centre is accompanied by a shifting of the winds from warm southerly to cold northwesterly, and this, consequently, gives to the interior of the United States its extreme variability of climate, so that even the southern extremity of the country scarcely belongs to the tropical zone. The cold air that flows in behind the storm centres is very dry and the sky is quite clear. Thus originate the so-called anti-cyclones or cold waves which follow each other rapidly, moving southeastwardly toward the Gulf of Mexico, while the areas of low pressure or cyclones move northeast over and beyond the lake region. The absolute annual range of temperature is therefore greatest in the interior of the country, being about 150° in the upper portion of the Missouri Valley and diminishing to 60° in the southern part of Florida and the northwestern part of Oregon. The suddenness with which the air temperature falls is an important consideration from many points of view. Thus changes of 20° in twenty-four hours occur far more frequently in the lake region than in the Ohio Valley, and there again oftener than on the southern coasts.
The average date of killing frosts in the spring and autumn determines the average length of the growing season for most of the important crops. The autumnal date is September 1st for the region from North Dakota to Lake Huron, and October 1st for the region from Colorado to Pennsylvania and northeastward along the New England coast. The latest spring frosts occur on May 15th from Idaho to Lake Superior and February 15th along the south Atlantic and east Gulf coasts. The growing season may be considered as the interval between the last frost of spring and the first of autumn, or it may also be defined as the season within which the average daily temperature does not fall below 40°. (See Maps accompanying article Frost.) From this point of view the growing season diminishes as one goes northward and amounts to about 120 days at the northern border of the United States. By a natural process of selection, stimulated by culture, plants that formerly required this length of time for maturity are now pushing northward beyond the borders into Canadian regions where the growing period is as much as twenty days shorter. Mr. Pennywitt of the Weather Bureau has published, in the Monthly Weather Review for February, 1901, three charts showing the dates on which the normal daily temperature at any place equals the annual mean at that place. There are two such dates for each station, dividing the year into the warmer half and the colder half. These are not quite the dates of frost, but generally come earlier than the frost in spring and later in autumn. His chart reveals the interesting fact that in general over the United States the warmer portion of the year is decidedly longer in duration than the colder season. This difference is greatest on the eastern slope of the Rocky Mountain range, and increases from thirty days in Texas and Kansas to forty-eight days on the northern border of Montana and probably increases somewhat as one goes north into Canada.
One of the most notable features in the climate of the United States is the great contrast between the Atlantic and Pacific coasts. This is due to the fact that the cold, dry air following each area of low pressure is confined in its movement southward between the Rocky Mountains and the Appalachian ranges. Occasionally dry, cold air descends from the north or northeast upon ‘The Great Valley’ of California with disastrous consequences, but in general the Pacific coast is under the influence of south and west winds, which bring abundant winter rains to northern California, Oregon, and Washington without severe cold weather. On the Atlantic coast the winter rains and snows are followed by very cold weather. The contrast between the mean temperatures on the immediate coast of the United States in January and July are shown in the following table. The temperatures here given are as read off on charts of isotherms, and refer to locations on the land at sea level near the shore. These temperatures are controlled largely by the prevailing winds and the adjacent oceans.
| LATITUDE NORTH |
January | July | ||||
| Atlantic Coast |
Pacific Coast |
A—P | Atlantic Coast |
Pacific Coast |
A—P | |
| 50° | 14°F. | 41°F. | −27°F. | 57°F. | 55°F. | + 2°F. |
| 48 | 20 | 39 | −19 | 58 | 57 | + 1 |
| 46 | 21 | 39 | −18 | 61 | 59 | + 2 |
| 44 | 25 | 42 | −17 | 64 | 59 | + 5 |
| 42 | 32 | 45 | −13 | 68 | 59 | + 9 |
| 40 | 36 | 47 | −11 | 75 | 57 | +18 |
| 38 | 39 | 49 | −10 | 75 | 57 | +18 |
| 36 | 42 | 61 | − 9 | 75 | 59 | +16 |
| 34 | 47 | 54 | − 7 | 76 | 62 | +14 |
| 32 | 50 | 54 | − 4 | 79 | 68 | +11 |
| 30 | 58 | 56 | + 2 | 79 | 70 | + 9 |
| 28 | 60 | 60 | 0 | 79 | 75 | + 4 |
| 26 | 66 | 66 | 0 | 80 | 79 | + 1 |
| 24 | 71 | 68 | + 3 | 81 | 80 | + 1 |
Of course a similar great contrast prevails between the temperatures on the east side of Asia and west side of Europe, but nothing like this occurs in the Southern Hemisphere, owing to the presence of the great southern ocean and the small area and low altitude of the lands.
Another striking peculiarity in the climate of the United States is the great contrasts of dryness and rainfall. The heaviest precipitation and also a fairly uniform humidity prevail on the coast of Oregon and Washington; next to this are the humidity and rainfall of the coasts of Florida and the adjacent States. Throughout the Mississippi Valley and along the Atlantic coast from Cape Hatteras to Nova Scotia there are great fluctuations in moisture and evaporation. On the average the driest portion of the continent extends from southern California and Arizona eastward over the eastern slopes of the Rocky Mountain region. Not only is the rainfall here a minimum, but the humidity of the atmosphere is also very small and the proportion of the clear sky very large. Yuma, in Arizona, Santa Fe, in New Mexico, and Pueblo, in Colorado, have mean annual relative humidities of 43, 45, and 46 per cent. respectively. The amount of evaporation from the soil is correspondingly large in these regions, and artificial irrigation is quite essential for successful agriculture. Even the summits of the mountains of this portion of the continent show but little permanent snow, and that in sheltered spots. What would remain on the ground in an ordinary, moist climate is rapidly evaporated in this dry air. See map showing average annual relative humidity in the
