weather and water wisdom 251
this your starting point. At 2200, the reading has fallen 3 mb below the datum line.
Call this your endpoint. Th e total fall from the starting point to the endpoint is 6mb.
Th e barometric trend is thus a slow fall at 1 millibar per hour (6 mb in 6 hours).
ISOBARS: AREAS OF EQUAL PRESSURE
Meteorologists draw concentric circles, called isobars, on weather maps (and synoptic
charts, which is a term used for the wider area weather maps oft en used by mariners).
Th ese circles show areas of equal atmospheric pressure. Look along the isobar circle
for the pressure in millibars. You’ll most oft en fi nd isobars plotted at 2- or 4-millibar
intervals. Adjacent isobars increase toward the center of a high-pressure area and
decrease toward the center of a low. Highs normally bring gentle to moderate breezes
and cooler, dryer air. Lows generally bring squally, unsettled weather.
Gradient and Surface Winds
At an altitude of 1,500 feet above the earth’s surface, winds blow along (parallel with)
isobars. Th ese high-altitude winds are called gradient winds, since their behavior
depends entirely on the pressure gradient. At ground level, however, friction from
terrain and water slows the wind to about two-thirds of its pressure-gradient speed,
and the earth’s rotation defl ects these reduced surface winds so that they cross the
isobars at an angle of 15 to 20 degrees.
In the Northern Hemisphere, the surface winds rotate clockwise around a
center of high pressure (counterclockwise in the Southern Hemisphere) but are
defl ected 15 to 20 degrees away from the high-pressure center. Conversely, Northern
Hemisphere winds spin counterclockwise around a center of low pressure (clockwise
in the Southern Hemisphere) but are defl ected toward the low-pressure center. You
can think of the winds around a low as spiraling inward, and the winds around a
high as spiraling outward from the center.
SEA-CRET TIP
X The distance between adjacent concentric isobars tells you the
relative wind strength you can expect. Widely spaced isobars
indicate weak gradients, bringing light to moderate winds. Isobars
close together indicate steep gradients and higher winds.Using a Weather Pattern Log
For both sailboats and powerboats, weather remains one of the primary fac-
tors in decision making. Do we leave or stay? Do we change routes to avoid
a frontal system? How soon before precipitation begins and waves increase?