weather Word of the Day or head to the Learn hub.
A change in the temperature of a parcel of air caused only by it expanding as it rises or compressing as it sinks, with no heat added or removed from outside.
The horizontal movement of air, moisture, or heat by the wind. When warm air is carried over a cold surface it can create advection fog.
The fraction of incoming sunlight that a surface reflects. Fresh snow has a very high albedo; dark oceans and forests have a low one, absorbing more heat.
A region of high atmospheric pressure around which air rotates, clockwise in the Northern Hemisphere. Anticyclones usually bring settled, calm, dry weather.
Shimmering coloured light in the night sky near the poles, produced when charged particles from the Sun collide with gases high in the atmosphere.
A severe snowstorm combining strong winds of at least 56 km/h with falling or blowing snow that reduces visibility to near zero for a sustained period.
A storm that intensifies explosively, its central pressure dropping at least 24 millibars in 24 hours, a process forecasters call bombogenesis.
The height above the ground of the lowest layer of cloud that covers more than half the sky. A low ceiling is a key concern in aviation.
A warm, dry wind that flows down the eastern slopes of the Rocky Mountains, capable of raising temperatures dramatically in a matter of hours.
The average pattern of weather in a place measured over decades, its typical temperatures, rainfall and seasons, as opposed to day-to-day weather.
The leading edge of a colder air mass pushing into warmer air. It often brings a sharp burst of rain or storms, then clearer, cooler, fresher conditions.
The process by which water vapour turns back into liquid water, forming clouds, dew, or the mist on a cold window. It is how clouds are born.
The rising of warm, less dense air and sinking of cooler air. Convection lifts moisture to form cumulus clouds and, when strong, thunderstorms.
The apparent deflection of moving air caused by the Earth's rotation, curving winds to the right in the Northern Hemisphere and left in the Southern.
A set of coloured rings seen immediately around the Sun or Moon, caused by light bending around tiny water droplets in thin cloud.
Water droplets that form on surfaces overnight when they cool below the dew point and moisture in the air condenses onto them.
A belt of calm, windless ocean near the equator, historically feared by sailing ships that could be becalmed there for days.
Weather radar that detects not just where precipitation is but how it is moving, allowing forecasters to spot rotation inside storms and issue tornado warnings.
A powerful downward rush of air from a thunderstorm that spreads out violently on hitting the ground, capable of causing wind damage mistaken for a tornado.
Precipitation of very fine, closely spaced droplets, smaller than raindrops, that seems to float down from low stratus cloud.
A prolonged period of abnormally low rainfall, long enough to cause a serious shortage of water for people, agriculture or the environment.
A periodic warming of the surface waters of the eastern Pacific that shifts weather patterns across much of the world, altering rainfall and temperatures.
One of two moments each year when day and night are almost equal in length everywhere, as the Sun crosses the celestial equator, around 20 March and 22 September.
The change of liquid water into vapour, driven by heat. It lifts moisture from oceans and land into the air, beginning the water cycle.
The calm, often cloudless centre of a tropical cyclone, surrounded by the eyewall where the storm's most violent winds and heaviest rain are found.
The boundary between two air masses of different temperature and humidity. The clash along a front is where much of our changeable weather is generated.
A deposit of ice crystals that forms on surfaces when their temperature falls below freezing and water vapour turns directly into ice.
A sustained wind of force 8 on the Beaufort scale, between about 62 and 74 km/h, strong enough to break twigs off trees and impede walking.
Soft, small pellets of ice that form when supercooled droplets freeze onto a snowflake. Also called soft hail or snow pellets, it crumbles easily.
The warming of the Earth caused by gases such as carbon dioxide and water vapour trapping heat that would otherwise escape to space.
A brief, sudden increase in wind speed above the average, typically lasting only seconds but sometimes far stronger than the prevailing wind.
Balls or lumps of ice that form inside strong thunderstorms, carried up and down through freezing layers until they grow heavy enough to fall.
A ring of light encircling the Sun or Moon, formed when their light is refracted by ice crystals in high cirrus cloud. Often a sign of coming rain.
A dry, dusty trade wind that blows from the Sahara across West Africa in winter, carrying fine dust that can dim the sun for days.
A slight reduction in visibility caused by fine dry particles, dust, smoke or pollution, suspended in the air, giving the sky a milky look.
An instrument that measures the humidity of the air, essential for weather stations, greenhouses, museums and comfort control.
A storm in which rain falls through a freezing layer and coats every surface in a heavy glaze of ice, bringing down trees and power lines.
An unusual situation where air gets warmer with height instead of cooler, trapping cold air, fog and pollution near the ground beneath a warm lid.
A periodic cooling of the eastern Pacific, the counterpart to El Niño, which likewise reshapes rainfall and temperature patterns around the globe.
A sudden, powerful discharge of electricity within a cloud or between cloud and ground, heating the air so violently that it explodes as thunder.
