资料题：The thirty-day forecast is made by examining（　）。

A weather map is an important tool for geographers.A succession of three or four maps presents a continuous picture of weather changes.Weather forecasters are able to determine thespeed of air massage and fronts.They are also able to determine whether an air mass is retainingits original characteristics or talking on those of the surface over which it is moving.Thus, a mostsignificant function of the map is to reveal an synoptic picture of conditions in the atmosphere ata 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, droughts and all climate.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 outlooks are based on an analysis of the upper air levels.With the use of electronic instruments and satellites, enormous gains have taken place recently in identifying and tracking storms over regions which have but few meteorological stations.Experiments are also in progress for weather modification studies, although few successful results such as artificial rainmaking have been achieved so far.