Wx Watch: Isobars and Height Contours

Wx Briefs

Web site of the month

Check out this Web site for some unique new ways of looking at the weather. The site shows hourly temperature, sea-level pressure, and cloud-cover maps, and under the "Maps & 10-day Timeseries" heading on the left frame of the window you can click to see sea-level pressure changes (in millibars) and dewpoints at three-hour intervals. Weather enthusiasts are sure to enjoy the other, equally surfable data included on the site.

Where are the 'old' prog charts?

Those of you who rely on the ADDS (Aviation Digital Data Service) Web site may wonder what happened to the old, Difax-style prog charts that you grew up on. Well, they're still there, but you have to know where to look: After clicking on the "Prog Charts" tab, click to the "Low Level (SFC-FL240)" link under the "Significant Weather Charts (SIGWX)" heading at the lower right of the window. When the "Low Level SIGWX Charts" page appears, click the black buttons on the line labeled "4 Panel," and the "old" prog charts are right there.

Every pilot worth his salt should be able to grab a surface analysis chart, slap it down on the table, and make pretty good sense out of the meteorological "big picture." You know that a large H (in blue on color charts) represents a center of high pressure and that a big L (in red on color charts) stands for a low, and you know what cold (barbed lines, in blue) and warm (semicircular bumps, in red) frontal boundaries look like. But there are plenty of other sets of helpful symbology on surface charts, such as isobars and isotherms.

Isobars are lines that connect locations having the same surface barometric pressure. They can tell a great deal about the weather at the surface, so knowing some isobar basics can help in preflight planning.

First things first. As pilots, we relate to barometric pressure in terms of inches of pressure exerted on a column of mercury. But isobars are shown using an alternate measure — millibars (abbreviated mb). Most of us remember that 29.92 inches of mercury (abbreviated in Hg) is the standard surface pressure and that 1013.2 mb is its equivalent. (Some altimeters also show barometric pressure in millibars, but that's another story.)

Isobars are plotted with a four-mb contour interval. To see how this works, locate a low, then find the nearest isobar and its label. Let's say the isobar is labeled "1008." The next highest pressure value will be 1012, then 1016, and so on.

Isobars show pressure, and therefore, wind patterns. If a low is encircled by a tightly spaced series of isobars, then you know that the low is a deep one — one with strong, converging surface winds and therefore a lot of lifting power. If the low-level winds are converging, there's nowhere for air to go but up! So, chances are that during the ascent, there's condensation and cooling of water vapor. This situation often means you can count on low clouds and precipitation in the vicinity.

On the other hand, if the isobars are loosely spaced and scattered all over the chart, you know that the pressure gradient — the rate of change in barometric pressure over a given distance — is not very high at all. This means surface winds tend to be lighter in strength. Widely spaced isobars sometimes are seen ringing the center of a large area of high pressure. This phenomenon, along with the heavier, descending, and therefore drier air masses associated with highs, helps explain why high pressure systems are frequently associated with good flying weather and light surface winds. Where there's a center of high pressure — at the H on the chart — you'll see a millibar value for the highest pressure. Ditto with low pressure centers. One high may be a "1024 High," in meteorological jargon, while another may be a "1008 High." It's a way of keeping track of the comparative strength of a pressure system. On the other hand, if you see a "906 Low," then you are most likely looking at a hurricane!

What about surface winds? Of course, you'll want to consult AWOS, ASOS, and ATIS reports for precise information on surface wind strength and direction, but reading the isobars can sometimes give you a ballpark idea.

We're often told that wind flows parallel to isobars, but that is not always true. Wind behaves according to three main forces — pressure gradient, Coriolis, and friction. Pressure gradient force drives air toward low pressure centers, while Coriolis force acts opposite to this force. When pressure gradient and Coriolis forces are in balance, that's when air moves parallel to isobars. But friction can upset the balance by slowing wind speeds and reducing Coriolis force.

