1. Winds are caused by the rising of air over heated areas, allowing cooler currents to take its place. (Geography)
2. Rooms are ventilated by heating some of the air more than the rest, thus producing a current. (Hygiene) Winds are nature's means of ventilating the earth.
RADIATION OF HEAT
This should be taken up as an introduction to dew, frost, winds, climate, etc.
1. Make an iron ball hot (the end of a poker will answer). Hold the hand a few inches below the iron. Does the heat reach the hand by convection? By conduction? By means of suitable questions, lead the pupil to see that it is not by convection, for the hand is below the hot object while heated air rises; it is not by conduction, for air is one of the very poorest conductors; moreover, the heat is felt instantly from the poker, but it takes an appreciable time for it to come by conduction and convection. We say this heat is radiated from the iron. The velocity of radiated heat is about 186,000 miles a second.
2. The above experiment may be varied by bringing the hot iron gradually toward the bulb of the air thermometer and noting the greatest distance at which it will affect the thermometer.
It is by radiation that the sun's heat and light reach us. We get much of the heat of stoves, fire-places, and radiators by the same means.
Why does the earth cool off at night? Why does dew form? Why can no dew form on a cloudy night? Why is a mountain top or a desert so cold, especially at night?
3. Take two tin cans (baking powder boxes will answer) and make holes in the lids large enough to admit a thermometer. Blacken one box in the flame of an oil lamp. Fill both with boiling water and put in a cool place. Test with a thermometer from time to time. Which cools most rapidly?
4. Fill the tin cans with cold water, find the temperature, and then place them near a hot stove. Which warms faster? Usually dark or rough surfaces radiate heat and absorb heat faster than bright or smooth ones. An excellent way of testing this is to lay a black cloth and a white one side by side on the snow where the sun is shining brightly. The snow will melt more rapidly under the black cloth. Painted shingles may be substituted for the cloths. Try different colours. The day chosen should not be extremely cold.