新托福听写训练 第18期（在线收听）

Listen to a talk given by a professor of meteorology.

First of all, let's look at why temperatures tend to be higher in city than in the rural area. This happens because almost 50% of the urban areas are comprised of hard surfaces like paved street, parking lot, buildings and roof tops. As a result, any amount of rainfall is quickly repelled by this service and carried away by storm drains and gutters. Especially water just does not have the chance to stand around until evaporate and during the process of evaporation that heat is removed from the air. So in cities where there is less evaporation temperatures will be higher and of course there are also be issue of added heat coming from building heating system, from industry, cars even human body. Even within the city itself, temperatures can vary significantly. For example, in winter streets that get a lot of use will be 2 or 3 degrees warmer than less travel streets.

In place where car sits for a while like a spotlight can be another 3 degrees warmer. On the other hand, low spots in the city where cold air collect will much colder than higher places. Rain and snowfall are also affected by urbanization. Cities tend to quite less snowfall than the surroundings of countryside because of warmer temperatures in the city. But rainfall in the city can be 5 to 10 percent higher. That happens because of 2 factors. First the warmer city temperature, second the larger number of dust particles in the urban air. It seems dust particles are important requirement for condensation. Water vapor in the atmosphere is able to change to the liquid by planning to dust particles suspended in the air. So where there is the higher number of dust particles, condensation takes place more easily. That's why the fogs and clouds are usually more frequent around the city. Once condensation takes place, the rainfall is not far behind. In the London area for example, thunderstorms can produce 30 percent more rainfalls than the surrounding countryside. Some urban climatologists go so far as they argue that they can see a pattern increasing the rainfall during the workweek. They believe rainfalls amounts are small around weekend because dust particles generated by cars and factories are reduced.