1 16 ILLINOIS ACADEMY OF SCIENCE 



miles or more per day. Also, though the distribution of 

 clouds and precipitation about a Low is occasionally sym- 

 metrical and uniform, it is nearly always irregular and un- 

 even, and one scarcely sees two maps in ten years that are 

 exactly alike. Especially in the summer half of the year is 

 the arrangement of the clouds and rain of a Low usually very 

 erratic. The showers are generally of relatively small ex- 

 tent, with much clear or partly cloudy sky between, and there 

 is often little or nothing on the map to indicate even approx- 

 imately what localities will receive rain and which ones will 

 be missed. 



Since the occurrence or non-occurrence of precipitaation 

 is in many respects and for most people the most important of 

 the weather elements for the larger part of the year, and since 

 rain or snow is always associated with cloudiness, and since 

 this precipitation is usually preceded by certain diuinct types 

 of clouds, and attended by other types, and followed by still 

 others, it would seem that a study of the cloud forms and 

 developments and movements ought to yield some assistance 

 to one interested in foretelling the weather. This has resulted 

 in the experience of the writer. 



Rain is nearly always preceded several hours, or a day or 

 more, by cirrus of cirro stratus clouds, though sometimes 

 these are obscured by lower clouds and can only be seen 

 through an occasional break in the lower layer. The ap- 

 pearance and movements of cirrus and cirro-stratus should 

 be carefully noted, as these clouds do not always indicate 

 rain. Among numerous types of cirrus indications are the 

 following. 



Cirrus spreading rather rapidly from west and thicken- 

 ing toward the horizon usually indicate that a storm is ap- 

 proaching or developing in the west. If the movement of 

 the individual clouds is from the west the storm will probably 

 advance over your locality arriving in about average time. 

 But if under a similar arrangement and appearance of cirrus, 

 the movement of the individual clouds is from S. W. or S .S. 

 W., the approach and passing and departure of the storm will 

 usually be slower, there is likelihood of more rainfall, and the 

 clearing will be delayed longer, than with the first type of 

 cirrus, even though the weather maps in both instances should 

 be alike. 



" Cirrus or cirro-stratus moving rapidly from S. W. to N. 

 E., or from W. S. W. to E. N.E .. and covering or crossing the 

 upper half of the southern sky. will be followed by rain or 

 snow next day four times out of five. (There is one set of 

 conditions under which this rule will not hold). If the move- 

 ment of cirrus is from nearly due west and crosses only the 