A sky patterned with rows of small white clouds resembling fish scales, formed by altocumulus or cirrocumulus and often heralding changing weather.
The layer of the atmosphere above the stratosphere, where temperatures fall sharply and most meteors burn up on entering the air.
The scientific study of the atmosphere and its phenomena, especially weather and climate, and the practice of forecasting from that science.
A small but intense downburst from a storm, less than four kilometres across, whose sudden winds are a serious hazard to aircraft near the ground.
Like fog but thinner, mist reduces visibility to no less than one kilometre. It forms from the same suspension of fine water droplets.
A strong, cold, dry northwesterly wind that funnels down the Rhône valley of southern France toward the Mediterranean, often for days at a time.
A thick, dark, featureless layer of cloud that blankets the sky and produces steady, prolonged rain or snow rather than showers.
A front formed when a faster cold front catches up with a warm front and lifts the warm air entirely off the ground, often near the end of a storm's life.
Relating to mountains. Orographic rain falls when air is forced to rise over high ground, cooling and releasing its moisture on the windward side.
A sky completely, or almost completely, covered by cloud, technically when more than about seven eighths of the sky is obscured.
A form of oxygen whose layer high in the stratosphere absorbs most of the Sun's harmful ultraviolet radiation, shielding life at the surface.
The pleasant, earthy smell that rises when rain falls on dry ground, produced by plant oils and a compound made by soil bacteria being released into the air.
The weight of the atmosphere pressing down on a surface, measured in millibars or hectopascals. Its changes drive winds and signal weather shifts.
An arc of spectral colours produced when sunlight enters raindrops and is refracted and reflected back, always appearing opposite the Sun.
An elongated area of high pressure on a weather map, generally associated with settled, fine weather, the opposite of a trough.
A rough, white deposit of ice that forms when supercooled fog droplets freeze on contact with cold surfaces such as branches and railings.
A hot, often dust-laden wind that blows north from the Sahara across the Mediterranean, arriving in southern Europe warm and sometimes muddy with rain.
A choking mixture of smoke or pollution and fog or haze, worst in cities under a temperature inversion that traps dirty air near the ground.
Precipitation of ice crystals that form directly from water vapour in cold clouds and clump into flakes, each with a unique six-sided structure.
One of two moments each year when the Sun reaches its highest or lowest point in the sky, giving the longest and shortest days, around 21 June and 21 December.
A sudden, sharp increase in wind speed lasting at least a minute, often accompanied by rain, hail or a line of thunderstorms called a squall line.
The calm, stable layer of the atmosphere above the troposphere, home to the ozone layer and the cruising altitude of airliners.
The change of ice directly into water vapour without first melting, the reason snow can slowly vanish even when temperatures stay below freezing.
A severe, long-lived thunderstorm built around a deep, persistently rotating updraught called a mesocyclone, and the type most likely to spawn tornadoes.
Relating to a broad, large-scale view of the weather at one moment, as shown on the pressure charts forecasters use to see the whole picture.
A violent windstorm, especially one accompanied by rain, hail or snow, a poetic and historic word for the fiercest of weather.
An instrument for measuring temperature, traditionally using the expansion of a liquid such as mercury or alcohol, now often electronic.
The sound produced by the explosive expansion of air heated instantly by a lightning bolt. Counting the gap to the flash estimates its distance.
Steady winds that blow toward the equator from the northeast and southeast, which powered the great sailing trade routes and still shape tropical weather.
The lowest layer of the atmosphere, reaching up to about 10 kilometres, in which nearly all weather and almost all the atmosphere's water are found.
An elongated area of low pressure, often bringing cloud, showers and unsettled weather, the counterpart of a ridge.
The name given to a hurricane-strength tropical cyclone in the northwest Pacific, threatening East and Southeast Asia.
A current of rising air within a cloud or storm. Strong updraughts build towering thunderclouds and can suspend hailstones until they grow large.
Streaks of precipitation that fall from a cloud but evaporate in dry air before reaching the ground, appearing as trailing wisps beneath it.
The leading edge of a mass of warmer air riding up over cooler air, usually bringing a long spell of gradually thickening cloud and steady rain.
A rotating column of air and water mist, essentially a tornado occurring over a body of water, most common over warm seas.
A balloon that carries an instrument package called a radiosonde high into the atmosphere, radioing back temperature, humidity and pressure as it climbs.
A blizzard condition in which falling and blowing snow erase all contrast and horizon, leaving a traveller unable to tell up from down.
An instrument, often shaped like an arrow or cockerel, that pivots to point into the wind and show the direction from which it is blowing.
A tapered fabric cone flown at airfields that shows both the direction of the wind and, by how far it lifts, a rough idea of its strength.
A soft, gentle breeze. The word comes from Zephyrus, the ancient Greek personification of the mild west wind.
A pattern in which the upper winds blow strongly from west to east with little north-south movement, tending to carry weather systems quickly along.