Atop a tall mountain in Chile, two giant telescopes, each with a mirror measuring eight meters across, will eventually be linked to two more giant telescopes that are now under construction. The plan is for each of the four giant mirrors to collect starlight, which will then be channeled underground and focused onto a single light detector. Such a linked-telescope design will provide high-resolution images of distant stars equivalent to those produced by a single, hypothetical telescope which would have a light-collecting mirror measuring hundreds of meters across (the manufacturing of such a large, single mirror is not physically or economically feasible). Astronomers hope that this linked-telescope system will allow them to see, for the first time, faint images of planets circling distant stars outside our solar system.

Families of low-pressure systems in various stages of development over the mid-latitudes (Figure 11.1) share the spirit of a giant linked telescope. Indeed, these mid-latitude lows collectively draw warm air from the distant tropics and cold air from faraway polar regions and then focus warmth and chill in narrow zones of contrasting temperatures called fronts. In turn, lows strengthen as they derive energy from these sharpening temperature contrasts. With more isobars encircling and expanding around their centers, lows are able to extend their northward and southward reaches and gather increasingly warmer and colder air, thereby magnifying temperature contrasts. This process is a step in a positive feedback process that leads to further strengthening. Like mirrors in a powerful telescope, the overall image that emerges from a strengthening low becomes crystal clear - warm air travels farther poleward while cold air knifes farther equatorward. (Figure 11.2(a-b)).

Another way of thinking about mid-latitude lows is that they are atmospheric relay switches - they receive and process signals of warmth from the tropics and signals of chill from the poles. They then amplify these signals and transmit warm air farther toward polar regions and cold air farther toward tropical regions. In this way, lows act as important relay switches in the general circulation's intricate circuitry (which includes Hadley "Cells") that is designed to mitigate temperature contrasts between the poles and tropics.

Telescopes are mounted at high elevations for astronomers to best view the cosmos. Following suit, meteorologists look to the upper troposphere to gain clearer insight into the development of surface low and high-pressure systems.

In order for mid-latitude lows (and highs) to fulfill their commitment to the general circulation, they need a little help. The purpose of this chapter is to shed light on the role that upper-air patterns of wind and pressure play in aiding the intensification of surface lows and highs, allowing them to more effectively send cold air equatorward and warm air poleward.