A weather map is an important tool for geographers. A succession of three or four maps presents continuous picture of weather changes. Weather forecasters are able to determine the speed of air masses and fronts (气锋) to determine whether an individual pressure aroa is deepening or becoming shallow and whether a front is increasing or decreasing in intensity (强烈).They are also able to determine whether an air mass is retaining its original characteristics or taking on those of the surface over which it is moving. Thus, a most significant function of the map is to reveal a summary picture of conditions in the atmosphere at a given time.
All students of geography should be able to interpret a weather map accurately.Weather maps contain an enormous amount of information about weather conditions existing at the time of observation over a large geographical area. They reveal in a few minutes what otherwise would take hours to describe. The United States Weather Bureau issues information about approaching storms, floods, frosts and all climatic conditions in general. Twice a month it issues a 30-day"outlook" which is a rough guide to weather conditions likely to occur over broad areas of the United States. These 30-day outlooks are based upon an anaiysis of the upper air levels which often set the stage for development of air masses, fronts and storms.
Considerable effort is being exerted today to achieve more accurate weather
predictions, with identifying and tracking storms over regions which have but few meteorological (气象)stations. Extensive experiments are also in progress for weathef modification studies.
小题1: One characteristic of weather maps NOT mentioned by the author in this passage is_______.
A．floods
B．fronts
C．temperature changes
D．frosts小题2: The thirty-day forecast is determined by examining________.
A．daily weather maps
B．upper air levels
C．satellite reports
D．changing fronts小题3:The observation of weather conditions by satellites is helpful because it________
A．is modern
B．uses electronic instruments
C．makes weather prediction easier
D．shows things that would otherwise take hours to describe in a few minutes小题4:A weather map is a summary because it_______
A．sums up a great deal of information
B．appears daily
C．shows changing fronts
D．is a science book小题5:You may possibly read this article in_________.
A．a magazine
B．newspaper
C．a novel
D．a science book
匀速穿过宽为b的有界的匀强磁场区域，磁场的磁感应强度为B，从线圈开始进入磁场到线圈刚离开磁场的过程中，外力做功为W。若a>b，则W＝______，若a<b，则W=_______。