The Alaska daily empire., 1917-02-27
WEATHER FoREcAsTINe
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 In the last article of this series the statement was made that the barometer is the most important of all meteorological instruments so far as forecastlng is concerned We learned that the function of the bar. ometer is to measure the pressure Of the air and that the changes in the pressure are the chief cause of the changes in our weather con. ditions.
 Al the stations in continental United States the barometers and oth. CT instruments are read at 8 a. m. and s p, m, Washington time, and the data is sent at once by tele graph to Washington and other Western Bureau stations where they are charted for the study Of fore casters By 9 a. m. and 9 D. ma if the wires are working well the re. ports will all have been received and the charts will be completed The .forecaster then has before him a view or sketch of the weather con ditions throughout the cOuntry-just the view he would obtain were he 10 ascend in 8 balloon tO a height that would enable him to survey with his eye the entire country. On these charts are drawn lines that have the same barometrIc pressure, a line being drawn for each tenth Of an inch Of pressure. Lines are also sketchcd for each ten degrees tenn pcrature. Tho pressure lines are an usually concentric in character, something like the ripples made by casting a stone into pond of wa. ter, though they are generally clip. t!cal in shape rather than circular.
 On each days chart or weather map, as it is called, will be found station where the barometer is relatively low. and which constitutes the center of the low pressure area for that particular map. The pres. sure lines encircle this, the number of lines depending on the intensity of the low" that is, on the differ ence between the readings at tho center and at the edge of the area Another station on the map will be found to have a relatively high barO- meter reading, and it, too, will be encircled by pressure lines, the mum Der of which will depend on the diff ference in pressure between this cen tral station and the edge of the high" area. These lows" and highs" vary greatly in size. from a diameter of a hundred miles to two thousand miles or more.
 A study of the weather conditions within Q How" as compared with those within high" shows that they are widely divergent and in some respects directly opposite. In the How" the winds blow spirally inward toward the center, or point where the barometer is lowest, in a direction opposite to the move mcnt of the hands of a watch lr the eastern half of the area the weather is usually cloudy and unset tied. with some little precipitation and the temperature is relatively high, while in the western half clearing conditions prevail, with fall. ing temperature. Tn the high the winds blow spiralIy outward from the region with highest barometer. but the movement is clockwise. In the eastern half of the area the weather is generally fair and the temperature relatively low, while in the western half cloudlness develops. and the temperature rises.
 In the absence of a weather map to illustrate the above, a fair idea of the air circulation within low may be had from the movement of water in a tank or bowl when the plug at the bottom Is withdrawn The water leaves the receptacle ia spiral manner with a vortex for. nation at the center. It is inter. psting to note that with us the water swirls in a counter-2loCkwisc direction. but that if the experiment is made in the southern hemis. phere it will travel in the opposite. or clockwise, direction. The move. ment in low pressure areas in the southern hemisphere is therefore al- so clockwise while in highs it is counter.clockwise. Of course the air and water movements in the above comparison are not strictly identical. since the water has a downward component of motion in leaving the receptacle, while the air in a How" moves spirally upward.
 By comparing weather maps made for successive 12.hour or 24-hour per iods we find that they move across the country in an easterly direction the easterly movement being due to the rotation of the earth on it's axis from west to east. The velocity oi this easterly movement varies great ly and is one of the greatest factors to be considered in forecasting. In the United States the average veloc ity for flows" is 29 miles an hour but cases have been observed wher, the rate was only 4 miles and il