A milky, yellow or reddish sky is not enough to diagnose Saharan dust haze. Its appearance depends on the type and amount of particles, their altitude, humidity, cloud cover and the position of the sun. A sound interpretation separates three questions: is dust present somewhere in the atmospheric column, what concentration is expected near the ground, and how much might visibility decrease?
This is an educational guide, not a current warning. For health, transport or outdoor decisions, use the official alerts and observations for your location.
What “calima” or dust haze means
The Spanish term calima describes reduced air transparency caused by suspended dry particles such as dust or smoke. Around the Iberian Peninsula and the Canary Islands it is often linked to Saharan mineral dust. However, not every hazy horizon is Saharan dust, and not every dust intrusion produces severe surface haze.
Strong winds can lift mineral particles from arid ground. Atmospheric circulation may then carry them hundreds or thousands of kilometres. Some dust travels in elevated layers, while some mixes down towards the surface. It eventually dilutes or settles through dry deposition; when it meets precipitation, wet deposition can leave muddy marks.
Dust haze, fog and high cloud are different
| Phenomenon | Main component | Useful clue |
|---|---|---|
| Dust haze | Suspended dry particles | Washed-out horizon, muted contrast and sometimes ochre or whitish tones |
| Fog | Water droplets or ice crystals near the surface | Very moist air and poor visibility inside the layer |
| High cloud | Droplets or crystals higher in the atmosphere | May veil the sun without strongly reducing horizontal visibility |
Humidity can change how aerosols scatter light, so visual identification is not always simple. Our guide to how fog forms and why visibility changes explains the role of saturation and water droplets.
Four products that answer different questions
1. Probability of mineral dust at the surface
AEMET’s Saharan dust intrusion maps estimate the probability that selected daily mean surface dust thresholds will be exceeded in Spain. They help assess whether dust may affect near-surface air. A probability is not an observed concentration, however, and it does not guarantee one uniform value throughout a coloured area.
2. PM10 concentration
PM10 covers particles with an aerodynamic diameter up to 10 micrometres. Mineral dust contributes to PM10, but so do other sources. A total PM10 map cannot by itself attribute every particle to the Sahara. Compare it with a dust-specific tracer, monitoring stations and information from the responsible authority.
3. Aerosol optical depth (AOD)
AOD indicates how strongly aerosols attenuate light through the full atmospheric column. A high value may come from an elevated dust layer that changes the colour of the sky without creating the same concentration at street level. AOD must therefore not be converted directly into surface PM10. The EU’s Copernicus Atmosphere Monitoring Service explains how desert dust is tracked with observations and models.
4. Meteorological visibility
Visibility depends on the quantity and size of particles and droplets, as well as humidity, fog, precipitation and local conditions. A dust forecast is not a substitute for a specific visibility observation or forecast used for roads, ports or aviation.
A reliable way to read a map sequence
- Check the valid time and time zone. Do not compare a midday forecast with a night-time observation.
- Identify the variable and unit. Probability, micrograms per cubic metre, AOD and kilometres of visibility are not interchangeable.
- Check altitude. A plume at 3,000 metres does not automatically mean intense dust at the surface.
- Follow the evolution. Agreement across several consecutive forecast times is more useful than one distant frame.
- Compare forecasts with observations. Satellites, air-quality stations and visibility sensors provide different, complementary evidence.
- Read local alerts. Operational advice comes from the competent authorities, not from the colour of one map.
Why can rain leave mud?
Raindrops crossing a dusty layer can capture particles and carry them to the ground. The water may look dirty or leave marks after drying. This demonstrates deposition, but it does not by itself reveal the surface concentration throughout the event. Dust can also occur without rain, and rain can fall without visible dust deposits.
Practical decisions
- For air quality, check the official regional index and monitoring stations as well as the forecast.
- If health authorities advise reducing outdoor exertion, follow that advice, especially if you are in a vulnerable group.
- For driving or marine activity, use actual visibility and transport warnings rather than estimating distance from sky colour.
- For photography, solar energy or cleaning, consider both airborne dust and the chance of dry or wet deposition.
Copernicus provides European forecasts for PM10, PM2.5 and dust tracers. The strongest interpretation combines those layers with the national weather service and air-quality network.
Key points
- Dust haze is reduced transparency caused by dry particles; it is not fog.
- Dust may remain aloft without producing the same concentration at the surface.
- AOD, PM10, exceedance probability and visibility are different variables.
- Model forecasts should be checked against observations and official alerts.