The chinook winds of the western portion of Canada and the United States occur principally to the eastward of high mountains, for the reason that the prevailing air-currents of that region are from the west.
_Thunder-Storms._--In the eastern portion of the United States and adjacent parts of Canada during the summer season the heating of the lower portion of the atmosphere, especially on still, sultry afternoons, causes ascending currents of warm, moist air, which become cooled as they rise, and give origin to vast masses of cumulus clouds. These magnificent "thunder-heads," as they are sometimes termed, illuminated by the full sunlight are most magnificent, and usually herald the coming of heavy showers, accompanied by frequently destructive lightning and heavy thunder. The bases of the clouds when seen from a distance are usually horizontal and may have curtain-like festoons beneath, due to falling rain; while aloft the white vapour boils upward in fleece-like masses, revealing a strong convectional ascent of moist air. The immediate cause of a thunder-storm is the rapid ascent of a column of warm moist air, which becomes cooled as it rises and the moisture contained in it condensed. The cause of the ascent of the air column, at least over plains and plateaus, is the heating of the air in contact with the earth. A layer of warm, and consequently light, air beneath a layer of cooler and heavier air furnishes unstable conditions which favour an overturning and an escape upward of the lighter air, which is forced to ascend much as the hot air in a chimney is made to flow upward by the pressure of cooler and heavier air around. The conditions preceding a thunder-storm are a stagnant atmosphere over a broad region where the lower layer of hot air is also charged with moisture. These conditions are frequently fulfilled on the plains of the Atlantic slope and southeastern portion of the continental basin in summer when warm moist air is drawn in from the Gulf region towards the centre of an area of low atmospheric pressure, and thunder-storms are there a characteristic feature. The storms usually advance northeastward, the direction being determined by the flow of upper air-currents, and move over the country with a breadth of from 10 to perhaps 100 miles, and send down copious supplies of refreshing rain.