7
which is all the more copious the steadier the winds remain. Occasionally it happens that thecloud masses are driven across the whole island, and in such cases they bring heavy rains over thewest coast also. But these are exceptions; there usually prevails at this time of the year fine anddry weather on the west coast of Luzon and in the greater part of the interior of the Archipelago.
(2) The second group.—“Variable,” having, as stated, a range of 4 millimeters (0.15 inch),corresponds to “changeable” of the ordinary barometer, and needs no explanation.. This divisioncomprises the normal barometric readings for the months from May to October. The hand of theaneroid will also recede to it whenever a cyclone is more than 500 nautical miles distant andconsequently still very far off. As the months May to October show a great tendency for atmos-pheric disturbances, the weather is unsteady or changeable if the needle moves in this division.
With northerly winds the weather is always unreliable, whatever may he the time of the year,as soon as the needle enters this group, except perhaps during October. The reason for this isthat during the months from May to September northerly winds are abnormal for the region of thePhilippines, and hence indicate abnormal conditions of the atmosphere, while from November toApril this whole section of the barometric scale lies below the normal level of the barometer.
Adjustment of the barometer.—Special attention is called to the limit of this second grouptoward the left, which is marked by a red line or arrow and serves as index for adapting theinstrument to latitude and season, as previously stated. It may also be used to correct the weatherindications for elevation above the level of the sea.
The mean barometric readings on the outskirts of a typhoon—or in other words, the upperlimits of atmospheric pressure for “stormy weather”—differs not only for different latitudes but inthe majority of instances also for the same parallel during the various parts of the year. As tothe latter, careful observation has proved beyond doubt that certain months have common character-istics as regards cyclonic disturbances, differing more or less from others which again agree amongthemselves. On the basis of these similarities we divide the year into three groups of months,namely, December, January, February, March forming the first group; April, May, October,November, the second; and June, July, August, September, the third. Whenever in the followingpages “groups” are mentioned, reference is made to this division.
These variations of pressure in the outermost part of a cyclone, ascertained by long experienceand not inconsiderable comparative study, we have laid down in the following table:
HEIGHT OF BAROMETER CORRESPONDING TO THE OUTER LIMITOF ZONE A OF A CYCLONE.
Betweenparallels—
Inches.
Milli-
meters.
Season.
0 °-ll°N_
29. 76
756
Throughout the year.
11° 17°N
f 29.76
756
During months of first group.
1 29. 73
755
During months of second and third groups.
29. 80
757
During months of first group.
170 - 21 ° n.
( 29.76
756
During months of second group.
l 29. 73
755
During months of third group.
( 29.92
760
During months of first group.
21°-25° N_
( 29.80
757
During months of second group.
l 29. 65
753
During months of third group.
30.12
765
During months of first group.
30.00
762
During October and November.
29.84
758
During April and May.
[ 29. 65
753
During months of third group.
I 30.04
763
During months of first group.
32°-35° N_
1 29.84
758
During months of second group.
| 29.69
754
During months of third group.
29.96
761
During months of first group.
35°_40°
{ 29.80
757
During months of second group.
l 29. 69
754
During months of third group.
40°-50° N_
29. 76
756
Throughout the year.
I