Fig. 64.
On a clear day place this apparatus out of doors on a table freely exposed to the sun with a piece of paper on the baseboard under the end of the tube. Point the tube directly at the sun in the early morning, in the middle of the forenoon, at noon, in the middle of the afternoon and about sunset. Mark on the paper the amount of surface illuminated by the sunlight passing through the tube at each of these different times. Why are different amounts of surface covered at these different times?
Place a thermometer in the centers of the surfaces covered by the sunlight passing through the tube at these different times. Note the different readings of the thermometer. Can you suggest a reason why they are not alike? The opening exposed to the rays has been the same throughout the experiment. Draw diagrams illustrating the action of the sun"s rays in the different positions.
The number of rays of the sun which fall upon a given area depends upon the angle at which they strike the surface. Figure 65 shows that the same number of rays fall upon a much smaller surface when the direction of the sun is vertical than when it is nearly horizontal. In the 30- degree arcs there are 2?, 7, and 9? ray spaces respectively. TheFig. 65.
sun is here considered to be vertical at the equator, as it is on March 20 and September 23. Thus on these days, other conditions being the same, about one fourth as much heat from the sun falls upon the 30° aboutthe pole as upon the 30° north of the equator.
The latitude of a place has much to do with the amount of heat that it receives. As the sun becomes vertical to places north of the equator, the length of the day in the northernhemisphere increases and the time that a place is in the sunshine is greater, so that it receives more heat from the sun. On the 21st of June all points within 23.5° of the north pole, as at North Cape, have 24 hours of sunshine, and the amount of heat received at the pole during these 24 hours is greater thanthat received at the equator where the
day is only about half as long.
A WINTER SCENE IN VENICE.
Although the latitude of a place has much to do with the amount of heat received, there are also many other things which affect its temperature. This will appear when we consider that Venice, Italy, with its mild and equable climate is in almost the same latitude as Montreal, Canada.
As has been seen, the height above the sea makes a difference with the temperature, since there is less thickness of air above and therefore a thinner blanket to hold the heat. Then, too, the kind of soil affects the temperature. If the soil is sandy and there is little or no vegetation, it becomes rapidly heated in the daytime and radiates back the heat into the air very rapidly, thus making the temperature of the air near the surface very hot during the day; while at night, when the sun is not adding heat, it rapidly loses the heat acquired during the day, and sothe temperature of the air becomes low. In the daytime on great sandy deserts the heat is almost unbearable, but at night it is so cold that heavy blankets are needed to keep the traveler warm.
The nearness to the sea and the direction of the wind also greatly affect the temperature of a place. In some parts of the earth these are the principal causes in determining the temperature. Thus theA WINTER SCENE IN MONTREAL.
The famous Ice Palace, built entirely of blocks of ice.
temperature of the atmosphere at any place is not due to a single cause, but is the result of many and complex causes, such as latitude, height, direction of prevailing winds, ocean currents, nearness to the sea, and kind of soil.
Maps are sometimes constructed showing heat belts where tropical, temperate and frigid conditions are found. These belts do not correspond very closely to the torrid, temperate and frigid latitude zones.
66.Graphic Method of Showing the Temperature of a Region. -Itis often quite essential that the temperature over a considerable region should be known and a record of it made and preserved. This might be done by taking a map and writing their temperatures above the dif- ferent places marked on the map. This would make a map full of small figures and very difficult to read.
HEAT BELTS.
Notice how these heat belts vary from the latitude zones shown on Figure 10.
A much better method has been developed and is now almost universally used. In making this map the temperatures are first written on the map and then lines aredrawn through places which have the same temperature. These lines are called isotherms and the map is called an isothermal map. By the use of such a map it is possible at a glance to determine the temperature prevailing at any place and to see the relation which this has to the temperature of other places on themap. As a rule the isotherms are not
Fig. 66.
drawn for each degree, but only for each ten degrees.
When the map has been constructed, copies are made in which the figures are left off and only the isotherms are preserved. In Figure 66 we have a plan before the isotherms are drawn, and in Figure 67 after the isotherms are drawn. Figure 68 is a typical isothermal diagram. If the map itself were sketched, it would be an isothermal map.
Maps recording barometric conditions are made in the same way as the isothermal maps, only their lines pass through places of equal barometric pressure instead of places of equal temperature. These lines are called isobars.
Weather maps are prepared by the United States Weather Bureauevery day, on which are both the isotherms and isobars for that day.
The data for these maps are telegraphed each morning from stations scattered all over the settled part of North America.