Early History -6- 



standpoint of national defense. But they didn't get along very fast. They 

 carried the research along on their own to a large extent, testing instruments 

 on Mount Washington, but they never got tests in aircraft. 



NUCLEATION 



By this time they were deeply interested in their cloud study. They 

 investigated and learned a lot of things. But the thing that struck them most 

 was that, if there are any snow crystals in a supercooled cloud, they must 

 grow rapidly and should tend to fall out. They came to the conclusion that 

 in winter, if there are supercooled stratus clouds from which no snow is 

 falling, even though the temperatures in the clouds are below freezing, there 

 simply are no appreciable numbers of effective snow nuclei. Such clouds can 

 apparently be supercooled to very low temperatures. 



They thought this presented a problem that should be investigated. Why 

 was it that sometimes snow forms so easily, with apparently no lack of nuclei 

 on which crystals can grow, and at other times there seem to be none? They 

 concluded there must be something in the atmosphere that causes water drop- 

 lets to change to ice only at certain times and under various conditions. They 

 decided to make some careful experiments in the laboratory in an attempt to 

 duplicate those conditions. 



SCHAEFER'S COLD BOX 



During Langmuir's absence in California for three or four months in 1946. 

 Schaefer made what Langmuir has described as "some beautiful experiments".^-'-' 

 During the previous winter he had been studying the behavior of droplets on cold 

 surfaces to see how they supercooled or froze as the temperature dropped. He 

 had found he could supercool water drops to as low as -20°C on surfaces coated 

 with polystyrene and similar materials. He had realized, however, that such 

 experiments were not simulating supercooled clouds and had sought a better 

 method of experiment. 



He decided to try a home freezing unit of the type used for food storage. 

 He lined it with black velvet so he could get a good view of what happened inside 

 when he directed a beam of light down into the box. He then breathed into the box, 

 and the moisture condensed and formed fog particles which were just like ordin- 

 ary cloud particles, although the temperature was about -23°C. No ice crystals 

 formed. He tried many different substances dusted into the box to get ice crys- 

 tals to form, but almost never got any. He got just enough to convince him that, 

 if he did get them he could easily see them. 



Finally, one July day when the temperature of the chamber was not low 

 enough, he put a big piece of dry ice into it to lower the temperature. In an 

 instant the air was full of ice crystals. The crystals persisted for a while 



