CONTRIBUTIONS TO METEOROLOGY. 



By Elias Loomis. 



AREAS OF LOW PRESSURE — THEIR FORJl, MAGNITUDE, DIRECTION, AND VELOCITY OF MOVE- 

 MENT. 



1. Tbe pressure of the atmosphere is coutinually chauging. In the michlle and higiier 

 latitudes of tlie Northern Hemisphere these cliaiiges are very great and sometimes very sudden. 

 In the northern jiart of tlie United Slates tlie barometer frequently rises a half inch above its 

 mean height, and it has been known to rise a whole inch above tiie mean. It frequently sinks a 

 half inch and sometimes more than an im^h below the mean. These changes take place simul- 

 taneonsly over regions of vast extent. In order to exhibit these plienonieua in tlie simplest 

 manner, we draw lines connecting all tliose places where the pressure at a given instant is the 

 same. Such lines are called lines of equal pressure, or isobaric lines, or simply isobars. Plate I 

 shows the isobars for the United States on the loth of January, 1877, at 4'' 35'" p. m., Washington 

 time, the isobars being drawn at intervals of one-tenth of an inch. It must be understood that 

 the barometric observations here represented are not the actual readings of the barometer, but a 

 correction has been applied to all of them to reduce them to sea-level. We see that the region 

 over which the pressure was less than 30 inches, is of an elongated form, about 1,000 miles in 

 diameter, measured in a direction from XW. to SE., and about 1,800 miles in diameter measured 

 in a direction from SW. to NE. This region, over which the pressure is less than the mean, is 

 called an area of low pressure; tlie point wliere the barometer is lowest is called the center of the 

 low area; and on the Signal Service maps this center is marked Low. 



2. If the atmosphere were of uniform density from the surface of the earth to its upper limit, 

 these differences of pressure wouhl indicate differences in the height of the atmosi)here; and if an 

 observer could be elevated above the earth so as to see the whole area of low i)ressure at one 

 view, be would notice a depression in the upjjer surface of the atmosphere somewhat similar to 

 that produced when a vessel of water is rotated rapidly about a vertical axis. The upper surface 

 of the atmosphere over the low area would appear dei)ressed below a horizontal surface, and would 

 appear to slope upwards from the low center. This slope is called the atmospheric gradient, or 

 barometric gradient, and the steepness of the slope is indicated by the increase of height in a given 

 distance, or the change of barometric jiressure in a given distance. The unit of distance now 

 generally adopted is a degree of the meridian, or 60 nautical miles. We notice that on Plate I 

 the gradient is uot the same in all directions from the low center, but is steepest on the northwest 

 side. Here the isobars are crowded close together, their average distance from each other being 

 43 nautical miles. The change of pressure for a distance of 60 nautical miles, measured iu a 

 direction perpendicular to the isobars, is 0.14 inch, and this is the barometric gradient for that 

 part of the low area. This is a very steej) gradient, and only occurs in the case of violent storms. 



3 The direction of the wind within this low area is indicated by arrows, and the velocity of 



the wind by the number of feather* on the tail of each arrow ; one feather indicating a velocity 



not exceeding 5 miles per hour; two feathers indicating a velocity from 5 to 10 miles per hour . 



three feathers a velocity from 10 to 15 miles, and so on up to ten feathers, indicating a velocity 



S. Mis. 154 2 9 



