Other Depressions — Page 359, Lesson 329

I want to walk you through a new topic: Other Depressions. This section covers several distinct weather systems that don't fit the classic frontal depression model. Let's start with secondary depressions. A secondary depression forms at the tip of the warm sector of a partly occluded depression. That tip is called the Point of Origin or the Triple Point — it's where the cold front, warm front, and occlusion all meet. While the primary depression is filling up and weakening, a secondary depression can spin up right at that triple point. One common trigger: when the primary depression and its occluded fronts are held up by a mountain barrier, like in southern Greenland or Norway. Once formed, these secondary depressions move in a cyclonic sense around the primary depression — meaning they circulate around it in the same direction as the low-pressure rotation. Now let's move to Cold Air Pools. Cold air pools exist within cold air masses. You can't see them on a standard surface analysis chart. To locate them, you have to examine thickness charts or upper contour charts. Let me explain what thickness means here. On Figure 19.17, the surface pressure is overlaid with a thickness chart. That thickness chart shows the vertical distance between the 1000 hPa level and the 500 hPa level. The thickness is colour-coded, with a scale on the right. The unit is decametres — that's tens of metres. So if over the north of the UK the thickness between 1000 hPa and 500 hPa is 524 decametres, that's 5,240 metres, or about 17,190 feet. These individual thickness values are also called isohypses. For reference, in the International Standard Atmosphere (ISA), the thickness between these two levels is approximately 18,000 feet. So when you see a thickness of 17,190 feet, that's significantly lower — and that indicates a cold air pool exists over the UK. The key point: a low thickness or low altitude on these charts tells you a cold air pool is present. Surface pressure charts give no indication of temperature, so you need the thickness data. Cold air pools can be quasi-stationary — like the one over northwest Greenland — or transitory, like those over the UK and Novaya Zemlya. In the northwest Atlantic and northwest Europe, cold air pools are often found in polar maritime air behind a cold front. If the cold pool is advecting — meaning it's moving horizontally — it will have a cold front at its leading edge. The weather associated with a cold pool is typically convective. That's especially true over land in summer, when you can expect thunderstorms. Now let's talk about Tornadoes. Here's the official definition from the Meteorological Glossary: 'A violent whirl, generally cyclonic in sense, averaging about 100 m in diameter and with an intense vertical current at the centre, capable of lifting heavy objects into the air.' So a tornado is a violent whirl, usually rotating cyclonically — that's counterclockwise in the northern hemisphere. It averages about 100 metres in diameter. At its centre is an intense vertical current — a powerful updraft — strong enough to lift heavy objects. The synoptic situation that gives rise to tornadoes in the USA is shown in Figure 19.18. Tornadoes occur when cold dry air from the northwest meets warm moist air from the Gulf of Mexico over the prairies of central USA. This happens in spring and early summer. 80% of tornadoes occur between 1400 and 2200 local time, with peak incidence at 1700. The precise mechanism of how the 'twister' forms is still open to considerable scientific debate, but computer modelling and the use of Doppler radars are making prediction more certain. Figures 19.19 through 19.22 show possible stages of tornado formation. Let me bring up the relevant diagrams so you can see the cross-section of a tropical revolving storm and the action to avoid one. `` ``