A. Because south and west winds come heavily laden with vapour; and vaporized air is lighter than dry air.
Q. What effect has wind on the mercury?
A. All winds make the barometer drop, except eastern winds: those winds which blow from the south, and south-west make it drop the lowest.
Q. Why do winds generally make the mercury of a barometer drop?
A. Wind is caused by a partial vacuum in some parts of the globe; and as the air rushes in to supply this deficiency, its general pressure is lessened, and the barometer falls.
Q. What is the 3RD SPECIAL RULE in regard to the barometer?
A. While the barometer stands above 30°, the air must be very dry or very cold, or perhaps both, and no rain may be expected.
Q. Why will there be no rain if the air be very dry?
A. If the air be very dry it will absorb moisture, and not part with what it has in rain.
Q. Why will there be no rain if the air be very cold?