These occur on the continents, especially
where they afford broad plains little interrupted by mountain ranges.
They are particularly well exhibited in that part of North America
north of Mexico and south of Hudson Bay. Like the hurricanes, they
appear to be due to the inrush of relatively warm air entering an
updraught which had been formed in the overlying, cooler portions of
the atmosphere. They are, however, much less energetic, and often of
greater size than the hurricane whirl. The lack of energy is probably
due to the comparative dryness of the air. The greater width of the
ascending column may perhaps be accounted for by the fact that,
originating at a considerable height above the sea, they have a less
thickness of air to break through, and so the upward setting column is
readily made broad.

The cyclones of North America appear generally to originate in the
region of the Rocky Mountains, though it is probable that in some
instances, perhaps in many, the upward set of the air which begins the
storm originates in the ocean along the Pacific coast. They gather
energy as they descend the great sloping plain leading eastward from
the Rocky Mountains to the central portion of the great continental
valley. Thence they move on across the country to the Atlantic coast.
Not infrequently they continue on over the ocean to the European
continent. The eastward passage of the storm centre is due to the
prevailing eastward movement of the air in its upper part throughout
that portion of the northern hemisphere. Commonly they incline
somewhat to the northward of east in their journey. In all cases the
winds appear to blow spirally into the common storm centre. There is
the same doldrum area or calm field in the centre of the storm that we
note between the trade winds and in the middle of a hurricane disk,
though this area is less defined than in the other instances, and the
forward motion of the storm at a considerable speed is in most cases
characteristic of the disturbance. On the front of one of these storms
in North America the winds commonly begin in the northeast, thence
they veer by the east to the southwest. At this stage in the movement
the storm centre has passed by, the rainfall commonly ceases, and
cold, dry winds setting to the northwestward set in. This is caused by
the fact that the ascending air, having attained a height above the
earth, settles down behind the storm, forming an anticyclone or mass
of dry ai