The Boy with the U. S. Weather Men Part 27

"But winds don't always blow the same way," said the farmer; "you talk as if they did."

"Some of them do," Anton replied. "There are lots of places where the winds hardly change, at all, but always blow in the same direction. You read of sailing ships taking the 'trade winds' when coming from Europe to America. Those are all easterly winds and blow towards the American coasts all the year round."

"I don't see how they can," the other objected.

"They do, Mr. Tighe," the Forecaster interrupted, endorsing Anton's statements; "the trade winds are the downflowing currents of cold air that Anton spoke of, which come down at either side of the equatorial belt to replace the warm air which is rising. The trade winds, however, form only a narrow belt and blow only near the surface of the earth.

Above them, you can see the lighter clouds blowing eastward with a westerly wind, so that, quite often, in the trade winds, you can look overhead and see two layers of clouds driving in opposite directions."

"You mean to say that there are different layers of wind?" queried the farmer.

"Sure," put in Ralph, the cloud expert, "I've got photographs that show that up clearly. You've seen clouds going at different rates, haven't you, Mr. Tighe, some fast and some slowly?"

The other nodded and turned to the Forecaster, who continued.

"There are always several layers of wind, and, except above the equatorial belt," he said, "the direction of the upper air winds is generally towards the east."

"How can you tell that?"

"By the clouds, or by kites and balloons. But we don't even need to do this, because there are a few places that rise above the lower layers of the trade winds. Thus, the Peak of Teneriffe, which is in the trade-wind belt, has a continuous easterly wind on its lower slopes and a continuous westerly wind right at the summit.

"This gives three belts of weather in the tropical and sub-tropical zones. The first of these is a light up-flowing east wind on or near the equator--it shifts a little to the north or south with the change of the seasons; a belt of heavy rains and calm, the rains being due to the warm, moist, uprising air cooling by expansion so that the moisture is condensed--this region is known to sailors as the 'doldrums' and many a sailing-vessel has been held for weeks there, without enough wind to carry her the few miles necessary to get into the next belt of winds; outside this, come the downflowing easterly currents, known as the trade winds, which form a belt between the tropics and the temperate zones.

Beyond this--to the north and south of the tropical zones--come the prevailing belts of strong west winds, which stretch almost to the Poles.

"The United States is in this west-wind zone and the strength and regularity of the eastward movement of the weather is because both the winds of the surface and of the upper air blow in the same direction.

Naturally, the same conditions are repeated on the other side of the equator. In the southern hemisphere the land ma.s.ses are not so large and the regularity of the winds is less disturbed. There, the west winds are so strong that certain lat.i.tudes are known as the 'roaring forties.'

These 'forties' correspond in lat.i.tude to the northern third of the United States. Chicago and New York are both in the 'roaring forties' of the northern hemisphere."

"The way you tell it, it sounds all right," the farmer objected, "but from my experience, winds blow from all over the place."

"Locally, perhaps, they seem to," the weather expert responded, "but if you watched them closely, you'd find that about seventy per cent of the winds come from a westerly direction."

"They do here, for a fact," put in Tom, who, as official wind-measurer of the League, had been following the explanation with the keenest attention. "I've noticed that in my kite-flying. The winds are from the southwest or from the northwest nearly all the time."

"You mean both in summer and winter?"

"Yes," answered Tom, "they're more from the northwest in winter, I think, but they're generally westerly."

"If the winds are due to the position of the equator and the poles," the old farmer said shrewdly, "I don't see why summer and winter ought to make any difference."

"That," said the Forecaster, "is due to an entirely different set of conditions. It's due to the difference in radiation. There's much greater change in temperature over the land than over the sea. Take an island like Bermuda, for example. From the hottest day in summer to the coldest day in winter there isn't a change of more than forty degrees, because Bermuda is surrounded by water and is near warm ocean currents.

In Arizona, on the other hand, there's a change of as much as fifty degrees of temperature in a single day. That is because land absorbs heat quickly and lets it go equally quickly. The interior of a continent in summer time heats and expands the air in the same way that the air is heated over the equator, and, in the same manner, sets in motion another system of winds, for cold air comes rushing down from all sides and forces up the rising warm air.

"Take Asia, for example, where the continental ma.s.s is large and the plateaus high. The interior becomes so hot that the air is sent up like the draught in a big chimney, and cool winds from the sea blow toward the interior from all sides in the summer time, and away from it, to all sides, in the winter time. That's what causes the famous Indian monsoons, which blow steadily to the north-east for the six months of summer and just as steadily to the south-west for the six months of winter. The native boats, there, are built on purpose for the monsoon, so that they can only sail with a fair wind and they make one round trip a year, going south with the monsoon in winter and returning with the summer monsoon."

The old farmer scratched his head.

"There's more to this than I thought," he said; "I always supposed that winds just happened."

"No, indeed," the Forecaster answered, "every place in the world has its own system of winds, though in some parts there are so many variations that it isn't always easy to distinguish between the regular and the irregular currents. In the United States the surface winds are very irregular, for we live in one of the stormiest regions of the entire world. Still, that doesn't alter the general rule that all our weather comes from the west."

"And yet," said the farmer, in a puzzled manner, "I don't see why it comes from the west."

