Weather Glossary

Air quality

Aerosol optical depth

Aerosol optical depth, or AOD, describes how particles in a column of air weaken light. It is not a direct measurement of particulate pollution at ground level.

Models & data

Air mass

An air mass is a large body of air with broadly similar temperature and moisture. Its origin explains much about the weather, while its journey changes it further.

Air pressure

Air pressure (mean sea level, QNH)

Pressure adjusted to sea level makes weather stations at different elevations comparable. QNH is a related aviation value calculated using different assumptions.

Air quality

Air quality index

An air quality index turns pollutant concentrations into an understandable rating. The meaning of a number or colour depends on the particular index.

Temperature & humidity

Air temperature

Air temperature describes how warm or cold the air is at the measuring site. It differs from ground-surface temperature and from a feels-like temperature.

Air quality

Ammonia

Ammonia, NH₃, is a nitrogen-containing gas. Agriculture is its main outdoor source, and it contributes to the formation of particulate matter.

Temperature & humidity

Apparent temperature

Apparent temperature is a calculated comparison of how warm or cold weather may feel to people. It considers more than the measured air temperature.

Astronomy

Astronomical twilight

Astronomical twilight occurs with the sun between 12° and 18° below the horizon. Astronomical night begins beyond that point, provided the sun sinks far enough.

Wind

Beaufort scale

The Beaufort scale describes wind strength in 13 forces, from 0 for calm to 12 for hurricane force. Each force corresponds to a range of wind speeds.

Wind

Boundary layer height

Boundary-layer height describes how far the ground strongly influences the air above it through heating, cooling and friction.

Thunderstorms

CAPE

CAPE describes energy potentially available to a rising air parcel through buoyancy. High values can support strong thunderstorm updraughts, but are not a probability of thunderstorms.

Air quality

Carbon monoxide

Carbon monoxide, CO, is a poisonous, colourless and odourless gas produced by incomplete combustion. Outdoor weather-page values cannot establish whether an indoor space is safe from CO.

Thunderstorms

CIN

CIN describes the energy needed to overcome a restraining layer of air. This inhibition can prevent thunderstorms even when buoyant energy would be available higher up.

Astronomy

Civil twilight

Civil twilight is the first twilight phase after sunset and the last before sunrise. Its darker boundary occurs when the sun’s centre is 6° below the horizon.

Models & data

Climate normal period

A climate normal period is a defined span of 30 years. Its averages provide a common reference for describing weather and climate.

Clouds & visibility

Cloud base

Cloud base is the height of a cloud’s visible underside. Always check whether the height is measured above the ground or above sea level.

Clouds & visibility

Cloud cover

Cloud cover is the fraction of the sky covered by cloud. It describes area, not cloud thickness or the probability of rain.

Radiation & sun

Daylight duration

Daylight duration is the time between sunrise and sunset. Clouds can make a day gloomy, but they cannot shorten its astronomical length.

Temperature & humidity

Dew point

The dew point is the temperature to which air would need to cool to become saturated with water vapour. It helps explain moisture, misted windows and fog.

Radiation & sun

Direct and diffuse radiation

Direct radiation arrives from the solar disc. Diffuse radiation has been scattered in the atmosphere and reaches us from many parts of the sky.

Models & data

Ensemble forecast

An ensemble is a collection of slightly different weather calculations. Their agreement and spread show plausible developments and help describe forecast uncertainty.

Soil & evaporation

Evapotranspiration

Evapotranspiration is the combined water loss through evaporation and release by plants. It is an important part of a soil or garden water balance.

Radiation & sun

Extraterrestrial solar radiation

Extraterrestrial radiation is solar radiation at the top of Earth’s atmosphere. It describes the available sunlight before clouds and air modify it.

Wind

Foehn

Foehn is a wind on the lee side of a mountain range. Descending air can warm while its relative humidity decreases.

Clouds & visibility

Fog

Fog is a cloud at ground level: tiny suspended droplets, or less often ice crystals, reduce visibility below one kilometre.

Precipitation & snow

Freezing level

The freezing level is the height where air temperature reaches 0 °C. It helps describe the atmosphere, but does not determine snowfall or ground frost on its own.

Temperature & humidity

Frost day

A frost day is a day when air temperature falls below 0 °C at least once. The afternoon can still become considerably warmer.

