Nicola Kennedy, age 11, of Saint Catharine's, Ontario, Canada, for her question:

What causes winds to blow from the north?

Most of North America lies within the belt of the prevailing westerlies. These global winds circle the globe, wafting windy pockets of gales or breezes. This is why most of our spells of balmy and stormy weather move over our heads from the west. But sometimes, especially in winter, a blustery blizzard pfoves down on us from almost due north. Its heavy air has been sitting on the cold polar ice, piling up in dense masses. Dense, heavy air of this kind tends to spread out and mingle with lighter masses of surrounding air. The icy wind begins to spread south from the polar region.

Sometimes the cold polar air mass builds up a mighty force. A smaller mass tends to veer to the west with the prevailing polar winds. As it moves southward, it turns eastward with the westerlies. But sometimes a wintry mass of polar air is dense and heavy enough to force its way due south across the prevailing wind belts. Then we get an Arctic blizzard with winds from the frozen north.