WIND AND WAVES DEPRESSIONS


For those of us who inhabit the mid to higher latitudes, few things make us groan so loudly as news that another depression is on its way. For mariners, these are particularly bad tidings for we know to expect strong winds, rough seas and lots of rain. So, how are they born? Well, in the higher latitudes the simple answer is to say that the fault lies with the polar fronts, but the subject deserves a fuller explanation. If viewed from the side, the Earth’s weather system (greatly simplified) would be seen as being divided into bands that are mirrored each side of the equator – or more accurately, north and south of the Intertropical Convergence Zone (ITCZ), more commonly known as the doldrums. The ITCZ is a band of low pressure where the northern and southern weather systems meet. You will see from Fig 10:1 that both poles are capped with areas of high pressure. These are the ‘polar highs’ and their boundaries are known as ‘polar fronts’. The air in the polar highs is relatively dry and cold, unlike in the neighbouring regions where it’s warm and moist, having been originally drawn up from the tropics. Note how the winds oppose each other along the polar fronts.


The development of a depression is shown in Fig 10:2.


This sequence shows the development and collapse of a depression in the northern hemisphere. The same occurs along the south polar front, but those latitudes are largely uninhabited. When you consider that the southern Australian city of Melbourne is about the same distance from the equator as is Alicante in Spain, the picture becomes clear. So, from here on, when referring to depressions it’s the north we’re talking about.


10


1. The cold air and warm air lie peacefully alongside each other


2. An atmospheric disturbance creates a kink in the polar front


3. The opposing winds and differences in pressure bring a rotational force and the wind starts to circulate within the depression, with cold and warm fronts leading the air masses they represent. That’s to say, the warm front


is the forward edge of the warm air and the cold front is the same for the cold.


4. Eventually, the cold front catches up with the warm front and they become one – the technical term is ‘occluded’. Although the depression is on its last legs, this final stage usually produces a band of heavy rain.


Fig 10:2


RYA Seamanship for Sea Anglers


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