"I think I can explain it to you," the weather expert replied. "You know that when water is running down a hole at the bottom of a basin, if it is in motion it doesn't go down straight but with a circular movement, finally making a whirlpool?"

"Of course," the farmer said.

"So does air," the Forecaster rejoined. "There is something the same sort of a whirl at the poles. The prevailing westerly winds of the United States are due to this circ.u.mpolar whirl, though modified and altered by the changes of the seasons, the differences of heat between day and night, the radiation from the land, the irregularity of the coastline, the currents of the ocean and a thousand other factors. Each of these the Weather Man has to study when he makes a forecast, but, in the United States, his work is aided by the fact that weather always travels eastward and that the storm follows regular tracks, sharply outlined, like Indian trails across the country."

"Roads in the air?" queried Fred.

"Yes, my boy," the Forecaster answered, "regular roads in the air. There used to be an old saying: 'American weather is made at Medicine Hat.' In a sense this was true, for about sixty per cent of the storm areas--'lows' or region of low barometric pressure--come from the Canadian Northwest. The St. Lawrence Valley is the outlet for our storms. You know the saying about the St. Lawrence, don't you?"

"No, tell us, Mr. Levin," begged Fred, always eager for some weather saying which he could put into the _Review_.

"Up there," the Forecaster rejoined, "they say that when a stranger complains about the weather, a native will reply, 'Don't mind this, we'll have another sample along in about five minutes.' And, sure enough, they do. The St. Lawrence Valley is a magnet for weather changes and has, perhaps, more storms than any other valley in the world."

"You spoke of the 'roads in the air,' sir," put in Ross, "how many are there?"

"Five regular trails," the Forecaster answered. "The northernmost one begins at the Canadian Northwest, runs along the International Boundary, crosses the Lake region and disappears up the St. Lawrence Valley. The second starts at the same point in the Canadian Northwest, travels southeast to the lower Mississippi Valley--a little north of where we are now, boys--curves up to the Ohio Valley and also escapes by the St.

Lawrence route.

"A third storm track strikes into the Pacific Coast a little north of San Francisco and runs east and a little south until it joins the Ohio Valley and St. Lawrence track. A fourth develops in the southwestern states and runs along Texas and the gulf states to the Florida coast, where it curves northward along the Atlantic coast, though a few storms take a sharp turn in the Mississippi Valley and go Ohiowards. The fifth storm track is that of the West Indian hurricanes, which whirl around the West Indies and enter the United States south of Cape Hatteras or from the Gulf of Mexico and pa.s.s north or northeastward. A few of these hurricanes--like the famous Galveston type--sweep westwards a long way before the northward movement sets in. This type also goes to the St.

Lawrence Valley.

"These five tracks are clearly marked, but as such areas are a thousand miles across, it follows that the country for five hundred miles on either side of the lines has its weather governed by them. Knowing these tracks is of great importance in forecasting weather, because, while you cannot always tell exactly what a storm is going to do, you definitely know some of the things that it will never do."

"What sort of things, sir?" asked Fred.

"Well, my boy," the Forecaster answered, "if there's an area of low pressure in Dakota, we know that it won't strike California; if there's one in New York, we know that Maryland is safe. A storm will never go down the Mississippi, nor up the St. Lawrence, but will always travel up the Mississippi and down the St. Lawrence."

"There does seem to be something regular about it," the farmer remarked, his interest growing, as the Forecaster took his pencil and sketched out, across the map of the United States, the five great storm tracks.

"That's all right for storms, maybe. But how about a cold wave? Fred, here, said that a cold wave was coming. Can you figure that out in the same way?"

"Certainly," the weather expert answered. "As a matter of fact, it is comparatively easy. A cold wave is simply a fall of temperature caused by the cold air from the upper atmosphere sweeping downwards after a cyclone of low pressure has pa.s.sed."

"A cyclone?" e.j.a.c.u.l.a.t.ed Ross, in surprise. "Is there always a cyclone before a cold wave?"

"Always," the Forecaster answered, "but, unless I'm mistaken, Ross, you're using the word 'cyclone' in the wrong sense. Most people do. I suppose you think a cyclone is some kind of a whirlwind, a particularly violent storm, eh?"

"Yes, sir," said Ross, "that's what I thought."

"Well, Anton can tell you better than that," the weather expert rejoined. "Tell him what a cyclone is, Anton."

"So far as I can make out," the crippled lad answered, "a cyclone is a whirl in the air, generally from five hundred to a thousand miles across, in the middle of which the barometer is very low, and on the edge of which the barometer rises. It always has winds that blow spirally inwards, those in the United States whirling in a direction opposite to the movement of the hands of a clock.

"So you see, Ross, to the east of a 'low' or ahead of it, the winds are southeasterly, to the north they are northeasterly, to the west, or behind it, they are northwesterly, and to the south, they are southeasterly, all curving into the centre and shifting as the 'low'

advances. As these 'lows' travel along the storm track at an average rate of four hundred miles a day, as mountains interfere, and as the shape of a 'low' in America isn't quite round, but looks like a sort of crooked oval, it takes close figuring to find out what the wind is going to do."

"And where does the cold wave come in?" persisted the farmer.