The cierzo is a cold, dry, often fierce northwesterly wind that channels through the Ebro valley in northeastern Spain. It occurs when an anticyclone or ridge sits over the Bay of Biscay or northern Spain while a depression lies over the Mediterranean. The pressure gradient forces air through the funnel-shaped Ebro valley — narrowing between the Cantabrian Mountains to the north and the Iberian System to the south — creating a powerful Venturi effect that accelerates the wind.
The cierzo is the Ebro valley's most characteristic meteorological phenomenon, blowing on over 100 days per year in Zaragoza, with gusts frequently exceeding 100 km/h. The maximum recorded gust in Zaragoza was approximately 160 km/h. The wind is strongest in winter and spring when the pressure contrasts between the Atlantic and Mediterranean are greatest. Because it descends from the Cantabrian mountains, it arrives dry and cold — often dropping temperatures by 5–10 °C in a few hours.
The cierzo has profoundly shaped Ebro valley culture, architecture, and agriculture. Traditional buildings present windowless walls to the northwest. Wind farms in Aragón — among Spain's most productive — harness the cierzo's energy. The wine industry (D.O. Campo de Borja, D.O. Cariñena) has adapted to the wind, which dries grapes rapidly, reducing fungal disease but also stressing vines. The cierzo is closely related to other European channelled winds: the Mistral (Rhône valley, France), the Tramontana (Catalonia and southern France), and the Bora (Adriatic coast).