512 SCIENCE OF HOME AND COMMUNITY 



-according to season, being about eight hundred miles a day 

 in winter and 'about five hundred miles in summer. In a 

 similar way, the high areas move in an easterly direction. 

 The weather experts are able to prophesy about how far 

 the areas will travel by the next day, and hence what weather 

 conditions will be brought to the various localities. 



To put it in another way, if one wishes to ascertain the 

 weather conditions that he will find in his locality at the end 

 of twenty-four hours, he can look at the weather conditions 

 found in the area situated several hundred miles west of 

 him, as far west as a low area travels in one day. 



These maps are made not only at Washington but in many 

 other large cities and, with the forecast printed on them, are 

 sent into the surrounding sections of the country. A brief 

 forecast is given to the daily newspapers. 



These forecasts are right in about ninety per cent of the 

 predictions and constitute the only reliable method of fore- 

 telling weather. Sometimes forecasts are made for two 

 days ahead; but the longer ahead the forecast is made, 

 the more unreliable it is, because the low areas undergo 

 so many changes as they pass across the country that it is 

 possible to foretell for only a short time what the weather 

 changes will be. 



LABORATORY EXERCISE 38 



Purpose. To keep a record of weather conditions by means of 

 instruments. 



Materials. Barometer, ordinary thermometer, maximum and 

 minimum thermometer, weather vane. 



Directions. I. Copy the table given on page 513 in your note- 

 book. Between eight and nine each morning make the observa- 

 tions called for and put the record in your notebook. 



The record of the force of the wind may be kept in accordance 

 with the following scale proposed by the U. S. Weather Bureau : 

 o, calm; I, light, just moving the leaves of trees; 2, moderate, 

 moving branches; 3, brisk, swaying branches, blowing up dust; 



