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Frontal Depressions By John Holmes


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  • Location: Aviemore
  • Location: Aviemore

    Fronts and Frontal Depressions

    This follows on from the article about Air Masses. It is the different air masses, being side by side to one another that cause our depressions, along with other factors. They develop along what we call the POLAR FRONT. This is where cold and warm air lie next to one another.

    Just delving very briefly into basic physics.

    At a given pressure cold air is denser than warm air. Thus if you were in an aircraft, or climbed up a mountain, the climb of 1,000feet in cold air will cause a greater drop in pressure than climbing the same hill in warm air. It is this fact which creates our major weather systems in the area of the world we live in. Warm air alongside cold air will try to ‘ride up’ over the cold air, due to the difference in density. Warm air is less dense than cold air and is thus ‘lighter’

    See the diagram below, showing the POLAR FRONT (diagram 1)


    The POLAR FRONT circles the whole globe in what is known as the TEMPERATE REGIONS, roughly between 40 and 70 degrees North, in our hemisphere. It is very similar in the southern hemisphere.

    Where warm and cold air meets, something called the THERMAL WIND develops which results in the JET STREAM. Near the Jet Stream ENTRANCE, air accelerates rapidly and near the EXIT the air rapidly decelerates. This, along with other factors, causes the pressure to fall in a particular area of the Polar Front. This is the beginning of a depression which we are so familiar with in the UK.

    Another feature is also affecting its development. This is a mixture of the PRESSURE GRADIENT FORCE and GEOSTROPHIC FORCE. PLEASE do not get worried about these terms.

    They are merely brought into this feature to give the background of how a depression forms. There are plenty of places for you to read further if you wish. I will give a short list at the end of this article. Perhaps it may be appropriate to do an article on these two important items another time.

    In some cases the disturbance may just run along the Polar Front without any significant development. On the other hand it may develop and deepen. Why it may do one or the other is of no concern here.

    See the diagrams below (2-5)


    In the four diagrams the warm and cold air start off both flowing west to east (left to right) with no break in their equilibrium. By diagram 3 this is starting to break down and by the diagram 5 a fully formed Frontal Depression has developed, with cold and warm fronts. Remember from diagram 1 that it is the difference in temperature and density that causes the warm air to try and move up over the cold air. This is the trigger to the possible development of a Frontal Depression, along with other factors which you need not worry about in this instance. These other factors will be explained in another section.

    What direction does this depression move then? On diagram 5 you can see that the direction (in this case only) is roughly left to right (west to east in the accepted sense). The speed of movement is governed by the spacing between the ISOBARS. These are the lines of pressure, seen in the diagram, and on the BBC weather charts. You have probably heard the forecaster talk about the closer the isobars then the stronger the wind. The forecaster has a special measuring scale, for each chart size, which allows a calculation to be made of the likely speed of the depression and its direction. Isobars are the lines on each of the diagrams 2-5. The depression also tends to move in, roughly, the direction of the warm sector isobars. The WARM SECTOR is that part on diagram 4 and 5 which lies between the COLD front and the WARM front. Other factors come into play but are beyond this explanation. All I am attempting to do in this instance is to give you a first idea of how depressions form, why, and how to do a quick approximation of where they may be in a few hours.

    In the next diagram you will see that the word SLOPE is used. This is another example of how warm and cold air has different characteristics due to their differing densities. It also affects the speed that the cold air and warm air travel at. Due to the effect of friction warm air slows down more than cold air. Thus it can be shown that eventually the cold front catches up the warm front and you have what is called an OCCLUSION.

    When this happens, the driving force for that particular weather system is largely removed and the depression tends to slow down and ‘fill’, meaning its central pressure begins to rise.

    Diagram showing Frontal slopes and surfaces - diagram 6


    Wind changes

    Looking at the next diagram again you can see that the isobars ‘kink’ at each front, this means that the wind direction also changes. All you need to know is that, in general, the wind BACKS ahead of a front and VEERS behind a front. (In the northern hemisphere, backing means going in the anti-clockwise direction, veering means going the same way as a clock fingers move)

    In the first diagram, diagram 7, we have put diagrams 2-6 together, whilst diagram 8 shows how the wind ‘backs’ ahead of the front, and ‘veers’ behind the front. This is the case whether it is a COLD front or a WARM front.

