The climate of the United States is influenced by the geographical situation, by the extension in latitude, by the arrangement and by the alternation of the high lands and plains. As for the latitude, we are already outside the tropical climates, which therefore are absent or are barely announced at the extreme S. continental climates; on the other hand, the Appalachians are not high enough to form a climatic dividing line, and therefore do not shield the coastal countries from cold inland influences; after all they run roughly parallel to the direction of the prevailing winds. On the western Cordillera the contrast between the humid Pacific side and the dry internal side is very remarkable; on the Appalachians it is much less pronounced. The influence of Hudson Bay, a very cold inland sea, which penetrates up to 51 ° lat. N., is still felt in the north-central United States, as in the extreme NE strip. of the country there is that of the cold Labrador current, whose ramifications go as far as Cape Hatteras. On the other hand, the southern Atlantic coasts are somewhat affected by the influence of the Gulf Stream and also the countries bordering the Gulf of Mexico are influenced by the mass of warm waters of this inland sea, where there are also exchanges of air currents, such as monsoon, between the continent and the sea. The Pacific coast has a typical oceanic climate, with relatively slight seasonal temperature fluctuations; the beneficial influences of Kuro shio do not reach the shores of the United States; on the contrary, the influence of cold waters rising from the depths is manifested on those of California. In the interior of the United States, the temperature variations are significant, because through the vast flat areas the exchange of masses of hot and cold air encounters no obstacles: hence also the violent and irregular fluctuations in temperature, hence the average values that are given usually for the various stations they have little meaning. In winter, when cyclonic areas form on the border between the United States and Canada, with a tendency to move towards the E., behind them there are cold waves (cold waves) that penetrate S. to the Gulf of Mexico (northens in Texas) signaled by icy winds, which often cause serious damage to subtropical crops. In the northern plains such waves are often accompanied by heavy snowfalls (blizzards). In summer, on the other hand, especially in the shelves of the S. and on the Atlantic coasts, heat waves, with winds from S. and SW, hot and humid, are frequent, so that the rapid rise in temperature and the unwelcome effect of the saturated air. of steam, which does not fade even at night, because the abundant humidity hinders the radiation. On the plains that flank the Cordillera and also in California, on the other hand, hot but dry winds of the föhn type, known as chinook, are often felt..
With the exception of the thin strip along the Pacific, well watered by the rains brought by the winds from the West, for the rest of the country the water reservoirs are the Gulf of Mexico and the Atlantic: therefore the amount of rain usually decreases from the Gulf to the North and from the Atlantic to the West. On the Pacific coast, rains prevail in winter; to E. of the Rocky Mountains they prevail in the warm season (spring and early summer). Further to the East., towards the Atlantic, rainfall is very uniformly distributed throughout the year: in the southern Atlantic and Florida, however, a maximum occurs in late summer. Only the highlands and intermontane basins of the western highlands have insufficient rainfall: this an arid climate area then overlooks the sea in the northern part of the Gulf of California, where indeed there is a typical desert climate. The coastal lowlands of the Gulf of Mexico and that along the South Atlantic are visited by hurricanes, which are frequent especially in autumn, but they do not penetrate much inland. Here, in the vast flat regions, instead, terrible for the disasters they cause, the tornados, characteristic for their very short duration and unprecedented violence: the tornado of May 27, 1896 in St Louis caused damage estimated at 10 million dollars and caused the loss of over 300 human lives. In Illinois, 79 tornados occurred between 1881 and 1896, an average of six per year.
On the basis of the characteristics now exposed, at least eight different climatic regions can be identified:
- The North Appalachian region, which includes the N part of the Hudson Valley and straddling the Appalachians, which do not form a climatic dividing line; in the mountain, however, the influence of height is naturally felt. Winters are rigid (with January averages from −1 ° to −4 °), hot summers (July 20 ° -23 °), but above all the instability of the weather is characteristic of winter, due to the abrupt alternation of air currents, due to the violent daily jumps of the thermometer (up to 20 ° -25 °) and the breaking out of cold waves even in summer, very harmful to crops. Rainfall is abundant everywhere (950-1200 mm.) And uniformly distributed in every season: in the first summer there are often copious showers, while in autumn, which is generally drier, there are often periods of clear days without rain, designated with the name of “summer of the Indians”. In the states of New England, where this phenomenon occurs more typically, popular belief admits that every year, in October, a similar period occurs for a more or less long number of days.
