The blessings bestowed upon our United States of America are almost numberless, and now, more than ever before, reports from the world beyond the seas serve to emphasize our good fortunes. At this time particularly, it is fitting that we pause to think about them. But where should we begin? Is it possible to single out from among our many national blessings that one which should receive highest honor? Should we be most thankful that our people from Maine can understand those in California or that there are no customs or monetary barriers? Should we give first praise to our inheritance of the democratic ideal, or that we are a great nation because of riches within our borders?
Possession of economic wealth has divided the world into two groups—the “haves” and the “have nots.” We are distinctly a “have” nation and as national wealth is one fundamental requirement for national greatness, we must salute that part of our country wherein lies the bulk of our abundant riches—the Mississippi Valley and the basins of its tributaries.
The chief physical characteristics of the United States are the two meridional mountain ranges and the broad, rich, well- drained area between them. So fertile was this region that before the white man invaded the North American Continent primitive men could have trekked from the Appalachians to the Western Plain States and at all times could have remained in the shade of the vast deciduous forests which covered that area. Of course these peoples had no knowledge of the tremendous reserves of coal and oil that were stored below the roots of this vast stand of hardwood or that the white man would someday develop this region into the world’s greatest bread basket out of which a mighty nation would feed not only itself, but many other parts of the world.
In order to grow the vegetation that stored the oil and coal, and later grew the vast forests, nature required a temperate climate and abundant rainfall well distributed throughout the growing season. What geographical arrangement provided this and what meteorological controls insures that the great Mississippi Valley will continue to be a land of milk and honey with waving wheat fields and giant stands of corn?
Somewhere back in the beginnings of our continent, a great surface depression 683 miles wide and 895 miles long was formed just south of the 30th parallel, along the border of our southern states. Extending southward to about the 18th parallel, it made a large indentation in our North American Continent which geographers later called the Gulf of Mexico. This area then filled with warm tropical water. To give an abundant flow within this gulf and to insure proper circulation, the Island of Cuba was, by coincidence, so located that it acts as a baffle to deflect the tropical waters, which are pushed out of the Caribbean by the continuously blowing northeast trade winds into the Gulf of Mexico. And to intensify the flow, the channel through which the Caribbean waters flow into the gulf is narrowed by the natural breakwaters of the Florida and Yucatan Peninsulas.
Ocean currents result largely from wind circulation which in turn is controlled by the atmospheric pressure distribution over the surface of the sea. The 30th parallel of latitude, between the eastern coast of the United States and the northern coast of Africa, forms the major axis of a large anti- cyclonic or high-pressure area (see Fig. 1). The wind flow about this “high” moves the ocean waters so that the southern part of the North Atlantic has a vast system of currents which flow to the southward off the African coast (see Fig. 2). As the southern periphery of the “high” is reached, the direction is changed to west and then to westnorthwest thus forming the North Equatorial Current. A similar flow from the South Atlantic, from the “high” along the 30th parallel between South America and southern Africa (see Fig. 3) produces a current which also flows westward and then northwestward to form the south Equatorial Current (see Fig. 4). Somewhere westward of the Windward Islands those two mighty ocean currents join and flow northwestward across the Caribbean towards the Gulf of Mexico to form one of the world’s greatest rivers, the Gulf Stream.
The island of Cuba, acting as a deflector or baffle, prevents the Gulf Stream from turning northward as it otherwise would, until it enters the eastern part of the Gulf of Mexico. If the Greater Antilles were located farther to the eastward, the Gulf Stream, motivated by the circulation along the western part of the Atlantic high- pressure system, would be turned northward at about the 70th meridian. But fortunately, largely because the longer axis of Cuba extends in an east-west direction, the Gulf Stream is forced to continue northwestward for another 600 miles and in so doing it narrows to a faster flowing stream. At -the Yucatan Channel, that 108-mile wide strip of water which separates Mexico from Cuba, the Gulf Stream divides—a part flowing northeastward through the Florida Straits and up along the Atlantic Seaboard. The other portion flows around the Yucatan Peninsula and on into the gulf where, as a peripheral flow, it swings around the whole gulf, later joining the parent stream off the Florida Keys (see Fig. 2). It is this portion of the Gulf Stream that is of greatest significance to the United States and its fertile Middle West.
The water temperature of the Gulf of Mexico remains between 68° F. to 82° F. throughout the year and the range from season to season is only about 13° F. The air masses over the gulf remain there long enough to acquire a high temperature and humidity. This is most fortunate, for it is the origin and the movements of these gulf air masses from their source region that exercise such a large control over the weather of the Mississippi Valley.
