问题 单项选择题

一种材料的导热系数的大小,与下列哪一条有关

A.材料的厚度
B.材料的颜色
C.材料的体积
D.材料的容重

答案

参考答案:D

解析: 导热系数是指在稳定传热条件下,1m厚的材料两侧表面的温差为度(K,℃),在1s内,通过1m2面积传递的热量,用λ表示,单位为瓦/(米·度)[W/(m·K),此处的K可用℃代替]。传热系数与材料的组成结构、密度、含水率、温度等因素有关。非晶体结构、密度较低的材料,导热系数较小。材料的含水率、温度较低时,导热系数较小。

单项选择题

The United States experienced some of the most extreme weather events in its history this spring, including deadly outbreaks of tornadoes, near-record flooding, drought and wildfires. Damages from these disasters have already passed $ 32 billion, and the hurricane season, which is just beginning, is projected to be above average, according to the National Oceanic and Atmospheric Administration.

Government scientists said Wednesday that the frequency of extreme weather has increased over the past two decades, in part as a result of global warming caused by the accumulation of carbon dioxide in the atmosphere. But they were careful not to blame humans for this year’s rash of deadly events, saying that in some ways weather patterns were returning to those seen at the beginning of the last century. "Looking at long-term patterns since 1980, indeed, extreme climatological and meteorological events have increased," said Thomas R. Karl, director of NOAA’s National Climatic Data Center. "But in the early part of the 20th century, there was also a tendency for more extreme events followed by a quiet couple of decades. "

Presenting a new NOAA report on 2011 extreme weather, Dr. Karl said that extremes of precipitation have increased as the planet warms and more water evaporates from the oceans. He also said models suggest that as carbon dioxide builds up in the atmosphere and heats the planet, droughts will increase in frequency and intensity. "But it is difficult and unlikely to discern a human fingerprint, if there is one, on the drought record of the United States," he said.

Some other climate scientists were more categorical about the human contribution to extreme climate events. Kevin Trenberth, distinguished senior scientist at the National Center for Atmospheric Research, said that when the greenhouse effect caused by burning fossil fuels is added to the natural variability of climate, weather disasters can be expected to occur more frequently. "Global warming is contributing to an increased incidence of extreme weather because the environment in which all storms form has changed from human activities," Dr. Trenberth said in a telephone interview Wednesday. "Records are not just broken, they are smashed. It is as clear a warning as we are going to get about prospects for the future. "

April was a particularly devastating month for tornadoes and rainfall, with 875 tornadoes reported during the month and heavy rain and snowmelt contributing to Mississippi River flooding later in the spring that surpassed the historic floods of 1927 and 1937. So far this year, there have been nearly 1,400 preliminary tornado reports nationwide; those reports will most likely be whittled down to about 900 confirmed tornadoes, the second-highest annual total recorded in modern times. The record is 1,011 confirmed tornadoes in 2008. The year also is on track to be one of the deadliest, with 536 fatalities so far from tornadoes, placing 2011 in sixth place in United States history and the deadliest since 1936.

Which of the following would blame human activities for the increase in extreme weather()

A. Thomas R. Karl

B. Kevin Trenberth

C. The NOAA

D. Government scientists

单项选择题