- The sudappalachiana region, to S. of Hudson, which is distinguished by the highest winter temperatures (i° -10 ° in January on the coast) as here too cold spells, with heavy snow, are not rare even in low. In the mountains the climate is more rigorous: drops in temperature rapid and violent, frequent here too, cause the formation of frost and hoarfrost (silverthaw ; mushfrost). The rainfall increases, proceeding from the coast towards the first Appalachian ranges, at the eastern foot of which the quantity (up to 2 m.) Is greater than in the interior of the mountain itself, where it does not reach m. 1.5. The seasonal distribution is also very uniform here, but there is a tendency to a summer maximum.
- The region of the eastern plateaus, with the southern part of the Great Lakes District and the upper Mississippi Basin. Here the climate is strongly continental, with excursions of 25 ° -33 ° and extremes between + 42 ° and −40 °. In winter, days with spring temperatures are often abruptly followed by periods of snow and intense cold. Summer is, apart from often repeated showers, rather dry, indeed very damaging droughts occur not infrequently; frequent is the phenomenon of the “Indian summer”. Rainfall rapidly decreases from W to E., falling from 1100 mm. to 600; the peaks occur in late spring and early summer, a generally favorable condition from an agricultural point of view. On the shores of the lakes, where the large masses of water mitigate the thermal extremes, there are particularly happy conditions: the southern shores form Mediterranean-type climatic oases with rich southern vegetation (vineyards, orchards); on the peninsula between Lakes Michigan and Huron, winter temperatures are even higher than in the Atlantic at the same latitude.
- The Gulf region, which encompasses the southern Atlantic belt, Florida and the lowlands surrounding the Gulf of Mexico. Temperatures are here already subtropical, with mild winters, sometimes interrupted by cold weather brought by north winds (northerns). Only in Key West does the thermometer drop below zero. The rainfall, high on the coast (1200-1700 mm.), Decreases rapidly towards the interior and is differently distributed: the Atlantic states of the S. have the maximum in the height of summer, without a well-characterized dry period, while in Florida winter is dry and in the Gulf states the least rainy period is early spring. Hurricanes from the Antilles are not uncommon.
- The region of the Prairies (approximately to the West of the 95th meridian), which has a very uniform climate, with very hot summers, severe winters (excursions up to 35 °), although the influence of latitude on temperatures is clearly evident. winter, which rapidly rise as you proceed towards S. Cold waves are terrible in winter, especially in Dakota and other states of the North, which are like the center of origin; they are accompanied by blizzards. The lower regions, where the winter air stagnates, are in worse condition than the higher ones, towards the foot of the Rocky Mountains. During the summer, generally hot, W winds are frequent. The rainfall is very unevenly distributed: three quarters or four fifths of the rain falls between April and September, a very favorable condition for agriculture and especially for the cultivation of cereals; but this condition is attenuated by the great fluctuations from one year to the next and also by the violent nature of the rains which often fall in the form of very copious downpours; after all, the quantity decreases rapidly towards the West, so that in the Western Prairies periods of drought lasting several months are not rare.
- The Cordillera region, with the plateaus and the intermontane basins, up to the crest of the Cascade Range and the Sierra Nevada. It is a region with an excessive climate, with very strong seasonal contrasts, especially in the interior of the plateaus, while at great altitudes the thermal trend is more uniform. The rainfall varies, both in quantity and in distribution, with great fluctuations from one year to the next.
The eastern side and the inland parts of the Rocky Mountains still have prevailing rains in spring and early summer (with maximums in May and June), while winter rains prevail in the basins and intermontane plateaus of the N., with a secondary maximum in May. and summer drought. Towards S., however, the rains in the inland region are rarefied, acquiring the character of occasional violent downpours, mainly winter.
- The Colorado region, with the inland basins of Arizona and southern California, has, with a climate always characterized by very hot summers and severe winters, rainfall so scarce and irregularly distributed (in exceptional showers), that it assumes a desert character, as is also attested by the poverty of the vegetation and the extreme scarcity of the cultivated areas.
- The Pacific region, between the crests of the Cascade Range and the Sierra Nevada and the sea. Although it extends considerably in latitude, this factor is of secondary importance with respect to distance from the sea and altitude. At the same latitude, winters are much milder and summers much cooler than on the Atlantic coast: the hottest month is August (sometimes even September, as in San Francisco), the coldest is February: fog is frequent on the coast, even in summer. The seasonal thermal contrasts are accentuated rapidly as we proceed towards the interior. The rains fall mainly in winter (70-80% between November and March); proceeding towards S., the quantity generally decreases and the summer drought period lengthens; but some longitudinal valleys of the interior also have little rain, so that the vegetation takes on a steppe character. A NW sub-region can be distinguished, with a purely maritime climate in cool summers, and a Californian sub-region with a Mediterranean climate, rather scarce rains, clearly concentrated in winter, long summer drought.