Reading 32

          

What makes it rain? Rain falls from clouds for the same reason anything falls to Earth. The Earth's gravity pulls it. But every cloud is made of water droplets or ice crystals. Why doesn't rain or snow fall constantly from all clouds? The droplets or ice crystals in clouds are exceedingly small. The effect of gravity on them is minute. Air currents move and lift droplets so that the net downward displacement is zero, even though the droplets 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 droplets and they finally fall. The cloud droplet of average size is only 1/2500 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 alt. Only when the droplet grows to a diameter of 1/125 inch or larger can it fall from the cloud. The average raindrop contains a million times as much water as a tiny cloud droplet. The growth of a cloud droplet 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."

1. What is the main topic of the passage?

(A) The mechanics of rain                                      (B) The climate of North America

(C) How gravity affects agriculture                         (D) Types of clouds

2. The word "minute" in line 4 is closest in meaning to which of the following?

(A) Second                     (B) Tiny                           (C) Slow                         (D) Predictable

3. Why don' t all ice crystals in clouds immediately fall to earth?

(A) They are balanced by the pressure of rain droplets.

(B) The effect of gravity at high altitude is random.

(C) They are kept aloft by air currents.

(D) The heat from the sun' S rays melts them.

4. The word 'motion" in line 6 is closest in meaning to which of the following?

(A) Wind                         (B) Descent                    (C) Movement                 (D) Humidity

5. What can be inferred about drops of water larger than 1/125 inch in diameter?

(A) They never occur.

(B) They are not affected by the force of gravity.

(C) In still air they would fall to earth.

(D) In moving air they fall at a speed of thirty-two miles per hour.

6. In this passage, what does the term "coalescence" refer to?

(A) The gathering of small clouds to form larger clouds

(B) The growth of droplets

(C) The fall of raindrops and other precipitation

(D) The movement of dust particles in the sunlight

7. What is the diameter of the average cloud droplet?

(A) 1/16 inch                                                         (B) 1/125 inch

(C) 1/2500 inch                                                      (D) One million of an inch

kunci jawaban:

1)A 2) B 3) C 4) C 5) C 6) B 7) C