stantaneous photograph of the weather of the whole country. From this map the expert can tell not only what the weather is, but what it is likely to be, for he can tell where the low areas are, in what direction they are traveling, whether rain, snow, or frost are on the way, whether it will be hot or cold. The coming of cold waves, or of storms that will injure shipping or cause floods on rivers, can be foretold in this way. Sometimes there is a mistake, because storms do not all act exactly the same, but usually the predictions are accurate, and they save many lives and many millions of dollars annually. When the weather expert thinks a cold wave is coming, he sends word to the cities of that region, and from these cities warnings are sent out by telephone to shippers and to farmers. Signs are also placed on trains, so that farmers are warned to do all they can to save crops. In a similar way warnings of coming storms are sent to ship captains. Weather Maps. On a weather map you will see heavy solid lines joining places that have the same atmospheric pressure in a given region; these are called isobars. We can see from the map that the isobars curve irregularly about the regions of low pressure and high pressure. The "lows" show cyclonic storms. The " highs" bring the sharp, bright weather which follows storms. There are also dotted lines to join places having the same temperature; these are isotherms. Generally only the isotherms for freezing and for zero weather are shown. The map shows also the temperature of the weather stations of the country, the direction of the wind, and the amount of rain or snow. 1. Examine weather maps for several successive 82. Exercises. days. In what general direction do the "lows" move? 2. What is a weather vane? Its shape? Why does it have this shape? 3. Fishermen on the seacoast often sail out at or before daybreak and return about noon. Do the land and sea breezes help or hinder them? 4. Find out, if you can, what are the various flags used as weather signals. 5. Why do men say, "If the wind doesn't 'come up' tonight, we shall have frost "? 6. Why are weather forecasts valuable? 7. What is the force which makes great waves rush up on a beach? 8. What is the difference between the terms weather and climate? 9. Why do we have hail in the summer time, but do not have snow? 10. Suppose you were watching a barometer while a tornado passed near you, what should you observe? 11. Did you ever watch a miniature tornado, or whirlwind? Describe it. |