These are something familiar to most of us, whether or not you know the ins and outs of the aviation world. You’ve seen isobar lines every time you see a weather report on television or online (Spain’s main weather agency Aemet even uses them as a background image on its website). But do you know exactly what isobaric formations are – and why it’s important for pilots to have thorough knowledge of them?
What Exactly Are Isobaric Formations?
Let’s start with a bit of etymology. Isobara comes from the Greek isos, meaning “equal”, and baros, “weight” or “heavy” – which in this context refers to atmospheric pressure (which is also measured in bars, a word with the same root). Therefore, the word “isobar” indicates equal atmospheric pressure. The isobar lines are represented by concentric circles with a letter inside: H or L in English, meaning high or low pressure. Other letters may be used elsewhere, depending on the language of the country.
They are not quite concentric circles because climatology is affected by many factors, such as differing elevations in the terrain or variations in surface temperature – which means that there will almost always be irregularities in these circles. These irregular and very recognisable shapes are the isobaric formations.
High-pressure areas (or anticyclones) can be recognised by the letter H, and low-pressure areas (or cyclones) by the letter L. Within these, there are other pressure systems that are identified by the arrangement of the isobar lines over an area. Each of these systems, or formations, has a specific name.
Types of Isobaric Formations
This is an extension of a low-pressure system, represented on a meteorological map by concentric Vs, since it’s an elongated area. Being low pressure, the air has a cyclonic circulation. In other words, it moves towards the low-pressure zone (towards the axis, where minimum pressure is), so anti-clockwise in the northern hemisphere and clockwise in the southern hemisphere. The climatological characteristics of a trough are the following:
- Wind and turbulence are moderate to strong.
- Vertically developing clouds,e. those forming from bottom to top.
- Precipitation, especially thunderstorms.
- Good visibility when there is no precipitation.
Therefore troughs are areas which make flying difficult.
The opposite of a trough, a ridge has an elongated shape – very similar to this one – and only in a high-pressure zone. In ridges the air circulation is opposite of that of troughs – that is, anticyclonic (towards the outside of it). In the northern hemisphere air circulates clockwise, and anti-clockwise in the southern hemisphere. Remember, the trough is represented by Vs, so the ridge is represented by inverted Vs. What characterises the weather in a ridge?
- Clear skies except when there’s temperature inversion, when fog, mist or haze are present.
- No precipitation.
- Light winds.
A ridge, not being at the centre of a high-pressure area, is somewhat more unstable than a trough. So there may be cumulus clouds around, but visibility is good, which makes flying easier.
This formation is located between two low-pressure areas (or depressions) and two anticyclones. Imagine each depression and anticyclone as these perfect concentric circles arranged on a square, one on each corner. The col is the area in between. Here the wind hardly moves, and the weather conditions will depend on what the surface temperature tends to be. In winter, when temperatures drop, conditions are stable, and fog or mist may arise. But in summer, when temperatures rise, conditions are more like those of a trough.
This is an area where the atmospheric pressure remains nearly constant over a large area. On meteorological maps, a barometric swamp is depicted with the isobar lines spaced far apart. These are areas with calm winds, where the weather conditions are very similar to those of cols. In other words, these areas depend on surface temperature variation, with more stable conditions in winter and unstable ones in summer. But the relative humidity of the air and the degree of atmospheric instability will also affect the different meteorological phenomena that occur in this type of formation.
The Surface Analysis Chart
All these formations are identified on chart compiled from simultaneous reports from different weather stations. With it, pilots gain maximum precision when navigating. They take all this information into account when creating the flight plan.