The Earth s gravity pulls it.But every cloud is made of water 0droplets or ice crystals. Why doesn t rain or snow fall constantly from all clouds? The 0droplets or ice crystals in clouds are exceedingly small.The effect or gravity on them is minute. Air currents move and lift 0droplets so that the net downward displacement is zero, even though the 0droplets are in constant motion.
Droplets and ice crystals behave somewhat like dust in the air made visible in a shaft of sunlight. To the casual observer, dust seems to act in a totally random fashion, moving about chaotically without fixed direction. But in fact dust particles are much larger than water 0droplets and they finally fall. The average size of a cloud 0droplet is only 0.0004 inch in diameter. It is so small that it would take sixteen hours to fall half a mile in perfectly still air,and it does not fall out of moving air at all. Only when the 0droplet grows to diameter of 0.008 inch or larger can it fall from the cloud. The average rain0drop contains a million times as much water as a tiny cloud 0droplet. The growth of a cloud 0droplet to a size large enough to fall out is the cause of rain and other forms of precipitation. This important growth process is called "coalescence.
23. What is the main topic of the passage? (A) The mechanics of rain (B) The weather patterns of North America (C) How Earth s gravity affects agriculture (D) Types of clouds
24.The word "minute in line 4 is closest in meaning to which of the following? (A) second (B) tiny (C) slow (D) steady
25 .Thc word "motion in line 5 is closest in meaning to (A) wind (B) change (C) movement (D) humidity
26.Ice crystals do NOT immediately fall to Earth because (A) they are kept aloft by air currents. (B) they combine with other chemicals in the atmosphere (C) most of them evaporate