Both at the surface and in the upper atmosphere, meteorologist constantly refer to "high" and "low" pressure systems. However, we look at them from two different perspectives.
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. Therefore, 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, the altitude at which any particular pressure value occurs will vary from reporting station to reporting station.
These changes in altitude represent different air densities in the atmosphere. Recall that as air temperature decreases, the air's density increases.
This means the altitude where any particular pressure occurs will be lower in the atmosphere when the air is colder. Conversely, higher air temperatures result in lower densities, which increases the altitude of pressure levels.
This is why, as a rule, the atmosphere decreases in height from the equator toward the poles.
Therefore, we draw lines on constant pressure chart to represent the altitude at which that particular pressure occurred. These lines are called isoheights - lines of equal height.
By convention, meteorologists simply refer to isoheight lines as "contours". These lines are analogous to topographic charts. In essence, upper air charts show the atmosphere in three dimensions.
By looking at these contours, we observe patterns of higher heights (called ridges) and lower heights (called troughs). These ridges and troughs drive the weather we experience at the surface.
Learning Lesson: Pie in the Sky
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 (gradient) in height of any particular pressure level.