weather bureaus are maintained not only by the United States, but by all the other highly civilized countries of the world. Records are kept also by sea captains and by other observers throughout the world, and these are gathered together by scientific men and from them are made charts of the weather conditions over the entire surface of the earth. Ev- ery year more and more data are being collected and these charts are becoming more and more reliable.
These charts are of great value, since they aid in the explanation of climaticFig. 67.
Fig. 68.
conditions in different parts of the world. The results of the data thus gathered together have been of untold service to commerce and each year have saved many lives and a vast amount of wealth. On the previous page are isothermal maps of the world for the months of January and July.
68.Land and Water Temperatures. -As was seen in Experi-ment 27, water has the power to hold a great amount of heat. During the summer, water is heated less rapidly than the air above it, so it con- tinually extracts heat from the air, making the air cooler than it other- wise would be. In the winter, water loses its heat less rapidly, so, being warmer than the air above it, it constantly gives heat to the air. Conse- quently the air over large bodies of water changes its temperature less rapidly than does the air over the land.
When air moves in wind from the ocean to the land, it cools the land in summer and warms it in winter. It is found therefore that landswhich border on the ocean usually have a smaller range of temperature than those which are far from the sea. On some islands the range of temperature throughout the year is almost imperceptible, whereas in the interiors of continents the average temperature of some of the summer months is more than a hundred degrees higher than that of some of the winter months.
69.Distribution of Air Pressure over the Earth. -An examina- tion of the isobar maps for January and July shows that atmospheric pressure, like temperature, is greatly affected by land masses. In the southern hemisphere, south of 40° latitude where there is little land, the isobars are very regular in their directions and nearly parallel to the parallels of latitude. North of this in the same hemisphere they are somewhat affected by the land, but the sea is still the predominant in- fluence.
In the northern hemisphere, however, the land and water aremuch more equally divided and here the effect of the land masses is at once apparent. In the winter the high-pressure areas and the low- temperature areas are found over the land, but in the summer, the low- pressureareas areas and the high-temperature areas are over the land. This illustrates what we have already learned; that land heats and coolsmuch more rapidly than water and that hot air is lighter than cold air.
In both summer and winter there is an area of comparatively high pressure on either side of the equator, but this area is not fixed; it moves north and south. In summer it is farthest north in the northern hemisphere.
The winds are simply a transfer of air from a place where the pressure is high to a place where it is low, or a transfer of air along what are called barometric gradients from a high barometer to a low barometer. So the above-mentioned changes in the relation between the pressure on the land and on the sea must have an effect upon the directions of the winds. As a rule the wind blows out from the land interiors in the winter and into these interiors in the summer. It is thus seen that isotherms and isobars are closely related to each other, and that the wind is but a result of the atmospheric conditions which they represent.
70.Wind.
Experiment 77. -On a day when the temperature in the room is considerably higher than that outside, open a window at the top and bottomand hold a strip of tissue paper in front of the opening. Is there an air current, and if so, in what direction does it move at the top and at the bottom of the window? What causes "drafts" in a room?
Experiment 78. -Procure two similar dishes about 15 cm. high and 5 or 6 cm. in diameter with short tubes of about 1 cm. in diameter opening outfrom near the top and bottom. Connect
the bottom tubes of the two dishes with a tightly fitting rubber tube. Do the same with the top tubes. Place a Hoffman"s screw upon each of the rubber tubes and screw it tight so that no liquid can flowFig. 69.
through either tube. Fill one of the dishes
with colored water and the other with kerosene or some light oil.
Although the two liquid columns are similar, yet the pressure at the bottom of the dish of water will be greater than that at the bottom of the dish of oil since the water is heavier than the oil. These are the conditions that exist on the surface of the earth at two places one of which has a high and the other a low barometer. Release the Hoffman"s screw upon the top tube and then the one at the bottom. Notice carefully what happens as the lower tube is allowed to open. The dishes are not now filled with oil and water respectively. In the transfer of the liquids, through which tube did each pass? (If part of each rubber tube is replaced by a glass tube, the action in the experiment can be seen to better advantage.)Experiment 79. -Fill a convection apparatus with water, putting in a little sawdust and mixing it well with the water. Heat one side of the tube and observe the convection currents set up.
In Experiment 78 the interflow from one dish to the other is due to the fact that the water is heavier than the oil and runs under it and pushes it up so that the oil overflows into the dish that the water has left. The same thing happens in the atmosphere when from any causeFig. 70.
the column of air above one place becomes heavier than that above another place. There will be under these conditions a transfer of air, along the surface, from the place where the pressure is greater to that where it is less great, and this movement of the air we call wind.