Almost all of the United States is in the region known as the prevailing westerlies. A characteristic of this belt is that a continuous procession of high- and low-pressure areas moves across the country in a general eastward direction with circulations that involve vast masses of air. As the wind circulation is anticlockwise about a low-pressure area, gulf air is dragged northward whenever a well-developed disturbance of this type—the so-called “low” —moves across the Mississippi Basin (see Fig. 5). Similarly, whenever a high-pressure area passes over the central United States, there will be a transport of gulf air to higher latitudes because the circulation on the rear side of a “high” produces south and southwesterly winds (see Fig. 6). The rate of translation of a high-pressure area being slower than a “low,” a vast amount of gulf air is transported frequently hundreds of miles northward by a large, slow drifting “anti-cyclone” circulation. Aviators flying northward from southern airports are frequently surprised when they get tail winds all the way from the gulf to the Canadian border. The wind system in the prevailing westerlies is not always from -west to east.
Present-day meteorologists, however, no longer think of weather solely in terms of pressure distribution, that is, of “highs” and “lows.” They are concerned chiefly with air masses directed by pressure influences and the line of demarcation or “fronts” which separate them.
The weather of the United States is influenced by air masses from several source regions. Polar maritime air from the Aleutian Islands moves across the Pacific Northwest during the entire year, but in the winter months tropical maritime air is imported from the ocean southwest of Lower California to give the west coast its well-advertised mild winter weather. Cold waves hit the Pacific coast only when there is an outbreak of large masses of polar Pacific air.
Eastern seaboard weather is controlled largely by air from other source regions. During all seasons tropical maritime air from the ocean areas north of the West Indies brings moisture-laden winds to the east coast. Cold waves in winter and a respite from summer heat are due to the southwesterly movement of air masses which originated over northern Canada. Polar maritime air from Labrador and beyond is also imported to the northern part of the eastern seaboard at all seasons. Throughout the year continental polar air from Canada pours into our Great Central Valley, but in the winter colder continental arctic air also moves into this area. In addition, during every month of the year, tropical maritime air from the Gulf of Mexico moves northward into this area frequently as far as the Great Lakes region, even into Canada and New England.
Large air masses remaining over or moving slowly across an area of uniform temperature and humidity, such as the Gulf of Mexico or snow-covered arctic latitudes, acquire the properties of those regions. Consequently air masses over the Gulf of Mexico are warm and laden with moisture while those remaining for any time over northern Canada are cold and dry. When such masses moving outside of their source regions finally meet, as they must under the pressure influence of the migratory “high” and “low” systems of the prevailing westerlies, a simple mixing does not occur. Instead, along the line of meeting, the cold continental air underruns the warmer gulf air and lifts it to an elevation where, cooled by expansion, clouds and precipitation are formed. So long as the prevailing pressure distribution feeds moisture-laden gulf air into colder masses, precipitation will continue, and usually, because great masses are involved, the rain or snow is not confined to small areas but to our whole national granary. It happens frequently that all the southern states may receive rain from a single well- developed frontal system.
During certain seasons of the year, particularly in the late fall, winter, and early spring months, disturbances often develop within the gulf itself. The source of humid air being at hand, polar air is dragged southward by the low-pressure whirl and as this drier, denser air elevates the tropical gulf air, a fast moving storm with precipitation soon develops which usually drenches the southern and eastern states as it moves on a northeasterly course out into the Atlantic.
In addition to widespread precipitation produced by the air masses of the gulf, there are other rain-producing disturbances for which the Gulf must be given credit. Oftentimes, and especially during the summer months, the Atlantic high- pressure system will push westward across the Appalachians, producing a flat pressure distribution with concomitant hot oppressive weather over the central United States. The position of the “high” is such
that the continental in draft draws moisture-laden gulf air northward where it is further heated by radiation from the earth. Such action results in rising vertical currents and the formation of thunderstorms, which are always good precipitation producers.
The Great Valley has high thunderstorm frequency. As is to be expected, it is highest along the Gulf Coast and decreases with distance from the gulf source region. The vicinity of Tampa has the highest U. S. thunderstorm frequency, averaging about 90 a year. New Orleans has between 70 and 80, St. Louis has 50, and St. Paul averages 32. Largely due to these local convectional storms, the rainfall in the Mississippi Valley is greatest in summer at a time when it is most economically valuable. Yet, in spite of the heavy summer rainfall, no month is without precipitation, and this happy alternation of sun and precipitation is one of our greatest national assets.
