Volcanic ash affects flights to Fez

On one of the busiest weekends this year in Fez, guesthouses are reporting cancellations as flights from northern Europe are grounded.

Eyjafjallajokull eruption, 27 March

Lonely Planet reports that airports in the UK and on mainland Europe are closed due to concerns over volcanic ash drifting from Iceland's Eyjafjallajokull glacier. London, Paris, Frankfurt and Amsterdam are all affected.

Volcanic ash is a major hazard to aircraft. The presence and location of the plume depend upon the state of the eruption and the winds.

The estimated ash cloud over Europe at 18:00 GMT on 15 April (UK Met Office)

There has been extensive air travel disruption following the second eruption. The eruption occurred beneath glacial ice and the cold water from melting ice chilled the lava quickly causing it to fragment into glass, creating small glass particles that get carried into the eruption plume. This, together with the magnitude of the eruption, created a glass-rich plume in the upper atmosphere, very hazardous to aircraft.

So what does a pilot do when up against a volcanic cloud? The first trick is to know it's there, which is tougher than it might seem. Volcanic plumes generally are not picked up on weather radar, so pilots have to rely on other tools to spot them, like a color-coded tracking system that assigns every known volcano a colour based on its level of activity. Green is normal, yellow raises the threat, and red signifies an imminent eruption. From there, pilots check the wind direction to gauge where the plume will blow and make a call—either over or around—based on their altitude. "You definitely want a good 10,000 feet between you and the cloud, and even then you'd want to be cautious," says veteran international airline pilot, Robert Schapiro. Planes generally fly at about 35,000 feet, so small eruptions don't pose a significant problem. Major eruptions, which can reach up to 60,000 feet, are more troublesome. The ash cloud from the Iceland eruption is hovering between 20,000 and 30,000 feet.