, the symbol for a thunder-storm. Nearly every weather component has a distinctive symbol, and since a great part of the meteorologist's work consists in going over records of observations to search for the number of times the different phenomena occur during each week or month, the task is much simplified when observers employ the symbols, as it is easier to pick out a symbol from a printed or written page than it is to recognize a word. These symbols, moreover, have been agreed upon as a sort of international notation, and make it easier for the meteorologists of different countries to understand the records of foreign meteorological services.
Everybody does not know the Russian word for snow, or the Dutch for hail, or the Bosnian for rain, but all who run, may read when "snow" is universally written, and hail represented by a wedge-shaped figure with lines drawn across. Time and space being limited, nearly all published records of weather merely set forth the number of days throughout the year on which the different phenomena occurred, and should snow, hail or thunder happen two or three times in one day, it would still be counted only as one day. The yearly totals, therefore, show the number of days on which these conditions have been observed. It is now an almost universal custom to count .01 inches or more during the twenty-four hours as a day of rain. Accordingly, where observers read their rain-gauge to three places of decimals, that on which less than .005 inch fell would not be counted as a rainy day. Smaller amounts would, however, be included in the total. Dew may sometimes fall to the amount of .01 in. or more; and that is counted as a rainy day, the rule being to consider the amount of precipitation, irrespective of the manner in which it has fallen. If you wish to make these observations comparable with published records you would do well to conform to these rules.
HAIL
Hail, the next weather component to be considered, presents many difficulties when the attempt is made to explain its origin and formation. Those who have anything to do with scientific matters are well acquainted with the hypothesis, which explains a given fact, and in considering the subject of hail, the meteorologist hears of many hypotheses which are put forward as complete explanations of this phenomenon. Caution is, therefore, to be exercised and every reported statement severely questioned. Remembering the aphorism: "The man or boy who never makes a mistake will never make anything," meteorologists have attacked the question of hail formation, and, although many mistakes have probably been made, the subject has lost a good deal of its mystery. For many years, it was customary to be content with a recognition of the fact that hail and lightning very often occur together, and the conclusion was drawn that the one was in some way responsible for the other. Sufficient corroboration of this hypothesis was to some meteorologists, found in the fact that thunder and lightning are said to be almost unknown in the Arctic regions, and this supposed companion, hail, almost unknown. Roughly speaking, the assumption was that lightning, as it flashed through a cloud laden with watery particles, caused hail to form. Such an explanation only tended to make the subject more mysterious, and the question, How is hail formed? practically remained unanswered. Many simpler explanations of hail have been propounded as the result of modern research, and, like rain and lightning, it has been demonstrated that hail owes its origin to the movement of the minute watery particles found everywhere in the atmosphere.
The clouds from which hail fall are ordinarily of great height above the earth, 40,000 feet or even higher. These are the well-known cirrus. The first condition necessary to the formation of hail is a powerful ascending current of hot, moist air, which may condense its moisture in the shape of the large woolly cloud, known as cumulus. Such a cloud may be 100 cubic miles in volume, and as long as it retains its shape nothing is likely to fall from it to the earth beneath. Before the formation of a thunder-shower, cirriform fibres in some instances break away from the upper portion of this cloud, the electrical tension is lowered, and rain falls. The coalescing of the particles of moisture has a great deal to do with the changes which take place in a cloud. All these changes take place in the higher clouds in a marked degree, and the varying strata through which the watery particles pass in ascending to and descending from this great height bring about the violent change essential to the formation of hail. The necessary conditions for hail are, therefore, a powerful, hot, ascending current of air and great variation in the strata of the atmosphere as regards moisture and temperature. Mountains assist in forcing currents of air upwards, and one mass of air impinging on another is also thrown upwards, so that condensation of moisture rapidly takes place. A hail cloud may be described as a tower of hot air, from the top of which, vapor is ejected into a frosty region. Hot plains are accordingly the most favourable spots for the formation of hail, and in mountainous districts, more hail falls at a distance from the mountains than among them. Snow is observed in all latitudes and at all heights, but hail is confined to middle latitudes, and is rare in high latitudes. The places most affected by hail are those in which, the temperature and humidity of the air are high, while above, at a great height, there is a cold area below the temperature of freezing point; but, as in the case of the rain drop, before anything can be definitely stated, it must be shown how the particles of moisture coalesce to form hail.
SNOW
Snow is frozen water which falls instead of rain when the temperature is below the freezing point. The ultimate constituents of snow are tiny, six-pointed crystals of ice. They assume in combination a thousand different figures ([Fig. 210]), all exceedingly beautiful. Professor Tyndall has shown, further, that the ultimate particles of ice are also these six-pointed stars. The white colour of snow is caused by the commingling of rays of all the prismatic colours from the minute snow crystals. Separately the crystals exhibit different colours.
