Plotting Fronts

[Bob Hartig]

I've tried my hand at creating my own surface maps, but the process stumps me. It's easy enough to plot a boundary where I can see a sharp, clear change in temperature. But I often encounter areas of more gradual change that don't suggest an air mass boundary. Yet when I look at a surface map, darned if some meteorologist hasn't slapped a cold front or a warm front across that sector. I examine the station data and can't figure out the logic behind it. I know more is involved than the flip of a coin in these cases, but it sure seems pretty arbitrary.

How do you:
* Know when a front exists, and where, when the temperature changes are gradual?

* Know where a front ends? I.e. where do I stop sketching in the boundary line?

 

[David Draun]

I learned in my basic atmospheric science course that if there is no major temperature change, look for wind shifts in the surface observations. Let's say Springfield, MO has a SW wind, and Kansas City has a NW wind, however the temperature difference is not too noticeable. We can still see that a cold front is there between the two surface obs due to the winds veering from SW to NW. Usually warm front passages will show winds changing from E or NE to SE or S as the front moves north. So on the weather map, you would sketch the front where that wind shift is very noticeable. North of a triple point, where the front is occluded you get more of a SE to SW wind shift as the front passes. And of course a dryline is pretty self explanatory, just look for rapid decrease in dewpoint as you move west. The wind shift here can also be SE or S to SW or W. North of a low pressure center, the winds back from SE to more NE as the low passes to the south.

Pressure change can be another indicator of a front. As a warm front approaches, pressure slowly falls. At arrival and passage the pressure tends to level off. With the approach of a cold front, surface pressure fall is a bit faster. Then at arrival and passage, there is a rapid rise in pressure.

I hope I was helpful.

 

[Bob Hartig]

That does help, David. Thank you!

 

[Bobby Prentice]

front—1. In meteorology, generally, the interface or transition zone between two air masses of different density.

Since the temperature distribution is the most important regulator of atmospheric density, a front almost invariably separates air masses of different temperature. Along with the basic density criterion and the common temperature criterion, many other features may distinguish a front, such as a pressure trough, a change in wind direction, a moisture discontinuity, and certain characteristic cloud and precipitation forms. The term front is used ambiguously for 1) frontal zone, the three- dimensional zone or layer of large horizontal density gradient, bounded by 2) frontal surfaces across which the horizontal density gradient is discontinuous (frontal surface usually refers specifically to the warmer side of the frontal zone); and 3) surface front, the line of intersection of a frontal surface or frontal zone with the earth's surface or, less frequently, with a specified constant-pressure surface. Types of front include polar front, arctic front, cold front, warm front, and occluded front. See also anafront, katafront, intertropical front, secondary front, upper front. 2. See wave front.

Perhaps the best method to detect surface fronts is via potential temperature (theta) gradients since theta minimizes the effects of station elevation on temperature. The Latest U.S. Analysis of Potential Temperature & Gradient from the State University of New York at Albany is the best online resource for this. Meteorologist generally draw the front along the warm side of the (potential) temperature gradient where the wind shift/pressure trough are typically located. If the cold air is advancing (retreating) they draw it as a cold (warm) front. If the cold air is neither advancing or retreating, the draw it as a stationary front. There are variations on this theme such as frontogenesis and frontolysis.

How do you:
* Know where a front ends? I.e. where do I stop sketching in the boundary line?

This is subjective. The front "ends" when the (potential) temperature gradients become insignificant.

The FAA/NWS Advisory Circular AC 00-6A - Aviation Weather provides further explanation:
AC 00-6A, Aviation Weather: Chapters 7-9. Look at Chapter 8 - Air Masses and Fronts.

 

[Bill Schintler]

RE: frontal identification

Often it helps if you start your surface analysis by drawing isobars (based on sea-level pressure). I sometimes augment the isobars with streamlines which often typically become closer and converge at fronts and other boundaries such as pressure troughs and outflow boundaries... To draw streamlines, sketch lines parallel to the wind barbs from the observations.

http://weather.cod.edu/surface/mthaest.gif

- bill

 

[Robert Speta]

I agree with bill on this one. First start off by drawing isobars. You can often find the postion of a front by this alone by observeing where the lows are and where the sharp bends in the troughs occur. Of course use your other factors. " the backing and veering of the winds"

Then I usually look at temperature after that. Since sometimes on occasion there may be little temperature variation. Expecially in the dying moments of a front.

 

[Glen Romine]

Then I usually look at temperature after that. Since sometimes on occasion there may be little temperature variation. Expecially in the dying moments of a front.

As a note of caution, if you are really looking for fronts - then by definition it must be a boundary between air masses with different characteristics, such as temperature and/or moisture. Often, these are easily recognized by a pressure trough that accompanies the boundary between the air masses, so a pressure analysis can aid in finding fronts. However, if there is only a wind shift and no significant cross gradient in temperature and/or moisture, then it is technically a trough and not a front.

 

[Robert Speta]

Good point glenn. I guess whatever way suites one best. But ya If one does only look at pressure then your right its only a trough not a front.

Thanks