Constant Pressure vs. Constant Elevation
At ground level, we seek air pressure values as they relate to "sea level" which provides us with a picture of the weather patterns at the surface. Using sea level (elevation = zero) as the common baseline we are able to make meaning of different pressure values between stations. So, on all surface charts, the elevation of the "surface" is considered zero feet.
The lines drawn on surface charts connecting areas of equal pressure are called isobars. "Iso" means equal and "bar" is the unit by which we measure pressure. Therefore, an isobar is a line representing the location where the pressure is equal (the same) along that line.
When we examine the atmosphere however, since air pressure decreases with increasing altitude, the elevation at which any particular pressure value occurs will vary from reporting station to reporting station.
These changes in elevation represent different densities (and ultimately air temperature) in the atmosphere. The height of any pressure level is determined by the density of the air. As the air temperature decreases the air's density increases.
Therefore, the altitude where any particular pressure occurs will be lower in the atmosphere is regions of colder air. Conversely, higher air temperatures result in lower densities with the corresponding altitude of various pressure levels higher.
This is why, as a rule, the altitude of constant pressure levels decrease from the equator toward the poles simply because it is colder at the poles than at the equator.
Therefore, we look at the atmosphere at fixed pressure levels and see the altitude at which these specific pressure levels occur. So for upper air weather maps, instead of looking at the pressure at the sea level elevation, we look at the altitudes at which constant pressures occur.
The lines drawn on constant pressure charts are called isoheights, lines of equal height. For example, on the 500 millibar pressure chart, the air pressure is a constant 500 millibars all across the map. Therefore, the lines on that chart represent the altitude at which the air pressure of 500 millibars was reached. In essence, upper air charts show the atmosphere in three dimensions.
By convention meteorologists simply refer to isoheight lines as 'contours'. By looking at these contours we observe the upper air weather patterns such as ridges of higher heights and troughs of lower heights. And it is these ridges and troughs that govern the weather we experience at the surface.
Analogous to topographic charts, ridges are areas where the elevation of any particular pressure value is higher than the surrounding heights of that same pressure value. And troughs are like valleys in that they lie along areas of the lowest elevations of any particular pressure value.
Learning Lesson: Pie in the Sky
So, wind flowing from a ridge toward a trough is decreasing in height above the surface. Conversely, wind flowing from a trough into a ridge is increasing in height.
Between the colder, more dense air and the warmer, less dense air is the location of the greatest change in heights of any particular pressure level. With change in density comes the change in temperature. Therefore, the wind speed is the strongest at this location and where we find jet streams.