A sea of clouds

 Foggy downing Mount Ezkanda (Irati-Aezkoa)

Eastern Irati,  from Mount Mendilatz (Aezkoa Valley, Navarre)

There are no goats in Irati.  This is Belagoa valley , from the stone of St. Martin.(French border)

In summer is quite common to look the fog falling down when north wind (cold air) is blowing , and therefore heavier, spreads down the slopes to the south after the northern valleys of the Pyrenees have been filled fog.

Fog is not alwais dangerous. Can be beautifull-

The advance of fog to the south  isdepending of  the heat  that its find this on the southern slope,with hot air going up that tends to slow it when it is falling down.
In these photos, in the end of the day, hot air loses strength and when it cooled, begins to accumulate in the bottom of the valleys, as I have told, by the weight of the cold air.This is called termical inversion because in  normal circumstances, at higher altitudes, colder.
In the next day, when the sun rises and warms the air, the fog will tend to rise and disappear.
There is a great saying I heard that my great-grandmother, who says: "Fog in the morning , afternoon for walking"

Valle de Belagoa, valley with Larra on the left side.

Fog can be very dangerous if you do not know the terrain, and even if we know it. I know stories of shepherds who have spent the night in the open pasture lost by those who walk every day.
It is highly recommended to always carry a map of the area and a compass. If you carry GPS also much better, but do not confide everything to technology, because if this fails, you will see a serious problem.
If you like the exciting world of meteorology, I highly recommend this book. Besides good information, it's funny and readable:
:GUIA DEL OBSERVADOR DE NUBES.
Autor: GAVIN PRETOR-PINNEY
Ed: SALAMANDRA
http://cloudappreciationsociety.org/