fronts). The centers of low-pressure systems parallel to isobar lines, turning slightly in-
are marked with an L and a notation of the ward around low-pressure systems (approxi-
central barometric pressure. The centers of mately 20 degrees, though somewhat more
high-pressure systems are marked with an at surface levels and less at higher levels)
H and a notation of the central barometric and outward around high-pressure systems.
pressure. Arrows projecting from the lows Wind speed is related to the spacing of the
and highs ending in an X show the systems’ isobars—the closer the isobars, the greater
projected tracks and both the predicted loca- the pressure gradient over a given distance
tions and pressure in 24 hours. and, therefore, the stronger the winds.
A low-pressure area is labeled “rapidly in- Data about surface winds as reported by
tensifying” if the low’s central pressure drops buoys and ships is also included; wind direc-
24 Mb or more in a 12- or 24-hour period. If tion is indicated on an eight-point compass
a low-pressure system is forecast to become a rose along with notations about wind speed
gale (Force 8: 34 - 40 knots) or storm (Force and cloud cover.
10: 48 - 55 knots), it is labeled “developing
500 Mb Charts
gale” or “developing storm” (for more on the
Beaufort Scale, see page 297). Pronounced
Upper-air jet stream winds act as steering
thunderstorm activity is shown with com-
currents for surface weather and influence
ments on coverage and intensity. Due to the
the formation of lows, highs, troughs, ridges,
uncertain movement of tropical weather
and fronts. When doing your own weather
systems, including hurricanes, forecast posi-
interpretation, you should compare surface
tions for these systems are often not shown
analysis charts with upper-air charts to pre-
on long-range forecast charts, but are only
dict what forces will likely drive the surface
depicted on 24-hour charts.
weather for the next few days or longer.
On surface analysis charts, atmospheric
The NWS issues many upper-air charts,
pressure is indicated by isobars (lines con-
each one identified by a specified atmo-
necting points with equal barometric pres-
spheric pressure. Mariners get the most
sure). Isobars are drawn at intervals of 4 Mb
useful information from the 500 Mb chart, r
of pressure. Outside the tropics, winds blow
which depicts the height above sea level
where the atmospheric pressure equals 500
e
athe
W
r
e
athe
r
standing W
nde
U
Figure 5
Figure 5 On a 500 Mb chart, lines are called height contours and are similar to lines on sur-
face “topo” maps. The line labeled 582 on the accompanying chart indicates a pressure of
500 mb at a point 5,280 meters above sea level.
789