UNFAVORABLE TEMPERATURE 123 



is whirled about ia the air a few times, after which 

 the thermometers are quickly read and the difference 

 in the readings noted. When the air is comparatively 

 dry, evaporation from the muslin proceeds rapidly, and 

 on account of the heat absorbed, the wet bulb indicates a 

 lower temperature than the dry one. When the air is 

 damp, evaporation is slower, and the difference between 

 the readings of the two thermometers is less. In saturated 

 air, evaporation ceases and the two thermometers read 

 alike. By means of the following table, the dew point 

 for any ordinary out-door temperature and atmospheric 

 himiidity during the growing season may be readily de- 

 termined. 



208. Computing the dew point. — The wet-bulb de- 

 pression is found by subtracting the wet-bulb reading from 

 that of the dry-bulb ; this is located in the top line of the 

 table, and the dry-bulb reading is found in the left-hand 

 vertical column. Opposite the dry-bulb reading, in the 

 column headed with the number indicating the wet-bulb 

 depression, is the dew point sought. 



Opposite 47°, in the left-hand column, and under 7 

 in the top liije, we find 28° — the dew point. If these 

 readings are obtained toward sunset on a clear, still eve- 

 ning, we should expect frost, because the dew point is 4° 

 below the freezing point of water. A slight wind, a hazy 

 atmosphere, or a few fleecy clouds would render frost 

 doubtful. With a dry-bulb reading of 45° and a dew 

 point of 25°, a killing frost might be expected. 



209. Use of weather maps in predicting frost. — The 

 still, clear weather favorable to rapid cooling of the soil 

 at night, to air drainage and frost is almost always due 

 to a definite type of atmospheric circulation — the anti- 

 cyclone. Anticyclones are of great extent, sometimes cov- 



