. The American farm register. Agriculture. THE AMERICAN FARM REGISTER. 10 ISOBAROMETRIC LINES SHOWING IN INCHES THE MEAN ANNUAL ATMOS- PHERIC PRESSURE OF THE UNITED STATES. If we compare the indications of the barometer as they exist siniiiltaneously over wide areas of country (as is done three times daily, from telegraphic reports, by the Signal Service), it will be found that there are cer- tain areas of high, and other areas of low pressure of the barometer. Lines drawn through points of equal pressure are called " Isobarometric Lines," or simply " Isobaries" or " I
. The American farm register. Agriculture. THE AMERICAN FARM REGISTER. 10 ISOBAROMETRIC LINES SHOWING IN INCHES THE MEAN ANNUAL ATMOS- PHERIC PRESSURE OF THE UNITED STATES. If we compare the indications of the barometer as they exist siniiiltaneously over wide areas of country (as is done three times daily, from telegraphic reports, by the Signal Service), it will be found that there are cer- tain areas of high, and other areas of low pressure of the barometer. Lines drawn through points of equal pressure are called " Isobarometric Lines," or simply " Isobaries" or " ; The areas that these lines in- close may vary from a few miles in diameter, as in tornadoes, to several hundred miles, as in general and widely extending storms. Usually areas of low pressure are much less in extent and more regular in form than those of high pressure. If we could obtain instantaneous reports from the whole surface of the earth, we would always find several of these areas of low pressure surrounded by those of high pressure, and one or more of them ari; constantlv seea with more or less distinctness upon the weather-maps published thrice daily by the Signal Service office. Taking the mean results of months and years, we find that in certain great regions the mean annual atmospheric pressure is habitually lower than in others, as is shown by the following map: Areas of low jiressure are usu- ally areas of cloudy and stormy weather, if the moisture of the at- mosphere be sufficient. They vary in extent according to the prevail- ing conditions of temperature and moisture of the regions over which they 2)ass, becoming smaller where the air is dryer. It does not follow that every area of low pressure is a storm, or even an area of cloudy weather; but it may be safely said that no extensive storm occurs ex- cept within such areas. North of 30° north latitude these areas of high and low pres- sure, as well as of extreme depres- sions of the temperature,
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectagriculture, bookyear
