Drawing Isobars

[Andrew Pastore]

I have been trying to draw isobars on unanalyzed upper air charts, and I have been getting somewhat close to the computer analyzed maps, which I use to check my work. However, I often miss low pressure centers and such. My question is, how do you know whether a isobar makes a circle (as in a low or high) or if it continues on?

 

[Tony Laubach]

That's a hard question to answer in terms of writing a reply.. however, I think I can go for this... when you draw isobars on a map, you should have wind directions on there as well. Using those, you can generally see where these centers are based upon the circulating winds. As for closing circles, as you follow contours and draw them, you will see that towards the center, they eventually close themselves off. This goes mainly for surface map obs.

Upper air isn't so easy, but again, you have to rely a lot on placement of those bars and how they flow across a map. Since stations are more spread out at these levels, you obviously have a harder time judging, especially in instances where you may or may not have a closed low. In those cases, sometimes you have to use your own judgement to determine that; this goes back to observing maps at other elevations to pull this answer.

Again, this is a difficult thing to explain. If surface maps are the issue, you're better off double checking things with the wind directions to see where the low would be centered. From there, you can work your way out to see how those isobars close off. For higher elevations where data is less, you're using a lot of your own judgement in combination with the various level maps.

Hope that helps a bit.
Tony

 

[mikegeukes]

Andrew wrote:

I have been trying to draw isobars on unanalyzed upper air charts,

Defintions:
Surface Charts
Isobars -A line of equal or constant pressure;

Upper Air Charts
I usually call them height contours, but also are called isoheight or isohypse.
Some definitons below, basically all mean the same thing.

Contour -A line of constant elevation above a certain reference
level on a previosuly deined surface,
Isoheight -A line of constant height.
Isohypse -A line of equal geopotential height

Other terms:
Interpolation -Estimating data values,
Isoline -A line of equal value.
Isopleth -A line of equal value.
----------------------------------------------------------------------------------
Are you using the following contour intervals when making your charts:
200, 250, 300 charts: contour interval of 120 meters (12dam)
500 mb charts: contour interval of 60 meters (6dam)
700, 850, 925 mb charts: contour interval of 30 meters (3dam)
-----------------------------------------------------------------------------------
Upper Air Data Plots and Contour intervals on Constant Pressure Charts:

200 mb chart, the first digit is 0 or 1 and is not usd
and the last digit is 0 and is not used. Uses the 3 middle digits.
Example: 200=12000 meters
188, 200, 212, 224, 236, etc would be the height contours you would draw.
Also you can label them if you want like 12000, 12120, etc

250 mb chart, the first digit is 0 or 1 and is not used
and the last digit is 0 and is not used. Uses the 3 middle digits.
Example: 960=9960 meters 008=10080 meters, 012=10200 meters
960, 008, 020, 032, 044, etc would be the height contours you would draw.
Also, you can label them if you want like 9960, 10200, 10320, etc.

300 mb chart: the last digit is 0 and is not used. Uses first three digits.
Example: 912=9120 meters
888, 900. 912, 924, 936, etc, would be the height contours you would draw.
Also you can label the contours, 9000, 9120, etc,

500 mb chart: the last digit is 0 and is not used. Uses first three digits.
Example: 558=5580 meters
540, 546, 552, 558, 564, etc, would be the height contours you would draw.
Also you can label the contours, 5400, 5460, etc.

700mb chart: the first digit is 2 or a 3, and that is not used. uses last three digits.
Example: 997=2997 meters, 000=3000 meters
294 ,297, 300, 303, 306, etc, would be the height contours you would draw.
Also, you can label them 2970, 3000, etc.

For 850mb the first digit is a 1 and is not used. Uses last three digits.
Example: 550=1550 meters,
147, 150, 153, 156, 159 etc, would be the height contours you would draw.
Also, you can label them 1470. 1500, etc.

For 925 mb, uses only three digits.
Example: 780=780 meters, 810=810 meters
780 ,810, 840, 870, 900, etc, would be the height contours you would draw.
--------------------------------------------------------------------------------
Some tips:
-Always use a pencil and sketch lightly, when first making your charts
-Height contours (isoheights) should not branch or fork
-Height contours (isoheights should not end abruptly, except for the edge of the map,
-Height contours (isoheights) cannot touch or cross
-Interpolate between values to place the height contour (isoheight)
-Label your height contours (isoheights)
-Locate high and low values of geopotential heights.
-Locate ridges and troughs
-Smooth your height contours (isoheights)

Some hints in locating lows:
-Locate low values of geopotential heights
-Look for troughs, cyclonic (counterclockwise) circulation
for possible upper level lows

-Remember that wind flow (geostrophic) is nearly parallel
to the height contours (isoheights), think of yourself standing
with your back to the wind, lower heights (pressure) will be
on your left, Buys-Ballot's law, exception to this might be
some tall mountains.

Source: Various notes, I have collected. Hope this will help.

Mike

 

[Andrew Pastore]

Thank you for your help. I was beginning to wonder if I was going to get some kind of reply. I will put your suggestions to work. Hopefully I will improve.

 

[Tony Laubach]

Andrew.. it takes some time to learn.. but the more you do it, the more you'll improve.. just stick with it! It actually adds a lot to this very complicated subject.. good luck!