The mystery of cloud formation is as ancient as mankind. Throughout history, man prayed for rain and feared snow and lightning. Only in the last few decades, was man able to experiment and answer many questions as to how clouds are formed. Lately, it has seemed that no almost queries were left in that field. However, a recent look at old experimental data is threatening to overturn a longstanding theory about how water droplets freeze within clouds.

Suspended water droplets can remain liquid even at temperatures far below the normal freezing point. Data collected in recent years show that clouds as cold as –37.5°C can still contain many liquid droplets of water. Such droplets freeze solid almost instantly if they bump into each other or are otherwise disturbed.   Most scientists have long assumed that a tiny globule of pure, liquid water, when disturbed, begins to freeze around an icy seed that suddenly forms inside it. According to this scenario, the time needed to freeze a given volume of water dispersed into a fine mist is independent of the size of the individual droplets because the formation of a seed particle is a chance event.
However, the results of recent laboratory experiments, when combined with information gathered from tests conducted as many as 30 years ago, do not support this scenario. Together, the data indicate that the time needed to freeze a given volume of liquid water varies drastically according to droplet size. This extreme variation makes sense if freezing begins at the surface of the drops, not at the cores. Dividing a given volume of water into a large number of small droplets yields more total surface area than if the volume is split into a small number of large drops. The freezing rate would then depend on the surface area.

The laws of thermodynamics also argue against ice nuclei forming inside liquid droplets. When water molecules begin to assemble into ice crystals, they release large amounts of latent heat. If that process had occurred in the center of a liquid droplet, the heat would have remained trapped within the globule, slowing the freezing process. But if crystallization begins at the droplet's surface, latent heat can more easily transfer to the surrounding air. In this case, the droplets are so cold that heat released internally as crystallization proceeds probably would not melt the developing ice.

Familiar as clouds are, the behavior of their constituent droplets remains only partly understood. Lightning, rainfall, and other meteorological phenomena vary with the ratio of water droplets and ice particles in clouds. Linking freezing rates of clouds to those atmospheric and other climate processes is one of the most unreliable areas in current climate simulations.

1.  According to the passage, if a given volume of water is divided into a large number of small droplets, as opposed to a smaller number or large droplets, it will freeze quicker.  This indicates that
(A) a given volume of water yields more droplets when below freezing
(B) a given volume of water yields more droplets when above freezing
(C) freezing begins simultaneously both at the cores and at the surface of the drops
(D) freezing begins not at the surface of the drops, but at the cores
(E) freezing begins not at the cores of the drops, but at the surface  

The best answer is E. If the relatively large amount of surface area provided by small droplets influences the time it takes to freeze, then freezing must begin on the surface of the droplets.   

2.  According to the passage, all of the following are characteristic of cloud droplets EXCEPT
(A) they periodically bump into each other
(B) the time needed to freeze a given volume of water dispersed into a fine mist is dependent on the size of the individual droplets
(C) can remain liquid even at temperatures far below the normal freezing point 
(D) they are only partly understood by scientists
(E) they appear in the same amount in clouds involved in various meteorological phenomena

The best answer is E. According to the passage, the ratio of water droplets and ice particles in clouds varies in lightning, rainfall, and other meteorological phenomena

3.  The passage suggests that many questions in the field of meteorology
(A) have been answered
(B) have no answers
(C) have yet to be answered
(D) have been answered incorrectly
(E) are uninteresting to scientists today

The best answer is A.  The author states that it has lately seemed that no almost queries were left in meteorology.

4.  The primary purpose of the passage is to
(A) present several explanations for a well-known fact
(B) argue in favor of a long-standing theory  
(C) discuss the implications of a new research finding
(D) present new evidence that overturns a long-standing theory
(E) question the methodology used in a study.

The best answer is D.  The passage presents new evidence on how water droplets freeze which overturns a long-standing theory that claims that droplets freeze from their center.