Over the last two days, much of Spain has been coated with dust (灰尘) , and the skies have turned bright orange. A huge cloud of dust from the Sahara Desert is causing the strange weather. The dust cloud is expected to spread to other parts of Europe. 

Across much of Spain on Tuesday morning, a thin layer (层) of red dust coated everything outside. From streets to sidewalks to balconies, everything was covered with a layer of dust. As the sun rose, the dust in the air turned the skies in some cities a strange orange. The cause of the dust and unusual sky colour wasn't a forest fire, but dust from the Sahara Desert. 

Every year, a large cloud of dust rises from the Sahara Desert. It's called the Saharan AirLayer. Usually, it crosses the Atlantic Ocean as part of a natural cycle that helps develop farmland and beaches in Central and South America. 

But sometimes, depending on the weather, the layer of dust gets forced to the north. Large sandstorms in northern Africa lift sand and dust into the air, where it gets blown towards Spain. Though the particles (微粒) in the air come from sand, they're much more like dust than sand at a beach. 

These kinds of storms aren't unknown in Spain. In fact, it has a special name. It's called a "calima". This calima is so large that it can be seen from space. A NASA scientist who studies weather in the atmosphere says it's like a river of dust in the air. That river of dust has met with a river of wet air driven by strong winds.

Spain's weather service described the dust storm as being "very intense (强烈) ". At one point on Tuesday, the air quality in Madrid was the worst in the world. Government health experts warned people not to go outside without a mask, and to keep their windows and doors closed at home.