Air Masses — Page 313, Lesson 302

I want to walk you through the warm sector of a polar front depression — this is the area of tropical air that sits between the two fronts. Let me start with the definition. The area lying between the two fronts — that is, between the warm front and the cold front — is known as the warm sector, because it is covered by tropical air. So the warm sector is the wedge of warm, moist tropical air that has been lifted up from lower latitudes and is now trapped between the advancing cold front behind it and the retreating warm front ahead of it. Now, the warm sector does not stay the same shape. As the warm front and cold front move, the warm sector will move with them, and it will in fact narrow. Why? Because the cold front moves faster than the warm front. The cold front is chasing the warm front, and as it catches up, the warm sector between them gets squeezed into a narrower and narrower wedge. The depression itself — the low-pressure centre — sits at the tip of the warm sector, where the warm front and cold front meet. That depression will move parallel to the isobars in the warm sector. Its speed is given by the distance between the first and second isobars. In other words, the tighter the isobars are packed in the warm sector, the faster the depression moves. Now let's look at the weather you experience as a polar front depression passes over you. I'll take you through each stage in order: ahead of the warm front, at the warm front, and then in the warm sector. First, ahead of the warm front. The surface wind velocity — the wind direction and speed — is increasing, with a slight backing. Backing means the wind direction is turning anticlockwise, and here it's usually from the south. The temperature is steady and low. The dew point — that's the temperature to which air must be cooled for saturation to occur — is also steady and low. Pressure is falling steadily. Cloud cover is increasing to 8/8 — that's completely overcast — and the cloud base is lowering. You'll see cirrus, then cirrostratus, then altostratus, then nimbostratus in sequence. Precipitation is light and continuous from the altostratus, becoming moderate to heavy and continuous from the nimbostratus. Visibility is reducing to poor. Second, at the warm front itself. The surface wind shows a sharp veer — that's a clockwise change in direction. Temperature rises suddenly. Dew point also rises suddenly. Pressure stops falling — it levels off. Cloud cover is still 8/8, with a very low base, and the cloud types are nimbostratus and stratus. Precipitation is moderate or heavy and continuous. Visibility is very poor, and fog can occur. Third, in the warm sector. The surface wind is steady, usually from the southwest. Temperature is steady. Dew point is steady. Pressure shows only a slight fall. Cloud cover is 6/8 to 8/8 — mostly overcast but with some large breaks possible. The cloud base is low, and the types are stratus and stratocumulus. Precipitation is light rain or drizzle. Visibility — the excerpt cuts off there, but you can expect it to be reduced, often poor, in the warm sector due to the moisture and low cloud. So to summarise the key contrasts: ahead of the warm front you get backing wind, falling pressure, and lowering cloud with continuous rain. At the front you get a sharp veer, a temperature and dew point jump, and pressure stops falling. In the warm sector the wind is steady from the southwest, pressure falls only slightly, and you get light rain or drizzle with low stratus and stratocumulus.