Page:The National Geographic Magazine Vol 16 1905.djvu/298
262 The National Geographic Magazine
which, like the dark shading, do not appear on chart I, which was purposely left clear of these symbols, so that the movement of wind in accordance with pressure gradients could be the better shown. These red lines connect places having the same temperature. Note how, on both charts, they trend from the Atlantic coast northwestward into the southeast quarter of the cyclone, and where they leave the storm center how precipitately they drop away toward the southwest. A cause can be easily found for this by examining the direction of the arrows. In the first case the isothermals are being pushed northward by southerly winds, and in the other forced southward by winds from the northwest. As the cyclone proceeds eastward the regions now under the influence of warm southerly winds will be, in less than 24 hours, on the west side of the storm, and cold northwest winds will sweep over them.
The line of arrows leading from western Wyoming to the center of the storm on chart III shows the place where the cyclonic circulation of wind began that constitutes the storm and the course pursued by the storm center. The small circles surrounding crosses mark the places where the storm was central at each 12-hour interval. The figure above the circle indicates the date, and the letter below evening or morning.
As previously explained, the large figures give the average temperature for each of the four quarters of the storm within a radius of 500 miles from the center. The same information may be gathered from the isotherms, but cannot be so strikingly presented. Now, remembering that the air ascends as it spirally moves around the center, one may see how the cold air of the northwest quarter is mingled with the warm air of the southeast portion, which in each of the three cases presented by the charts so far brought into the discussion is more than three times as warm. On chart
III the two quarters are represented — one by 13 degrees and the other by 47 degrees. The mixing of such cold and such warm masses of air and the addition of cold due to expansion as the mixture rises is a fruitful cause of precipitation, but not the only one, for we see that rain has fallen in the Gulf states, as exhibited on chart III, probably only as the result of cold northwest winds flowing into and mingling with the warm air of the south. Precipitation may also occur as the result of the warm humid air of southerly winds under-running cold and heavier air, and by other processes not yet understood.
ANTI-CYCLONIC STORMS
Attention is now directed to the anti-cyclone or high-pressure area shown on these three charts as resting over the Rocky Mountain plateau. Here all the functions of the cyclone are reversed; hence the name anti-cyclone. The air has a downward component of motion at and for a considerable area about the center, instead of an upward component ; the winds blow spirally outward from the interior, instead of inward, and are deflected to the left of their initial direction, instead of to the right, and the air is mostly clear, cool, and dry, instead of cloudy, warm, and humid. The center of this high moved but little during the three 12-hour periods, but its area expanded eastward as the low advanced, and if the chart of December 17, 8 p. m., were shown the high pressure would be seen to cover with clear, cool weather the region now embraced within the limits of the low pressure.
These are winter conditions that are being described. The storms are general, not local, as in summer, when the highs and the lows exhibit small differences of pressure, move slowly, and seldom embrace large areas. The summer type of local storms gradually merges into general storms as the heat of sum-