Hurricanes are another type of proficient precipitation producer. While they seldom originate in the Gulf of Mexico, the air masses there enable such disturbances originating in other localities to maintain their intensity as they move across gulf waters. And whenever a hurricane moves inland out of the Gulf of Mexico to become an extratropical disturbance, the precipitation produced is usually heavy and of widespread distribution.
The Good Gulf has one blot on an otherwise outstanding record of immeasurable service. The air masses which it sends northward are often responsible for the formation of tornadoes, the smallest, most violent type of terrestrial storm. It has been estimated that wind velocities in these disturbances may range from 100 to 500 miles per hour. Strangely these destructive whirls which occur frequently in many of the states in our Great Central Valley, particularly in Iowa, Kansas, Arkansas, and northwestern Missouri, are a phenomenon largely peculiar to the United States.
Tornadoes develop in humid, often sultry, southerly winds. Whenever the isobars in a low-pressure trough are packed in a sharp “V” or elliptical shape, there is a well-marked front passing through the center of the “V” which separates the humid southerly gulf air on one side of the “V” from the cold, opposite northerly winds on the other. Strong, vertically rising, warm air masses in the proximity of opposing currents are essential for the formation of the destructive tornadoes, and it is the Gulf of Mexico which supplies this warm air. On the books of Good Gulf Incorporated, tornadoes must be entered on the debit side of the ledger.
During the fall months of 1941, there was a threatened shortage of electrical power in the southeastern section of the country due to low-water levels in the rivers of that area. If there had not been a deficiency of rainfall there would have been sufficient hydroelectric power. And if the humid air from the gulf had not been cut off, there would have been sufficient rainfall. Let us now examine what happened.
It is not an unusual occurrence, particularly in the fall of the year, for large high- pressure areas to drift slowly out of the northwest and to remain for days over the Great Valley of the Mississippi. This condition gives dry Indian Summer weather, for the winds on the south side of these slowly moving “highs” will, of course, be easterly. Under these conditions such high- pressure areas act as a barrier to the transportation northward into the Great Valley of the humid “rain-making” gulf air (see Fig. 7). And the longer it takes for the “high” to drift off, disintegrate or merge with the Bermuda “high,” the longer the dry weather will prevail over the southern and central sections of the Mississippi Valley.
It is frequently stated that in the region of the prevailing westerlies there is a continuous procession of alternating high- and low-pressure areas. That conception is somewhat misleading. In the case of slowly drifting high-pressure areas, only a slight pressure trough may separate successive “highs” and frequently these troughs are so slight that it is difficult to locate them on a surface weather chart. If the fronts in the troughs which separate such “highs” were more marked, gulf air would be transported northward in sufficient quantities to give rain with the frontal passage, but whenever these air masses are shut out of the southern states for a protracted period, drought conditions will prevail.
The Gulf of Mexico comprises 700,000 square miles, an area nearly one-fourth the size of the continental United States. If its waters were a land mass, the “highs” and “lows” which trek across the United States would need to reach far down to the Caribbean for their supply of moisture-laden tropical air in order to produce abundant precipitation. Because their circulations are seldom so extensive, only dry air would be drawn northward from the 700,000 square miles of semi-arid land mass. In consequence there would be little precipitation in the Great Valley of the Mississippi because its southerly winds would not be moisture-laden and such frontal action as occurred would be weak and ineffectual.
“One’s loss is another’s gain” is an old adage particularly applicable to Mexico and the United States in the matter of rainfall. Because Mexico lies below the prevailing westerlies and because it is corrugated by the Rocky Mountains which extend in a general north-south direction, only the eastern littoral of that country benefits from the moisture-laden winds which lie over the large body of water which bears its name. Geographers have named it the Gulf of Mexico, but meteorologists know that it is continually working almost solely for the United States to keep the world’s greatest food basket continually full.
If by some volcanic upheaval, the Gulf of Mexico became a land mass, the southern part of the Mississippi Valley would be converted to a semi-desert, to an area not unlike that to the southwest of the Caspian Sea. How much it would mean to Russia if Iran had a gulf as large and warm as. America’s Gulf of Mexico!
Truly, for us it is a good gulf. To it we owe much of our abundant national life and the surplus which we share.