When the height contours bend strongly to the south, (as in the diagram below), it is called a TROUGH. Strong troughs are typically preceded by stormy weather and colder air at the surface.
Here is an example of a trough in an upper-level height field (red contours). The trough axis is denoted by the purple line.
Upper air troughs are also known as Upper-Level Lows.
When the height contours bend strongly to the north (as in the diagram below), this is known as a RIDGE. Strong ridges are accompanied by warm and dry weather conditions at the surface.
Here is an example of a ridge in an upper-level height field (red contours). The purple line denotes the ridge axis.
Upper air ridges are also known as Upper-Level Highs.
Therefore, by examining our 500 mb surface again, we can identify a couple of things:
This height field tells us that fair weather conditions probably exist along the East Coast (where an upper level ridge is found).
On the other hand, cloudy and cooler conditions (and possibly precipitation) exist over the central United States (due to the presence of an upper air trough).
So you see, the height field on an upper air chart can provide some initial insight about weather conditions at the surface.