CHECK THE ISOBARS

One of the first weather products a pilot seeks out before flying is a surface analysis chart, and one of the most eye-catching features of that chart is the pattern of isobars arranged around high- and low-pressure systems. Isobars, defined as lines of equal atmospheric pressure, are as informative as they are visually striking. Isobars provide a ready look at how strong the winds are and from what general direction they flow. That's because winds flow roughly parallel to isobars—clockwise around high-pressure systems and counterclockwise around lows in the Northern Hemisphere.
"Isobars show pressure, and therefore, wind patterns. If a low is encircled by a tightly spaced series of isobars, then you know that the low is deep—one with strong, converging surface winds and therefore a lot of lifting power. If the low-level winds are converging, there's nowhere for air to go but up! So, chances are that during the ascent, there's condensation and cooling of water vapor. This situation often means you can count on low clouds and precipitation in the vicinity,"
Why don't winds precisely parallel isobars? Several forces struggle for dominance, as Horne explains: "Wind behaves according to three main forces—pressure gradient, Coriolis, and friction. Pressure gradient force drives air toward low pressure centers, while Coriolis force acts opposite to this force. When pressure gradient and Coriolis forces are in balance, that's when air moves parallel to isobars. But friction can upset the balance by slowing wind speeds and reducing Coriolis force." That friction is introduced at the surface by terrain. Rougher terrain means more friction.
The link between isobars and winds is just one example of weather information available to any pilot with Internet access who does some homework before contacting flight service for a preflight weather briefing. To learn what else is available, and how to use it, read meteorologist Jack Williams' "Weather on the Web" at AOPA Flight Training Online.