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When storms arise, mariners are advised by maritime authorities or national weather services through periodic weather broadcasts and by signals in harbors. There are four levels of warnings:
Small craft advisory: winds up to 33 knots; one red pennant
Gale: winds from 34 to 47 knots; two red pennants
Storm: winds from 48 to 63 knots; square red flag with black square in center
Hurricane: winds 64 knots and above; two square red flags.
During the day, flags are hoisted in harbors; at night a system of red and white lights is used to signal the warnings. However, in more remote areas there may be no local weather service and mariners must have the capability to access high-frequency radio weather faxes or Internet-based weather data via satellite.
LOWS AND HIGHS
Earlier I mentioned the Pacific High, a point at which there is little or no wind. Somewhere out in the middle of the Pacific Ocean—maybe at 145 degrees west, 40 degrees north—there is that spot where the barometric pressure (a measure of the weight of the earth’s atmosphere bearing down on land and sea) hits 1,028 millibars, or maybe 1,030, or about 30.4 inches of mercury.17 Virtually all of the weight of the atmosphere is concentrated in the first 19 miles (30 kilometers); above this altitude, the emptiness of space begins. The pressure exerted by the atmosphere was first measured in 1643 by Evangelista Torricelli (1608-1647), an Italian physicist. He constructed a mercury barometer, basically a long glass tube filled with mercury and closed at one end. The open end was placed in a bowl full of mercury, the closed end standing up vertically (Figure 5). The pressure of the air pressing on the surface of the bowl of mercury was sufficient to maintain the column of mercury in the closed tube to a height of 29.92 inches, or in modern terms a pressure of 1,013 millibars. This is considered standard atmospheric pressure at sea level and a temperature of 15 degrees Celsius.
Mercury barometers would be impractical on a vessel in constant