    See diagrams below - diagrams 7 & 8


    We now need to consider what happens as; first, the Warm front, then the Warm sector and finally the Cold front pass over ‘you’. Every front is different in its amount of precipitation, how thick the cloud is etc. This is an ‘idealized and rather simplified version to try and explain what happens.

    Diagram 9-11


    On both diagrams 10 and 11, the heights shown on the right hand side are in 000’s of feet and the distances along the bottom of each are in miles.

    Please see the explanation text on page 6. Note that abbreviations for all clouds are used (See separate topic on clouds)

    In diagram 9 this shows a theoretical cross section through the two fronts of the frontal depression. Note that the slope of the warm front is less than the cold front. This is due to the two air masses having different temperatures, hence, different densities. The Polar being colder and denser so the warmer less dense Tropical air flows up the cold air for the warm front creating the characteristic LAYER type cloud. When the cold front moves in then because it is denser, it undercuts the warm air and as well as layer cloud it may also create deep instability causing convective cloud to be embedded in it.. Note the 0 degree isotherm and how that changes depending on the air mass. Also the TROPOPAUSE (see another text) which also changes with the air mass. Finally notice the two letter J’s. These indicate the position of the JETSTREAM (another topic) near each front.

    From Diagram 10:

    Taking the sequence of events for the WARM FRONT: As already mentioned because the air is STABLE (another topic) the cloud is formed in layers as the air ascends and gradually thickens as the front gets nearer. The first sign of an approaching warm front are wisps of Ci, often hooked, above some Cu. This thickens into Cs and may give a halo round the sun or moon. Continued thickening then gives Ac and As which can cause a corona round the sun/moon. At the same time as this is happening any lower cloud becomes more Sc in nature, the wind begins to BACK and the pressure falls.

    By the time the front is about 200 miles away, the wind is often quite strong, probably having backed to south or even south easterly, rain begins to fall and St clouds form, partly as a result of the rain(If the temperature is low enough then snow may fall). Due to the loss of heating and the rain falling the temperature may also fall. Another change which can be noted is that the DEWPOINT has begun to rise quite markedly. Also there is a noticeable drop in the VISIBILITY. As the front arrives, at the surface, then several things are likely to happen. After a period of rather heavier rain ahead of the front, it usually changes to drizzle; there is a marked rise in both temperature and dewpoint; the pressure eases off or stops falling and the wind VEERS more to the south or even south west and may decrease.

    In the WARM SECTOR the air mass has TROPICAL air characteristics. Unless the centre of the low is quite near, and/or you are near an exposed coast, then good cloud breaks will develop. Near exposed coasts and hills, low cloud (St), and Sc and some drizzle along with hill and coastal fog is likely.

    Diagram 11

    As the COLD FRONT approaches so the upper cloud thickens, in much the same sequence as ahead of the warm front, but over a much shorter distance and therefore time scale. The wind BACKS again and strengthens, rain begins, pressure falls. On the front itself very gusty winds are likely with a marked drop in temperature and dewpoint; heavy rain is probable, perhaps turning to snow in winter; hail and thunder are also possible, and the pressure starts to rise rapidly. Also there is usually a rapid VEER in the wind into the west, maybe even north west. Usually, quite quickly the cloud will clear away, and the tops of Cb clouds with the characteristic anvil may be seen.

    We are now back in the POLAR air mass and as the cold air deepens so convective clouds develop again, and showers can develop with large Cu and Cb around.

    Finally JETSREAMS (another topic) have been shown on each of diagrams 10 and 11. The Jetstream is almost always present when a Frontal Depression forms. Indeed they are a major factor in the speed and direction of movement of these depressions at the start of their existence.

    This I hope has given some understanding of, why depressions form on the boundary of cold and warm air masses. How they form, and the pattern of weather to be excepted as the depressions warm and cold fronts move across, usually from a westerly point to a more easterly.

    Several topics have been mentioned for future explanation and another one is to be able to read the WEATHER maps seen on TV which I try and present before too long.

    Please take time to read for those new to this topic and please do ask questions, either on the forum or in a PM to me. For those with greater experience may I ask that, unless you see an absolute major error you bear with my description? Again if you wish to raise any topic then please do so and I will attempt to answer you. Some of you on the forum are both more learned than I am, and more up to date in Meteorology, so it may be me that asks a question.

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