A CLOUDBURST NEAR CEDAR BRAKES, UTAH
A snapshot taken from the edge of the cañon.
Over the large land areas of the north temperate zone highs and lows show a tendency to travel over typical tracks, the locations of which vary a good deal with the season. One of the most remarkable facts about the lows of North America is that, wherever they come from, whether from the Canadian northwest, the western United States, or the West Indies, they nearly always leave the continent by way of the Gulf of St. Lawrence or the northeastern corner of this country. Our North American lows travel at an average speed of 600 miles a day. Highs travel somewhat more slowly; about 540 miles a day is the average in this country.
A tornado is a small vortex in the atmosphere, occurring generally in the southeastern part of a cyclonic area, where, in some cases, several separate tornadoes develop at the same time. The tornado, for some reason that is not altogether clear, is far more common in the interior of North America, east of the Rocky Mountains, than anywhere else in the world, though true tornadoes do occur in other countries. The West African storms bearing this name are merely thundersqualls, quite different from the American tornado. The chief visible feature of a tornado is the so-called funnel-shaped cloud (sometimes balloon-shaped or, again, like a great coiling serpent), which is always in contact with the ground when destruction is in progress. The passage of the storm is attended by a loud roaring or rumbling. The path of a tornado varies in width from a few rods to half a mile or (rarely) more. Within its borders buildings are blown to pieces, trees are uprooted and human beings only find safety underground; while even at a distance of a few yards outside the path no damage is done. The tornado travels at an average speed of about twenty-five miles an hour. Its speed of rotation has been estimated, from the effects produced, to amount to 500 miles an hour in some cases; a wind force far exceeding that of any other type of storm.
Waterspouts, which occur on the ocean and other large bodies of water, are similar in character to tornadoes, though much less violent. They range in height from 100 to 1,000 yards, or more. One measured recently from the British steamer War Hermit, near Cape Comorin, was 4,600 feet high to the base of the overlying cloud. The column tapered from 500 feet wide at the junction with the cloud to 150 feet wide at the sea. Spray was thrown up to a height of more than 800 feet over a region 250 feet in diameter.
Thunderstorms occur chiefly in warm climates and during the warm season in temperate climates, but they are by no means unknown in the polar regions. They are characterized by rapidly rising air currents, which may be either incidental to the circulation of a low, or due to local overheating of the lower atmosphere. In the former case they are called “cyclonic thunderstorms,” and in the latter “heat thunderstorms.” This is only a rough classification, however. Some thunderstorms partake of the features of both these types, and, on the other hand, additional classes are distinguished by many authorities. It is a common occurrence for thunderstorm conditions, starting in some small area, to travel across country at a speed of perhaps thirty or forty miles an hour, at the same time spreading out fanwise until the front of the storm is hundreds of miles in length. This front constitutes a “line squall” (so called from the long line or apparent arch of dark cloud that marks its location), and is attended by more or less thunder and lightning, but is not necessarily a continuous thunderstorm. The characteristic wind of a thunderstorm is the squall that rushes out in front of the storm when close at hand. This blast of wind, lightning, hail and torrential rain are all agencies of destruction in severe thunderstorms.
IDEAL CROSS SECTION OF A TYPICAL THUNDERSTORM
A, ascending air; D, descending air; C, storm collar; D’, wind gust; H, hail; T, thunderheads; R, primary rain; R’, secondary rain. (W. J. Humphreys.)
Concerning the winds of the globe in general and the remarkable atmospheric interchanges that they involve, Sir Napier Shaw writes: