50 BOARD OF AGRICULTURE. [Pub. Doc. 



ferent kinds of clouds, tornado effects and miscellaneous 

 phenomena were exhibited and explained. 



A sample of the weather map or chart referred to above 

 is included with the lecture, and the following explanation 

 of the map is issued by Prof, Willis L. Moore, chief of the 

 United States Weather Bureau, and is here appended : — 



The weather map presents an outline map of the United States 

 and Canada, showing the stations where weather observations are 

 taken daily at eight a.m. and eight p.m., seventy-fifth meridian 

 time, and telegraphed to Washington and other cities. These 

 observations consist of readings of the barometer, thermometer 

 (dry and wet), direction and velocity of wind, state of sky 

 (whether cloudy or otherwise) and amount of rain or snow. 

 Solid lines, called isobars, are drawn through points having the 

 same pressure, a sepai'ate line being drawn for each difference of 

 one-tenth of an inch in the height of the barometer. Dotted lines, 

 called isotherms, connecting places having the same temperature, 

 are drawn for each ten degrees of the thermometer. Heavy dotted 

 lines, inclosing areas where a great change in temperature has 

 occurred within the last twenty-four hours, are sometimes added. 

 The direction of the wind is indicated by an arrow flying with the 

 wind, or opposite to the ordinary vane, and has a square end when 

 storm signals are displayed. Shaded areas show where rain or 

 snow has been falling since the last observation. 



The general movement of storms in the United States is from 

 west to east, similar to a series of atmospheric waves, of which 

 the crests are designated on the map " highs," and the troughs or 

 depressions " lows." These alternating highs and lows have an 

 average easterly movement of about six hundred miles per day. 



High winds, with rain, or snow if cold enough, usually precede 

 the low area, often extending to a distance of six hundred miles 

 to the eastward of the centre of the storm. In advance of the 

 low centre the winds are generally southerly, and consequently 

 bring high temperature. When the centre passes to the east of a 

 place the wind at once shifts to the west or north-west, bringing 

 lower temperature. The temperature on a given parallel Avest of 

 the low may be reasonably looked for on the same parallel to 

 the east when the low has passed, and frost will occur along the 

 north of an isotherm of about forty degrees, if the night is clear 

 and there be but little wind. Following the low usually comes 

 an area of high, bringing sunshiny weather, which in its turn is 

 followed by another low. 



By bearing in mind a few general rules as to the direction and 



