Wherever eighteen thousand five hundred degrees can be accumulated before frost the olive will ripen, so that if this number of degrees is shown by October in any certain locality, then the olive will ripen there in that month; but in others, where frost intervenes, the olives must be left hanging on the trees for a longer period. This, however, should not extend beyond the end of February, as a later date than this is prejudicial to the welfare of the trees. The lesson derived from these tables seems to be that where the sum total of degrees for the year does not reach twenty-one thousand degrees Fahrenheit, the olive fails to ripen; this we see in the following table, is the case in Sienna, Benevento, Perugia, Bologna, Milan and Turin, and also that this is the fact where the temperature falls below fifty in November. Whether this will prove to be as true in California as in Italy experience alone can determine.
The following table on page 44 is presented in centigrade degrees, as in Fahrenheit it would fail to indicate the object intended, which is the exact period when the olive ripens. The reason of this is, that in Fahrenheit thirty-two degrees of cold are always included which are unnecessary and confusing in deciding the period when the olive ripens, as only degrees of heat are needed. This period is reckoned beginning from the first of June up to such a date as shall give three thousand nine hundred and eighty-two degrees of heat, which is the amount necessary to ripen the olive. As soon as this number of degrees is reached, even though it be the first of October, there the olive will ripen at that time. To change a daily or average temperature Fahrenheit to Centigrade subtract 32° and divide by 1.8, the result will be Centigrade degrees; thus 52° Fahrenheit—52°-32°1.8=11.11° Centigrade.
To change Centigrade to Fahrenheit multiply degrees of Centigrade by 1.8 add 32° and the result will be degrees Fahrenheit. Thus 20° Centigrade 20x1.8+32=68° Fahrenheit.
