THE EDGE OF THE SUN — MENZEL 217 



the spoon in a coffee cup gets hot ; convection, the circulation of warm 

 and cool layers of gas ; and radiation, the flow of light waves or heat 

 waves through space that may or may not contain matter. 



Prior to about 1900, astronomers generally supposed that convec- 

 tion was the major process controlling flow of energy, with heated gas 

 rising and cool gas descending, as in the earth's atmosphere. For 

 convection to occur, however, some very special conditions must be 

 fulfilled. 



Imagine a balloon — a weightless balloon — filled with air. I hold 

 one here in my hands, though you cannot see it, of course, because 

 it is invisible. The temperature and pressure are the same as in the 

 surrounding air. If I release the balloon, therefore, it shows no tend- 

 ency to rise or fall. 



But let me lift this balloon a few feet. Air pressure up here 

 is somewhat less than where the balloon was originally. And so the 

 balloon must expand slightly to equalize the external pressure. The 

 invisible, weightless skin of the balloon holds the gas together. As 

 the balloon expands, the air inside it must cool. We now measure the 

 outside temperature to see if it is higher than, equal to, or less than 

 that inside the balloon. If the balloon is colder than its surroundings, 

 the air inside is denser and heavier. The balloon tends to fall back to 

 its original position. If the two temperatures are equal, the balloon, 

 with its weightless skin, tends to stay in its new position. However, 

 if the balloon is hotter than its surroundings, the air inside will be 

 lighter than the air outside. The balloon will tend to rise faster and 

 faster, as long as this condition persists. A true hot-air balloon! 



On earth, the air around us is usually full of these rising and 

 falling invisible balloons. Of course I use the word "balloons" in a 

 figurative sense, because the gas really does not possess the "invisible, 

 weightless skin" I postulated. But this absence of an envelope does 

 not significantly change the picture. Volumes of gas will rise or fall, 

 whether they are enclosed or not. The rising masses are the "thermals" 

 used by birds or human pilots of gliders to soar to great heights. The 

 vast sea of air can be very bumpy — as many air travelers realize when 

 a rising or falling blast of air may tip even a large plane. 



Although we do not see the thermals in the lower atmosphere, we 

 often can see their upper boundaries, where the rising gas has cooled 

 so much that water vapor begins to condense and form a cloud. As 

 a matter of fact, the condensation process supplies new heat to the 

 rising gas — lights a fire in the balloon, figuratively speaking. And so 

 the gas ascends even more violently than before, like the Chinese "Fire 

 Balloons" that used to climax our Fourth of July celebrations. 

 Clouds, therefore, can be violently turbulent, which is why pilots, 

 especially of small planes, try to avoid them in flight. And you will 



