剑桥雅思4听力test1section4原文答案解析+文本精讲

[00:57.91]Good day, ladies and gentlemen.
[00:59.49]I have been asked today to talk to you about the urban landscape.
[01:03.57]There are two major areas that I will focus on in my talk: how vegetation can have a significant effect on urban climate, and how we can better plan our cities using trees to provide a more comfortable environment for us to live in.
[01:20.41]Trees can have a significant impact on our cities.
[01:24.16]They can make a city, as a whole, a bit less windy or a bit more windy, if that's what you want.
[01:31.12]They can make it a bit cooler if it's a hot summer day in an Australian city, or they can make it a bit more humid if it's a dry inland city.
[01:41.75]On the local scale - that is, in particular areas within the city - trees can make the local area more shady, cooler, more humid and much less windy.
[01:54.60]In fact trees and planting of various kinds can be used to make city streets actually less dangerous in particular areas.
[02:04.35]How do trees do all that, you ask?
[02:08.34]Well, the main difference between a tree and a building is a tree has got an internal mechanism to keep the temperature regulated.
[02:19.18]It evaporates water through its leaves and that means that the temperature of the leaves is never very far from our own body temperature.
[02:29.45]The temperature of a building surface on a hot sunny day can easily be twenty degrees more than our temperature.
[02:37.23]Trees, on the other hand, remain cooler than buildings because they sweat.
[02:44.16]This means that they can humidify the air and cool it - a property which can be exploited to improve the local climate.
[02:57.52]Trees can also help break the force of winds.
[03:00.80]The reason that high buildings make it windier at ground level is that, as the wind goes higher and higher, it goes faster and faster.
[03:11.53]When the wind hits the building, it has to go somewhere.
[03:15.24]Some of it goes over the top and some goes around the sides of the building, forcing those high level winds down to ground level.
[03:24.83]That doesn't happen when you have trees.
[03:27.58]Trees filter the wind and considerably reduce it, preventing those very large strong gusts that you so often find around tall buildings.
[03:38.60]Another problem in built-up areas is that traffic noise is intensified by tall buildings.
[03:45.70]By planting a belt of trees at the side of the road, you can make things a little quieter, but much of the vehicle noise still goes through the trees.
[03:56.76]Trees can also help reduce the amount of noise in the surroundings, although the effect is not as large as people like to think.
[04:04.45]Low- frequency noise, in particular, just goes through the trees as though they aren't there.
[04:11.46]Although trees can significantly improve the local climate, they do however take up a lot of space.
[04:19.00]There are root systems to consider and branches blocking windows and so on.
[04:24.30]It may therefore be difficult to fit trees into the local landscape.
[04:29.24]There is not a great deal you can do if you have what we call a street canyon - a whole set of high-rises enclosed in a narrow street.
[04:39.26]Trees need water to grow.
[04:42.70]They also need some sunlight to grow and you need room to put them.
[04:47.69]If you have the chance of knocking buildings down and replacing them, then suddenly you can start looking at different ways to design the streets and to introduce...