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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.