Thunderstorms,with their jagged bursts of lightning and roaring thunder,are actually one of nature's primary mechanisms for transferring heat from the surface of the earth into the atmosphere.A thunderstorm starts when low-lying pockets of warm air from the surface of earth begin to rice.The pockets of warm air float upward through the air above that is both cooler and heavier.The rising pockets cool as their pressure decreases,and their latent heat is released above the condensation line through the formation of cumulus clouds.
What will happen with these clouds depends on the temperature of the atmosphere.In winter,the air temperature differential between higher and lower altitudes is not extremely great,and the temperature of the rising air mass drops more slowly.During these colder months,the atmosphere,therefore,tends to remain rather stable.In summer,however,when there is a high accumulation of heat near the earth's surface,in direct contrast to the considerably colder air higher up,the temperature differential between higher and lower altitudes is much more pronounced..As warm air rises in this type of environment,the temperature drops much more rapidly than it does in winter,when the temperature drops more than four degrees Fahrenheit per thousand feet of altitude,cumulus clouds aggregate into a single massive cumulonimbus cloud,or thunderhead.
In isolation,a single thunderstorm is an impressive but fairly benign way fo Mother Earth todefuse trapped heat from her surface,thunderstorm ,however,can appear in correct,and the resulting show,while extremely impressive,can also prove extraordinarily destructive.When there is a large-scale collision between cold air and warm air masses during the summer months,a squall line,or series of thundersheads,may develop.It is common for a squall line line to begin when an advancing cold front meets up with and forces itself a layer of warm and moist air,creating a line of thunderstorms that races forward at speeds of approximately forty miles per hour.A squall line,which can be hundreds of miles long and can contain fifty district thunderheads,is a magnificent of nature with incredible potential for distruction.Whithin the squall line,often near its southern end,can be found supercells,long-lived rotating storms of exceptional strength that serve as the source of tornadoes.
1.The topic of the passage is
a.the development of thunderstorms and squall lines
b.the devastating effects of tornadoes
c.cumulus and cumulonimbus clouds
d.the power of tornadoes
2."Mechanisms"in line 2 are most likely
a.machines
b.motions
c.methods
d.materials
3.It can be inferred from the passage that,in summer,
a.there is not a great temperature differential between higher and lower altitudes
b.the greater temperature differential between higher and lower altitudes makes thunderstorms more likely to occur
c.there is not much cold air higher up in the atmosphere
d.the temperature of rising air drops more slowly than it does in winter
4.The word"benign"in line 16 is closest in meaning to
a.harmless
b.benefical
c.ferocious
d.spectacular
5.The expression"in concert"in line 17 could best be replaced by
a.as a chorus
b.with other musicians
c.as a cluster
d.in a performance
6.According to the passage,a"squall line"in line 20 is
a.a lengthy cold front
b.a serious thunderstorms
c.a line of supercells
d.a string of thunderstorms
7.The pronoun"itself"in line 21 refers to
a.a large-scale collision
b.a squall line
c.an advancing cold front
d.a layer of warm and moist air
8.All of following are mentioned in the passage about supercells EXCEPT that they
a.are of short duration
b.have circling winds
c.have extraordinary power
d.can give birth to tornadoes
9.This reading would most probably be assigned in which of the following couses?
a.Geology
b.Meteorology
c.Marine Biology
d.Chemistry
10.The paragraph following the passage most likely discusses
a.the lightning and thunder associated with thunderstorms
b.various types of cloud formations
c.the forces that contribute to the formation of squall lines
d.the development of tornadoes within supercells
reference:
LONGMAN PREPARATION COURSE FOR THE TOEFL TEST
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