Friction near the surface is caused by terrain effects — especially high terrain. When friction enters the picture, the advantage goes to pressure gradient force, and wind crosses isobars at an angle — and toward any low pressure centers. Usually, winds cross isobars at up to 30- to 45-degree angles when the terrain is roughest and the isobars are steeply packed.

So by looking at a surface analysis chart's isobars, we can quickly see the strength of any lows or highs, the prevailing winds, and the origin and movement of any air masses. Is an air mass flowing from the Gulf of Mexico? Expect warmer temperatures and cloudy conditions. Is it flowing from the Pacific Northwest or New England coasts? Expect cooler temperatures and widespread low cloudiness. Some surface analysis charts help by showing areas of precipitation, as well as station models that give more detailed information on temperature, dew point, pressure changes in the past three hours, and yes, wind speed and direction. Many times, you can compare the wind barbs to the isobar orientation to see how surface winds respond to friction effects.

Height contours

Up to now, we've been talking about pressure patterns at the surface. But when we check certain types of winds aloft charts — those that show contour lines, not just wind barbs — we're looking at a whole different method of plotting pressure. These charts depict levels of a constant pressure surface, hence their name — constant pressure charts. These charts also have isotherms — lines having equal temperature values.

You've probably noticed that these charts are labeled 850 mb, 700 mb, 500 mb, 300 mb, 250 mb, or 200 mb. These values correspond roughly to 5,000 feet, 10,000 feet, 18,000 feet, 30,000 feet, 35,000 feet, and 40,000 feet msl, respectively.

The lines on these charts show the heights of the referenced pressure surfaces. In other words, all of the contour lines on a constant pressure chart have the same pressure value! What varies is the height of the pressure surface. You'll notice that on a 500-mb chart, for example, consecutive lines may be labeled "5520," then "5460," then "5400." This labeling translates into the height of the 500 mb pressure surface, in meters. The contour interval is 60 meters on 500-mb charts. It's 60 meters on 700-mb charts, too; 850-mb charts have 60-meter height contour intervals in the winter months and 30-meter intervals in the summer.

What influences the variation in height contours? Mainly it is temperature. Warmer air takes up more volume than colder air, so the height of a warm-pressure surface will be higher than that of a cold-pressure surface. Conversely, molecules of colder air are packed more densely, so cold air masses register as having relatively low heights. Meteorologists look at the steepness of a pressure surface's height profiles to determine the movement and intensity of clashing air masses, as well as to look for evidence of frontal activity. If heights are rising next to a trough aloft, for example, then you have confirmation that warm air is moving north. Most likely, a warm front is on the move.

On the other hand, troughs aloft appear as southward bulges in height contours. Those contours have comparatively low height values, reflecting the cold air contained within the trough. What happens when daytime heating occurs beneath a trough aloft? Answer: Instability, as the warm air rises into the colder air aloft and keeps on rising. Thunderstorms could be in the offing.

Meteorologists also check constant pressure charts for signs of diverging air aloft and for other signs of upper-level energy that can intensify the weather below. Divergent patterns appear as a fanning-out of height contours. These patterns typically are found in the southeast quadrants of troughs aloft.

Because constant pressure charts show conditions aloft, surface friction plays less of a role in the direction of winds aloft. For this reason, pressure gradient and Coriolis forces are more balanced, so winds generally parallel height contours. What's more, closely spaced height contours mean stronger winds aloft — the same way closely packed isobars mean strong winds on a surface analysis chart. This relationship, in turn, means you can use constant pressure charts to help get an idea of the winds aloft. The station models on constant pressure charts add even more information. The downside to constant pressure charts is that they come out only twice a day — at 0000 Zulu and 1200 Zulu. Surface analysis charts sometimes are created as often as every three hours, although six- and 12-hour intervals are more common.

Most of us relate fairly well to surface analysis charts, but knowing the ins and outs of constant pressure charts is equally important for rounding out our meteorological savvy. Next month we'll look at how height contours can indicate certain types of winter precipitation.

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