Air pressure

Geopotential height

Geopotential height describes height while accounting for gravity. On weather charts it usually shows how high a chosen pressure surface lies.

Radiation & sun

Global radiation

Global horizontal irradiance is the total shortwave solar radiation reaching a horizontal surface at ground level. It includes direct sunlight and scattered light from the sky.

Radiation & sun

GTI (global tilted irradiance)

GTI describes solar radiation received by a tilted surface such as a solar panel. The surface’s angle and orientation must therefore accompany the value.

Temperature & humidity

Heat index

The heat index combines air temperature and relative humidity into a feels-like value for warm to hot weather. It explains why humid heat can be especially demanding.

Air pressure

High-pressure system

A high-pressure system has higher air pressure than its surroundings. It often brings settled weather, but not necessarily sunshine.

Temperature & humidity

Hot day

In German weather statistics, a hot day reaches at least 30 °C air temperature. The threshold applies to a shielded measurement, not a thermometer in direct sunshine.

Temperature & humidity

Ice day

On an ice day, air temperature remains below 0 °C throughout the day. The daily maximum defines it, not the presence of ice.

Models & data

ICON

ICON is a family of weather and climate models used by the German Weather Service and others. Global and regional versions calculate different areas and levels of detail.

Thunderstorms

Lifted index

The lifted index compares a theoretically lifted air parcel with its surroundings around five to six kilometres high. A negative value indicates potential buoyancy.

Thunderstorms

Lightning

Lightning is a powerful electrical discharge in the atmosphere. Its hazards are real even though individual accidents are rare, and comparisons with lotteries or sharks can be misleading.

Thunderstorms

Lightning potential

Lightning potential is a calculated indication of how favourable cloud conditions are for electrical discharges. It is not a measurement of lightning that has already occurred.

Air pressure

Low-pressure system

A low-pressure system has lower air pressure than its surroundings. Rising air often favours cloud and rain, but a low is not automatically a windstorm.

Air quality

Mineral dust

Mineral dust consists of small particles of soil, sand, rock or mineral building materials. It may be lifted locally or transported over long distances.

Models & data

Model run

A model run is one new calculation by a weather model. It starts with an analysis of current conditions and calculates how the atmosphere may develop.

Astronomy

Moon phase

The Moon’s phase shows how much of its sunlit half we can see from Earth. It results from the positions of the Sun, Earth and Moon, not Earth’s shadow.

Astronomy

Nautical twilight

During nautical twilight, the sun’s centre lies between 6° and 12° below the horizon. Many stars are visible while, in good conditions, the horizon can still be distinguished.

Air quality

Nitrogen dioxide

Nitrogen dioxide, NO₂, is an air pollutant mainly associated with combustion. Concentrations can be substantially higher near traffic and other emission sources.

Models & data

Nowcast

A nowcast predicts the next few minutes to a few hours. It relies especially closely on recent observations.

Air quality

Ozone

Ozone is a gas made of three oxygen atoms. High in the atmosphere it protects against UV radiation; near the ground it can harm health.

Air quality

Particulate matter

Particulate matter consists of small particles that remain suspended in air. PM10 and PM2.5 describe size groups; the finer PM2.5 fraction is included within PM10.

Air quality

Pollen count

Airborne pollen is plant pollen carried through the air. For people with allergies, the species, concentration and individual sensitivity all matter.

Precipitation & snow

Precipitation amount

Precipitation amount describes how much water falls on an area. One millimetre of precipitation equals exactly one litre per square metre.

Precipitation & snow

Precipitation probability

Probability of precipitation is the chance of precipitation at a particular place during a particular period. It does not describe how long or how heavily it will rain.

Air pressure

Pressure levels

A pressure surface connects points with the same air pressure. Weather charts use these levels to compare temperature, wind and moisture through the atmosphere.

Air pressure

Pressure tendency

Pressure tendency is the amount air pressure rises or falls over a specified period. It describes the change, not the pressure itself.

Models & data

Radar reflectivity

Radar reflectivity describes how strongly particles in the atmosphere scatter a radar signal back. It helps detect precipitation but is not yet a direct rainfall measurement.

