Our Weather Bureau gives much attention to the prediction of frosts and to the prompt and widespread distribution of frost warnings. Growing crops and fruits are often seriously injured by frosts, and farmers are naturally anxious to have as early warning as possible of their occurrence. Various methods of protecting crops and trees against frost are used. The method most commonly employed consists in the building of fires of brush or other inflammable material on the windward side of the field or the orchard when a frost is expected. The smoke from the fire is blown to leeward across the field, and acts as an artificial cloud, affording protection to the vegetation underneath. Such fires are known as _smudges_. CHAPTER XXIII. CLOUDS AND UPPER AIR CURRENTS. Attentive observation of clouds will soon lead to a familiarity with their common type forms. A series of cloud views,[7] with accompanying descriptive accounts, will teach the names of the clouds and give definiteness to the record. The directions of movement of clouds are determined by means of the nephoscope. Cloud observations should be made at different hours, in different weather conditions, and in different seasons. The following problems are concerned with clouds and upper air currents:—— [Footnote 7: See _Hydrographic Office Cloud Types_, Appendix B.] _A._ =The Typical Cloud Forms and their Changes.=——Note carefully the characteristic forms assumed by clouds; their mode of occurrence, whether in single clots, or in groups, in lines, or all over the sky; their changes in form and in mode of occurrence. Classify and summarize your results. Compare the clouds of the warm months with those of the cold months. Observations have shown that clouds have certain definite characteristic forms which are substantially the same in all parts of the world. This fact makes it possible to give names to the different typical forms, and these names are used by observers the world over. Hence cloud observations, wherever made, are comparable. The first classification of clouds was proposed by Luke Howard, in 1803. The classification at present in use is known as the _International Classification_, and was adopted by the International Meteorological Congress in 1896. _B._ =The Prevailing Direction of Cloud Movements.=——The use of the nephoscope is necessary in the accurate determination of cloud movements. Study the prevailing directions of movement of the clouds, by means of frequent observations with the nephoscope, in different weather conditions. Separate the upper and lower clouds in this study. Summarize your results according to the weather conditions and the kinds of clouds.