TOEFL Question

The Atmosphere

The Earth’s atmosphere is a complex and dynamic environment with many physical and chemical processes occurring and interacting simultaneously. Technically, there is no definitive upper limit to the reaches of the atmosphere, since the gases from it is comprised simply grow thinner as the Earth’s gravitational influence lessens; however, for everyday purposes, the atmosphere can be said to include two distinct layers, the troposphere and the stratosphere. The regions above this need only be considered for the purposes of space travel.

The troposphere, which extends from the surface of the Earth to an average altitude of around 17 km, is the portion of the atmosphere we most often encounter in our everyday lives. The troposphere contains nearly 80% of the entire mass of the atmosphere. This is the region where most of the weather events we experience, such as rain, winds, and storms, occur, and it is the most physically dynamic section of the atmosphere. The main energy source in the troposphere is convective heating, in which the sun’s heat is radiated back into the atmosphere from the surface of the Earth. Thus, its lower reaches, which are located closest to the heat source, are the warmest. The physical dynamics of the troposphere result from the tendency of warm air to rise and cooler air to sink. This means that convective heating creates a constant and complex vertical movement in the air of the troposphere, which in turn creates winds and most of the weather patterns we are familiar with. The greenhouse effect, the warming of the Earth that has become the focus of environmentalists in recent decades, occurs because the buildup of pollutants in the troposphere allows for greater convective heating.

The stratosphere extends above the troposphere to a height of 50 km. Here, the major heat source is the ultraviolet rays of the sun. Ultraviolet rays are weakly penetrating rays and therefore their effect is greatest in the upper reaches of the stratosphere. The stratosphere is segregated into distinct temperature layers, with littler vertical interaction. For this reason, there is far less air turbulence in the stratosphere and airliners travel in this region to take advantage of the smoother air. The primary environmental concern in the stratosphere is the conservation of the ozone layer. Many airborne pollutants, most notably CFCs, react chemically to break down ozone as they reach the stratosphere, exposing the troposphere and the Earth’s surface to greater levels of ultraviolet radiation.

Q1. According to paragraph 2, most weather events are due to

(A) the greenhouse effect

(B) the dispersal of excess heat from the sun

(C) the formation of static layers in the troposphere

(D) the rise and fall of air masses in the troposphere

Q2. Based on the information in paragraph 3, what can be inferred about the stratosphere?

(A) Its environmental problems are the most significant we face in modern times.

(B) Its temperature rises with each progressively higher layer

(C) It is totally devoid of wind and weather patterns.

(D) Its air never interacts with the air of the troposphere.

A1. (D) A2. (B)