Mid-latitude storms known as nortes (northers) disturb the normal weather patterns up to 20 times a year during winter, from November to March. They occur when northern polar air moves south into northern and central Mexico.
They bring low pressure (cyclonic) conditions, heralded by the arrival of a cold front. The polar air displaces the warmer surface air, forcing it to rise as the cool air pushes its way underneath. At the surface, a sudden drop in temperature and the advent of cold winds marks the passage of the front, followed by several days of overcast skies with light rains or drizzle, onomatopoeically called chipichipis in some areas.
Rains from nortes are heavier on the northern or eastern sides of mountains where the cool air is forced to rise. As the front passes, the temperature can drop by 5–8 degrees C (9–14 degrees F) in a few hours.
From an agricultural perspective these rains are a welcome sight for farmers, helping to improve grazing land and reduce the chances of wind-blown soil erosion. However, the winds can play havoc with shipping in the Gulf of Mexico and result in ports being temporarily closed. Veracruz and Tampico are regularly affected.