56 HEAT. 



quantity of the sun's light is reflected from the surface of these currents 

 especially at great angles of incidence and this gives rise to the glare 

 seen, especially, towards the sun. It is very probable that in some cases 

 what is termed " haze" is due to convection-currents started, either by 

 lighter air ascending from the surface, or by heavier cold air descending 

 from upper currents.* 



Winds. Convection also occurs on a much greater scale in the 

 atmosphere, the currents formed being recognised as winds. A well- 

 known example is given by 



Land and Sea Breezes. It is very often noticed at the seaside 

 that there is, during the daytime, a sea-breeze, which changes to a land- 

 breeze at night. In tropical regions these land and sea breezes are even 

 more marked than in higher latitudes. We may explain them as con- 

 vection-currents. During the day the surface of the land becomes much 

 hotter through the sun's rays than the surface of the sea, the higher 

 specific heat of water, and the mixing up of the surface layers by the 

 waves, both combining to lessen the rise of temperature. The air over 

 the land is, therefore, more heated, and expanding upwards tends to 

 overflow above. The overflow in the upper strata takes place towards 

 the sea, and so the pressure at the sea surface is increased while that at 

 the land surface is diminished. There is therefore a tendency for the 

 surface layer of air to move from sea to land, the motion constituting 

 a sea-breeze. At night, however, the land radiates out its heat more 

 rapidly than the sea, the high specific heat and the agitation of .the sea 

 both tending to keep up its temperature. There is, therefore, a con- 

 traction of the air over the land, and an overflow in the upper strata 

 from sea to land, accompanied by an opposite flow in the surface strata 

 from land to sea, constituting the land-breeze. 



Trade- Winds. We may also explain in a similar way the well- 

 known trade-winds, which blow in certain latitudes, in our hemisphere, 

 from the north-east towards the equator. As in the land and sea breezes, 

 the equatorial heat expands the air, which overflows in the upper strata 

 towards the polar regions, tending to decrease the pressure at the surface 

 near the equator, and increase the surface pressure in higher latitudes. 

 The surface layers of air are therefore pressed from the north and south 

 towards the equator. The north-easterly direction in the Northern 

 Hemisphere, and the south-easterly direction in the Southern Hemi- 

 sphere, of these lower currents arise from the rotation of the earth. 

 For, taking the northern trade-winds, the mass of air moving towards 

 the equator continually comes into regions moving faster from west to 

 east than the region just left. There is, therefore, a tendency on the 

 part of the winds to lag behind the earth's surface in its west to east 

 motion ; or, the wind has a motion towards the west as well as towards 

 the south, making it a north-east wind. 



The corresponding upper current, in its journey towards the pole, 

 ultimately comes down to the surface somewhere about 35 N. latitude, 

 and constitutes a south-west wind. But it is not nearly so constant as 

 the trade wind. 



* An explanation of the twinkling of the stars as due to convection- currents in 

 the air has been given by Montigny, Exner, and Rayleigh (Phil. Mag., xxxvi. p. 129, 

 1893). 