Temperature & humidity

Relative humidity

Relative humidity is the percentage that describes how close air is to saturation with water vapour at its current temperature. Heating or cooling alone can change the value.

Astronomy

Seeing

Seeing describes astronomical image disturbance caused by Earth’s atmosphere. Good seeing makes stars and planetary detail appear steadier and sharper; it is not the same as clear visibility.

Precipitation & snow

Shower

A shower is precipitation that often starts suddenly, changes intensity quickly and affects a limited area. It may bring rain, snow, graupel or hail.

Precipitation & snow

Snow depth

Snow depth measures the thickness of snow already on the ground. It is different from newly fallen snow and from the snow’s water content.

Precipitation & snow

Snowfall

Snowfall occurs when ice crystals or flakes formed from them reach the ground from clouds. The air at ground level does not always need to be below 0 °C.

Precipitation & snow

Snowfall level

The snowfall level is approximately the height above which precipitation falls mainly as snow. It is a transition zone that can move while precipitation is falling.

Soil & evaporation

Soil moisture

Soil moisture describes water held in the ground. Soil type, depth and the stated unit matter as much as the number itself.

Soil & evaporation

Soil temperature

Soil temperature is the temperature at a specified depth in the ground. Depth belongs with the value because the surface and deeper soil warm at different rates.

Air quality

Sulphur dioxide

Sulphur dioxide, SO₂, is an air pollutant from sulphur-containing fuels and other sources. It can affect the airways and contribute to particulate matter.

Temperature & humidity

Summer day

In German weather statistics, a summer day has a maximum air temperature of at least 25 °C. The calendar month does not determine the classification.

Radiation & sun

Sunshine duration

Sunshine duration counts time with sufficiently strong direct sunlight. Daylight alone does not count: a day with twelve hours of daylight can still have no sunshine hours.

Air pressure

Surface pressure (QFE)

Surface pressure is air pressure at the actual location and elevation. In aviation, QFE refers to pressure at a specified airfield reference level.

Temperature & humidity

Temperature inversion

In a temperature inversion, air becomes warmer with height through a layer instead of colder. This stable arrangement can restrict vertical mixing.

Thunderstorms

Thunderstorm

A thunderstorm is a weather event with lightning and thunder. Heavy rain, gusts or hail often accompany it, but not every storm produces all of them.

Temperature & humidity

Total column water vapour

Precipitable water expresses the water vapour in a column of air as an equivalent depth of liquid water. It describes the moisture supply, not the expected rainfall total.

Astronomy

Transparency

Astronomical transparency describes how readily light passes through the atmosphere. It helps determine whether faint stars, nebulae and galaxies can be seen.

Temperature & humidity

Urban climate

Urban climate describes the typical climatic features of built-up areas. Buildings, paved surfaces, green spaces and human waste heat alter temperature, wind and radiation.

Radiation & sun

UV index

The UV index describes the strength of sunburn-producing ultraviolet radiation at ground level. Higher values mean unprotected skin can be damaged more quickly.

Temperature & humidity

Vapour pressure deficit

Vapour pressure deficit, or VPD, describes the gap between current water vapour and saturation. It helps indicate how strongly the air drives evaporation.

Clouds & visibility

Visibility

Visibility describes how far away objects can still be distinguished through the air. Fog, precipitation and suspended particles can reduce it sharply.

Air pressure

Weather front

A weather front is a transition zone between air masses with different temperatures or moisture. Wind, cloud and precipitation often change around it.

Models & data

Weather model

A weather model calculates how the atmosphere develops using physical equations. It divides Earth and the air above it into a three-dimensional computational grid.

Temperature & humidity

Wet-bulb temperature

Wet-bulb temperature shows how far evaporation can cool under the surrounding air conditions. It is especially useful for understanding the effect of humidity.

Temperature & humidity

Wind chill

Wind chill describes the extra heat lost from exposed skin in cold wind. The comparison temperature can be much lower than the measured air temperature.

Wind

Wind direction

Wind direction tells you where the air comes from. A westerly wind arrives from the west and blows towards the east.

Wind

Wind gusts

A gust is a brief increase in wind speed. This peak can be much stronger than the wind averaged over several minutes.

Wind

Wind speed

Wind speed tells you how fast air is moving. To interpret it properly, also check the measurement height, averaging period and gust speed.