When the pressure falls in the centre of a depression, the pressure gradient… — Page 313, Lesson 300

I want to walk you through how unbalancing works in a depression, and then we’ll move into the polar front and the depressions that form on it. Let’s start with that unbalancing idea. When the pressure falls in the centre of a depression, the pressure gradient changes. The winds are no longer geostrophic — that means they’re not blowing parallel to the isobars, balanced between pressure gradient force and Coriolis force. Instead, because the pressure has dropped, air starts to flow across the isobars, inward toward the deepening centre. So you get a net flow of air into the low, which is what helps the depression intensify. Now, let’s look at the bigger picture: the polar front. This is the boundary where polar air masses and tropical air masses meet. It’s located in the temperate latitudes — that’s the mid-latitudes — in both the northern and southern hemispheres. Its position shifts, especially with the seasons. For the British Isles, the part that matters most is the Atlantic polar front. The polar front is important because depressions form right on it. These depressions contain modified portions of the front, and those modified portions are responsible for much of the bad weather we get in the UK and Europe. Depressions that form on the polar front are called polar front depressions. They tend to form in families — one behind the other. The most common place for formation is on the tail of the cold front of an existing depression. The portions of the polar front that lie on either side of a polar front depression are labelled either warm or cold, depending on which air mass is advancing. Now, how do these polar front depressions move? They move parallel to the isobars in the warm sector, and their speed is equal to the geostrophic wind speed measured between the two central isobars in that warm sector. Let’s shift to the different types of fronts. First, warm fronts. If warm air is replacing cold air, the front is called a warm front. A warm front has an approximate slope of 1:150 — that means for every 150 units of horizontal distance, the front rises 1 unit vertically. The front moves at right angles to itself, and its speed is equal to two-thirds of the geostrophic interval measured along the front. Next, cold fronts. If cold air is replacing warm air, the front is called a cold front. The slope of a cold front is steeper — approximately 1:50 to 1:80. A winter cold front in Europe will usually produce more intense weather and precipitation. The cold front also moves at right angles to itself, but its speed is equal to the full geostrophic interval measured along the front — not two-thirds, but the full value. Finally, quasi-stationary fronts. When a front has little or no movement, it’s called a quasi-stationary front. Because there’s little movement, the weather conditions are comparatively quiet, though they can last longer. This situation can be described as geostrophic, because the front is parallel to the isobars. So to summarise: unbalancing from falling pressure drives air across the isobars into a deepening depression. The polar front is where polar and tropical air meet, and polar front depressions form there, often in families. Warm fronts have a gentle slope and move at two-thirds the geostrophic interval; cold fronts are steeper and move at the full geostrophic interval; quasi-stationary fronts barely move and bring longer-lasting, quieter weather.