colors used on surface and upper level streamline
analysis charts. Figure 4-14 shows the symbols and
colors generally used on low- and high-level Horizontal
Weather Depiction (HWD) analysis and prognosis
charts, although there are minor variations. Many of
these HWD chart symbols may also be used on surface
analysis charts, especially for a small area Local Area
Weather Chart (LAWC).
Contour Lines
In addition to standard symbols, most analysis and
prognosis charts have one, two, or sometimes several
sets of data contour lines displayed. Contour lines
normally connect areas with equal data values, so may
generally be called "isolines" or "isopleths"-lines of
the same value. Normally, the primary set of contour
lines is displayed as a solid line, while the secondary
contour is displayed as a &shed line. For example, a
standard 850-hPa, 24-hour prognosis chart has height
contours in solid lines, temperature contours in dashed
lines, and wind speed and direction as plotted data.
Colors may also be used with contour lines. Normally
contour lines are identified with the value at each end, or
at the top of closed circular systems, as shown in figure
4-15.
The change in numerical value between each
contour line is called the "contour interval." Table 4-1
lists several of the most frequently seen contours, the
Sometimes the distance between contours is so wide
that intermediate contours are drawn. For example, one
intermediate pressure contour using a 2-hPa interval
may be drawn between each 4-h.Pa standard interval
contour. Intermediate contours are normally dashed
lines that are only drawn where they are needed to show
a pattern and are normally labeled at each end.
Other Contour Lines
Other contour names not used as often as those in
table 4-1 are as follows:
Isochrone—lines of equal time occurrence of a
phenomenon, such as the start or end of rainfall
Isodrosotherm—lines of equal dew-point
temperature
Isopach—lines of equal thickness
Isohyet—lines of equal precipitation amount
Isohume—lines of equal humidity (relative,
specific, or mixing ratio)
Isopycnic—lines of equal or constant density
Isogon—lines of equal vector quantity, such as
wind direction
Isovel—lines of equal vertical velocity
Isovort—lines of equal constant absolute
basic contour value, and the standard contour interval.
vorticity
Figure 4-15.—Labeling of contour line values.
4-17
