234 PHYSIOGRAPHY 



southern hemisphere spring comes in September, October, and No- 

 vember, summer in December, January, and February, and so on. 

 The vernal equinox of the northern hemisphere is the autumnal 

 equinox of the southern, and the summer solstice of the northern 

 is the winter solstice of the southern. The definition of the seasons 

 given above is based on temperature, the summer being made up of 

 the three warmest months, so far as intermediate (temperate) lati- 

 tudes are concerned, and the winter of the three coldest. 



The seasons are sometimes defined in a different way. Thus 

 spring is sometimes regarded as the time between the vernal equinox 

 and the summer solstice; summer the time from the summer sol- 

 stice to the autumnal equinox, etc. 



In middle latitudes we think of the difference between seasons 

 as one of temperature; but in some parts of the earth, wet and 

 dry seasons are more distinct than warm and cold ones. In the 

 polar regions the temperature of the cold season is very much lower 

 than that of the warm one, but there is also a striking difference 

 in the matter of light. The warm season is the light season, and the 

 cold season is the dark one. 



Differences between summer and winter. Summers and winters 

 differ in ways other than temperature. Some of them are the fol- 

 lowing: (1) In our latitudes the summer days are more than 12 

 hours long, and the nights less. (2) The sun is much higher above 

 the horizon at noon in summer than at noon in winter. (3) In 

 summer the sun rises to the north of east and sets to the north of 

 west, while in winter it rises to the south of east and sets to the south 

 of west. (4) The amount of moisture in the air may vary with 

 the season; but in some regions it is the warm season which is wet, 

 while in others it is the cool season. (5) In some regions the winds 

 change their direction and force with the change of seasons, as will 

 be seen later. The first and second of these differences are the most 

 important, so far as concerns the seasons of most places in middle 

 latitudes. 



Why we have summer when we do. (1) Long days and short 

 nights give more hours of heating than of cooling each day, while 

 short days and long nights mean fewer hours of heating and more 

 hours of cooling. (2) The sun's rays are less oblique when the 



