The reason for the peak period for tornado formation being in the spring has much to do with temperature patterns in the U.S. Tornadoes often form when cool, polar air traveling southeastward from the Rockies overrides warm, moist, unstable Gulf of Mexico air in the eastern states. Tornadoes therefore tend to be commonly found in front of a cold front, along with heavy rains, hail, and damaging winds. Since both warm and cold weather are common during the springtime, the conflict between these two air masses tends to be most common in the spring. As the weather warms across the country, the occurrence of tornadoes spreads northward. Tornadoes are also common in the summer and early fall because they can also be triggered by hurricanes, although the tornadoes caused by hurricanes are often much weaker and harder to spot. Winter is the least common time for tornadoes to occur, since hurricane activity is virtually non-existent at this time, and it is more difficult for warm, moist maritime tropical air to take over the frigid Arctic air from Canada, occurrences are found mostly in the Gulf states and Florida during winter (although there have been some notable exceptions). There is a second active tornado season of the year, late October to mid-November. Autumn, like spring, is a time of the year when warm weather alternates with cold weather frequently, especially in the Midwest, but the season is not as active as it is during the springtime and tornado frequencies are higher along the Atlantic Coastal plain as opposed to the Midwest. They usually appear in late summer.