SOLUTION: Isobars and Isotherms on the Weather Map Lab Report

SOLUTION: Isobars and Isotherms on the Weather Map Lab Report.

Module 2 Lab
Drawing Isobars and Isotherms on the Weather Map
Isoplething, or contouring, is one of the most important skills
you will need to understand maps in meteorology, because it’s the
easiest way to spot spatial patterns in those maps. In this exercise,
you will make your own isopleths in order to better understand those
contoured maps. Isopleths are lines of constant value of any
parameter, such as temperature (isotherms), pressure (isobars), or
wind speed (isotachs). The technique of isoplething forms the basis
for the analysis of most meteorological parameters. By drawing
isopleths one generalizes, over an area, the data which is observed at
a finite number of places within the area. Any continuous field of
numbers can be isoplethed. All one needs to do is to connect all the
points having the same numerical value. To do this, one must
sometimes interpolate values between the observation places.
The first and most important rule is:
1. Draw lines which keep higher values on one side and lower values
on the other. It should look like the following example:
Example of isoplething. Observations which are exactly the same as the
isopleths must lie exactly on it. The contour interval is 2, and notice how the
contours go through data of the same value, and in between the others.
Other rules are as follows:
2. Interpolate, i.e., a “10” line should be midway between
observations of 9 and 11 and 1/3 of the way from 9 to 12.
3. Isopleths NEVER branch, fork, or intersect. Leave no loose ends
in the middle of a map. You may stop a line just in from the edge of
the map or where the data ends. Label all ends (see #5).
4. Avoid narrow necks. In general, do not pass an isopleth of value
N between two observations of value N+1 or N-1. The isopleth
below is a line of value 9:
5. Label all isopleths with their value:
6. Draw isopleths lightly in pencil so you can erase and redraw easily
if mistakes are made. Ink or draw over in colored pencil only after
you are sure of your analysis. (OK, that’s not a “rule”. It’s just a
The most common kinds of isopleths are isobars and isotherms.
Definition: Isobar: “A line of equal or constant pressure; an isopleth
of pressure…” (from the Glossary of Meteorology). Isobars are
often drawn on the surface map in increments of 4 millibars
(mb). We incorporate the suffix “bar” in the name.
Definition: Isotherm: “A line of equal or constant temperature
…” (also, from the Glossary of Meteorology; there is one in Milne
Library, REF QC854.G55 2000). The name isotherm tells you easily
what it is.
In this course we will also encounter isotachs (lines of constant wind
speed), streamlines (lines of constant wind direction), and isoheights,
which are like isobars, but for lines of constant pressure surface
height above mean sea level.
Remember from Module 1, weather observations are plotted on
station models, allowing us to plot and analyze several parameters at
Coded weather: Temperature = 65°F, Dew point = 60°F, Wind =
south at 10 knots, pressure = 1004.0 mb, pressure is rising steadily at
2.4 mb/3 hrs, sky is 6/10 cloudy with a slight rain shower.
IMPORTANT: When you draw isopleths, the value is at the center
of the station circle, NOT at the number. Here’s an example using
In order to produce a legible chart, it is useful to draw isopleths of
different parameters, such as temperature and pressure, in different
colors. The usual color scheme is: Lines of equal temperature
(isotherms) are red, lines of equal pressure (isobars) are black. For
black, pencil will be fine. Use the red pencils supplied for isotherms,
or use your own red pencil or pen. If you use pen, be sure to draw
the isopleths in erasable pencil first, then ink over when you are sure
of your analysis.
Isobars are in standard spacings – every 4 mb (992, 996, 1000,
1004, etc.). The spacing is often called a contour interval. That
contour interval for isotherms is not as standardized as for
isobars. We can draw them every 10°F (30, 40, 50, etc.).
Pressure centers (H’s and/or L’s). If an isobar makes a closed
figure like a circle, there must be a center to it, either a High (H) or a
Low (L) but if the isobar is not closed, Do NOT draw in a center!
Practice chart: please use the following map
Draw dewpoint contours from 45 to 70 degrees and use a 5 degree interval. If you did it
correctly it should look like this:
Graded Assignment: Print this chart. In the below weather chart, decode temperature
(F) station codes (recall, temperature in degrees F is the number to the upper left of the
“dot” for each station). Then using red pen, hand draw onto this chart, in red, the 20, 30,
40, 50- and 60-degree isotherms, using the rules of isoplething presented in Module Two.
Take a picture with your phone and send me the jpeg of your completed work using HCC email or Canvas. Worth 8 pts (8% final grade).
7. Lines of equal temperature are called _________________________________.
8. Lines of equal wind speed are called ___________________________________.
9. Lines of equal pressure change are called _____________________________________.
10. Isobar contour lines main features are to show what?

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SOLUTION: Isobars and Isotherms on the Weather Map Lab Report

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