This week you will begin the process of drawing isolines on
maps.
You may have seen similar lines on topographic maps indicating changes
in height. These isolines are a very useful way to look at meteorological
data; it allows us to ‘see’ the numerical data in more of a picture-like
way--It can be easier to look at lines and contours on a map rather
than to try and picture what all those individual numbers on the
station models mean.
Temperature is usually a bit easier than pressure to contour,
so we will start with it. Below you will find a list of contouring
"rules".
These rules apply to any line that you contour, whether it be
temperature, pressure, etc., so remember them every time you contour
! After these exercises you will be able to look at a
contoured map and obtain important meteorlogical information by
examining the contours. Good isolines take time, however, so please
practice practice!
There are 4 basic types of isolines:
Isotherms-- a line of constant temperature.
Isobar-- a line of constant barometric pressure.
Isohypse/Isopleth-- used mostly on upper level charts, is a
line of constant geopotential height above the surface.
Isodrosotherm-- a line of constant dewpoint temperature.
Some general rules to follow in ALL contouring:
- All data points along an isoline have the same value.
- The interval between isolines is a constant value on any one map.
Usually this interval is given (ex: 10°
F), but may be implied.(ex: 4 mb)
- You will be asked to choose your intervals by ‘centering’ on a value;
(For example, center on 0°
F and go up and down in 10°
F intervals)
- Lower values are always on one side of an isoline while higher
values are on the other side of the line-Remember which is which as you
go
- Isolines NEVER CROSS OR TOUCH, NEVER SPLIT
- Isolines will generally be SMOOTH AND PARALLEL
- Interpolate between points as exactly as possible!
- Do NOT draw lines where there are no data points
. (You
cannot interpolate where you have no data to go between!)
- LABEL each isoline on the ends for open and in the center for
closed isolines.
Isolines must be drawn to separate or include all data points.
Some hints:
Isotherms tend to run parallel to one another;
You will NOT typically see closed contours on a temperature contour map, except in mountains and desert regions where temps are more extreme-
Isobars tend to contour like circles around low and high pressure areas;
They might remind you of topographic maps with hills and valleys on them.
