Common Features of Constant Pressure Charts

Height Contours

One thing all upper air charts have in common are the height lines (contours) themselves. These lines represent the altitude (in meters) of various significant pressure levels. On any given constant pressure chart, the air pressure is the same at all locations.

What varies on constant pressure charts is the altitude where that particular pressure occurs. In effect, this means these constant pressure charts actually show three-dimensional undulations in the atmosphere.

These undulations represent different densities (due to different air temperatures) in the atmosphere. The height of any pressure level is determined by the density of the air below it. As the air temperature decreases, the air's density increases.

How density affects constant pressure levels.

Therefore, the altitude where any particular pressure occurs will be lower in regions of colder air. Conversely, higher air temperatures result in relatively higher altitudes of various pressure levels. This is why, as a rule, the altitude of constant pressure levels decrease from the equator toward the poles simply because it is warmer at the equator than at the poles.

The areas of lower heights (colder, more dense air) are called troughs. The regions of higher heights (warmer, less dense air) are called ridges.

Troughs are typically indicated by brown dashed lines with ridges indicated by brown zig-zag lines. We typically experience fair weather under upper level ridges and unsettled weather near upper level troughs.

In the upper air (usually 500 millibar level or higher), fair (or improving) weather conditions generally occur between the ridge and the down wind trough. Unsettled (or deteriorating) weather generally occurs between the trough and the down wind ridge.

All upper air charts use the metric system of units for heights (and temperatures).

Wind Direction

The motion of the air in the upper atmosphere is usually west to the east in both Northern and Southern Hemispheres and will parallel or closely parallel the contours. Why west to east? The reason has to do with momentum.

Your speed relative to the Earth's axis depends on your location. Someone standing on the equator is moving much faster around the planet compared to someone standing on a 45° latitude line. In the graphic (right) the person at the equator arrives at the yellow line sooner than the person at 45°N.

How the earth's rotation affects the west to east direction of the jet stream.

Someone standing on a pole is not moving at all (except that he or she would be slowly spinning). The speed of the rotation of the earth is great enough to cause you to weigh one pound less at the equator than you would at the poles.

The momentum the air as it travels around the earth is conserved, meaning if air over the equator starts moving toward the poles, it keeps its eastward motion constant. The result is that, relative to the Earth's surface, the air moves faster and faster from west to the east the farther it moves away from the equator.

Wind Speed

When the height contours (lines) are close to each other, it means there is a more rapid change in altitude of the constant pressure level, and indication of a large temperature gradient.

As the temperature gradient increases (more rapid change) so does the pressure gradient. Wind is created when there is a pressure gradient, and the stronger the gradient the stronger the wind.

Wind speeds on weather maps are in knots (kt) where one knot equals 1.15 mph and 1.85 km/h. Toggle the "Wind Barbs" check box to display the speed and direction of the wind across the map.

The wind barbs indicate wind direction and wind speed (rounded to the nearest 5 knots). The longest line (shaft) points in the direction FROM which that wind is blowing.

The shorter lines, called barbs, indicate the wind speed in knots (kt). The speed of the wind is determined by the barbs.

Each long barb represents 10 kt with short barbs representing 5 kt. A pennant is used to represent each 50 kt. Total the barbs and pennants to provide the wind speed at that location.

When lines of equal wind speed are drawn (called isotachs) the regions of greater and less wind speed stand out. The isotachs are typically drawn for every 20 kt.

On the map below the check boxes will toggle overlays that show some of the common features of upper air charts.

Map of the North America
Height contours
Locations of troughs and ridges.
Where weather occurs related to troughs and ridges.
Wind barbs
Wind contours

As a radiosonde ascends it reports the pressure levels of significant changes in temperature and humidity. However, there are also specific pressure reporting levels of temperature and humidity that are the basis for the creation of constant pressure charts.

By convention world-wide, constant pressure charts are typically created for the 200 mb, 300 mb, 500 mb, 700 mb and 850 mb pressure levels. Together, these five level can give a meteorologist a good representation of the state of the atmosphere.