Other Depressions — Page 359, Lesson 328

I want to walk you through the section on Other Depressions, starting with the action to avoid a revolving storm — that's a tropical revolving storm, or TRS. First, let's talk about what to do if you're flying near one. In the Northern Hemisphere, if you observe very high values of starboard drift, that means the wind is pushing your aircraft strongly to the right. In that situation, you should turn port or starboard until port drift occurs. Port drift means the wind is pushing you to the left. Once you have port drift, your aircraft is then heading away from the storm. The key point is that you're using the drift direction as your guide to fly out of danger. In the Southern Hemisphere, the drifts are reversed. So the same principle applies, but the direction you turn would be opposite. There's a diagram that illustrates this — it's Figure 19.12, titled "Action to Avoid a TRS". Now, let's move on to Secondary Depressions. This is a different type of depression, not a tropical revolving storm. A secondary depression is defined as a small depression that is enclosed within the circulation of a larger depression. The larger one is called the primary. Importantly, the isobars of the secondary need not show a closed centre — so it might just appear as a kink or bulge in the isobars, not necessarily a complete ring. Secondaries are particularly associated with frontal depressions — the kind you see on weather charts with warm and cold fronts. They form in two main ways. First way: They form on a trailing front from an occluded primary. Imagine a primary depression that has already occluded — its cold front has caught up with the warm front. On the trailing part of that front, a secondary can develop. This secondary may deepen — that means its central pressure falls — and it can grow to become the next depression along the Polar Front, eventually equalling the size of the primary. At that stage, the primary and the secondary tend to rotate around each other, until eventually they merge and the secondary becomes the new primary. So this is how a family of depressions can form along a front. Second way: They form on a trailing cold front well within the primary circulation. In this case, the secondary appears only as a disturbance on the front — it's not a fully developed low. It moves along the front without much development until it eventually becomes absorbed back into the primary circulation. Although this type of secondary produces little weather of its own, it can be significant because it may delay the movement of the cold front and make forecasting of frontal passage difficult. So even a weak secondary can cause problems for a forecaster trying to predict when the front will arrive. There are two figures to help visualise this. Figure 19.14 shows a secondary depression on the end of a cold front. And Figure 19.15 shows a secondary depression within the primary circulation.