Glossary

Glossary: weather terms explained

All the terms that turn up in weather forecasts and in our guides, gathered in one place.

Wind

Wind terms are described in more detail in the wind guide, but here are the most common terms in brief.

m/s: Metres per second, the standard unit for wind speed in Swedish forecasts. 1 m/s equals 3.6 km/h.
Knot: A nautical unit of speed, 1 knot = 1 nautical mile per hour ≈ 0.514 m/s. Common at sea and in aviation.
Gust: Brief, stronger bursts of wind lasting a few seconds. Often 30–60 percent higher than the mean wind.
Mean wind: Wind speed measured or calculated as an average over a period, usually 10 minutes.
Beaufort: An older 13-step scale (0–12) describing wind strength based on its visible effects, from calm to hurricane.
Gale: A wind strength with a mean wind of at least 14 m/s (Beaufort 7–8), a threshold that often decides whether leisure boats should stay in harbour.
Storm: A wind strength with a mean wind of at least 25 m/s (Beaufort 10–11).
Hurricane: A wind strength with a mean wind of at least 33 m/s (Beaufort 12), the highest level on the Beaufort scale.

Significant wave height: The average height of the highest third of the waves over a period. Individual waves can be nearly twice as high. See the wave guide.
Wavelength: The distance between two successive wave crests. Together with the boat's length, this determines how the sea feels on board.
Swell: Long, even waves that propagate from weather far away, often without local wind on site.
Fetch: The open stretch of water the wind has been able to act over. Longer fetch, together with wind strength and duration, produces higher waves.
Sea state: A combined description of the waves' height, length and direction in a sea area at a given time.

Annonsplats

mm precipitation: Millimetres of precipitation correspond to litres of water per square metre, whether it falls as rain or snow. See the rain guide and the snow guide.
Precipitation intensity: How much precipitation falls per unit of time, usually mm per hour. Determines whether rain feels like drizzle or a downpour.
Precipitation probability: The probability that measurable precipitation falls at a given location during a stated period, not how large a share of the time or area gets wet.
Snow water equivalent: How much water a layer of snow corresponds to if melted, which determines the snow's weight regardless of how deep it looks. See the snow guide.

hPa / air pressure: Hectopascal, the unit of air pressure. Normal air pressure at sea level is close to 1013 hPa.
Front: The boundary zone between two air masses with different temperature and humidity, often associated with weather changes and precipitation.
High pressure: An area of elevated air pressure, usually associated with sinking air, clear weather and calm winds.
Low pressure: An area of reduced air pressure, usually associated with rising air, cloud, precipitation and stronger winds.
Dew point: The temperature the air must be cooled to in order to become saturated with water vapour and form condensation, such as fog or dew.
Relative humidity: How close the air is to saturation with water vapour at the current temperature, given as a percentage.
Wind chill: How much colder the body feels due to wind stripping away the warm, insulating layer of air at the skin. See the wind chill guide.
Apparent temperature: The felt temperature after accounting for factors such as humidity, wind and solar radiation. See the temperature guide.
Fog / mist: Condensed water vapour near the ground giving visibility below 1 km (fog) or 1–10 km (mist). See the fog guide.
Visibility: The longest distance at which a clearly distinguishable object can be seen, an important measure in fog, snowfall and heavy rain.
Cloud base: The height above ground at which cloud starts to form, crucial for, among other things, air traffic and visibility at altitude.

UV index: A scale for the strength of the sun's ultraviolet radiation, where higher values mean a faster risk of sunburn.
Convection: Vertical air movement that occurs when heated air near the ground rises, which can build thunderclouds and showers.
CAPE: Convective Available Potential Energy, a measure of how much energy is available for convection. High values increase the risk of severe thunderstorms.
Forecast model (ECMWF/GFS/HARMONIE): Computer programs that calculate the future state of the atmosphere. ECMWF (European) and GFS (American) are global models, HARMONIE is a high-resolution Nordic model.
Ensemble forecast: A set of several forecast runs with small variations in the starting conditions, used to estimate the uncertainty in a forecast. See the forecast guide.

Term	Category	Explained further in
Gale	Wind	/guider/vind and /guider/sjosakerhet
Significant wave height	Waves	/guider/vagor
Wind chill	Temperature	/guider/vindkyla and /guider/temperatur

Why does Sweden use both m/s and knots?: Metres per second is the standard in land-based forecasts, while knots are the international standard at sea and in aviation. Both therefore occur in parallel.
What's the difference between air pressure and wind?: Air pressure drives the wind: air flows from high pressure towards low pressure, and the more tightly packed the pressure difference is geographically, the harder it blows.
Is CAPE the same as thunderstorm risk?: Not quite. A high CAPE value means a lot of energy is available once convection gets going, but a triggering mechanism is also needed, such as a front or ground heating, for a thunderstorm to actually form.